C++ & VISUAL C++
7 likes2,443 views
This document outlines the aims, design, objectives, and units of a Visual C++ programming course. The aims are to review object-oriented design and impart skills in developing VC++ applications using fundamental C++ features like classes, objects, inheritance, and encapsulation. The course is divided into units that cover developing applications using the VC++ IDE, object-oriented programming concepts, and Visual C++ basics like data types, expressions, and control structures. Key concepts like classes, objects, inheritance, polymorphism, encapsulation, and abstraction are defined. The document provides details on theory and practical lessons for each unit.
![VISUAL C++ PROGRAMMING VC++
small business 2.6%
big business 2.9%
gold business 3.0%
Now, the code for this could be written as:
rmmakaha@gmail.com 43
switch (account_value){
case 1:
interest = 2.3;
break;
case 2:
case 4:
interest = 2.6;
break;
case 3:
case 5:
interest = 2.9;
break;
case 6:
interest = 3.8;
break;
default:
interest = 0.0;
}
#include <iostream>
using namespace std;
int main(int argc, char *argv[])
{
switch( tolower( *argv[1] ) )
{
// Error. Unreachable declaration.
char szChEntered[] = "Character entered was: ";
case 'a' :
{
// Declaration of szChEntered OK. Local scope.](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-43-320.jpg)
![VISUAL C++ PROGRAMMING VC++
char szChEntered[] = "Character entered was: ";
cout << szChEntered << "an"; } break; case 'b' :
rmmakaha@gmail.com 44
// Value of szChEntered undefined.
cout << szChEntered << "bn";
break;
default:
// Value of szChEntered undefined.
cout << szChEntered << "neither a nor bn"; break;
}
}
Control Structures
A program is usually not limited to a linear sequence of instructions. During its process it may
bifurcate, repeat code or take decisions. For that purpose, C++ provides control structures that
serve to specify what has to be done by our program, when and under which circumstances.
With the introduction of control structures we are going to have to introduce a new concept: the
compound-statement or block. A block is a group of statements which are separated by semicolons (;)
like all C++ statements, but grouped together in a block enclosed in braces: { }:
{ statement1; statement2; statement3; }
Most of the control structures that we will see in this section require a generic statement as part
of its syntax. A statement can be either a simple statement (a simple instruction ending with a
semicolon) or a compund statement (several instructions grouped in a block), like the one just
described. In the case that we want the statement to be a simple statement, we do not need to
enclose it in braces ({}). But in the case that we want the statement to be a compund statement it
must be enclosed between braces ({}), forming a block.
Conditional structure: if and else
The if keyword is used to execute a statement or block only if a condition is fulfilled. Its form is:
if (condition) statement](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-44-320.jpg)
![VISUAL C++ PROGRAMMING VC++
rmmakaha@gmail.com 62
private:
double real;
double imag;
};
Complex Complex::operator+(const Complex& other)
{
double result_real = real + other.real;
double result_imaginary = imag + other.imag;
return Complex( result_real, result_imaginary );
}
The assignment operator can be overloaded similarly. Notice that we did not have to call any
accessor functions in order to get the real and imaginary parts from the parameter other since
the overloaded operator is a member of the class and has full access to all private data.
Alternatively, we could have defined the addition operator globally and called a member to
do the actual work. In that case, we'd also have to make the method a friend of the class, or
use an accessor method to get at the private data:
friend Complex operator+(Complex);
Complex operator+(const Complex &num1, const Complex &num2)
{
double result_real = num1.real + num2.real;
double result_imaginary = num1.imag + num2.imag;
return Complex( result_real, result_imaginary );
}
Why would you do this? when the operator is a class member, the first object in the
expression must be of that particular type. It's as if you were writing:
Complex a( 1, 2 );
Complex a( 2, 2 );
Complex c = a.operator=( b );
when it's a global function, the implicit or user-defined conversion can allow the operator to
act even if the first operand is not exactly of the same type:
Complex c = 2+b; //if the integer 2 can be converted by the Complex
class, this expression is valid
By the way, the number of operands to a function is fixed; that is, a binary operator takes two
operands, a unary only one, and you can't change it. The same is true for the precedence of
operators too; for example the multiplication operator is called before addition. There are
some operators that need the first operand to be assignable, such as : operator=, operator(),
operator[] and operator->, so their use is restricted just as member functions(non-static), they
can't be overloaded globally. The operator=, operator& and operator, (sequencing) have
already defined meanings by default for all objects, but their meanings can be changed by
overloading or erased by making them private.
Another intuitive meaning of the "+" operator from the STL string class which is overloaded
to do concatenation:
string prefix("de");
string word("composed");
string composed = prefix+word;
Using "+" to concatenate is also allowed in Java, but note that this is not extensible to other
classes, and it's not a user defined behavior. Almost all operators can be overloaded in C++:
+ - * / % ^ & |
~ ! , = =
++ -- << >> == != && ||
+= -= /= %= ^= & = |= *=
<<= >>= [ ] ( ) -> ->* new delete](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-62-320.jpg)
![VISUAL C++ PROGRAMMING VC++
Definition: Precedence is the order in which a program performs the operations in a
formula. If one operator has precedence over another operator, it is evaluated first.
Higher precedence operators "bind tighter" than lower precedence operators; thus, higher
precedence operators are evaluated first. The lower the rank in the following chart, the higher
the precedence.
The following table lists the precedence and associativity of C++ operators. Operators are
listed top to bottom, in descending precedence.
Precedence Operator Description Associativity
1 :: Scope resolution Left-to-right
rmmakaha@gmail.com 64
2
++ -- Suffix/postfix increment and decrement
() Function call
[] Array subscripting
. Element selection by reference
−> Element selection through pointer
3
++ -- Prefix increment and decrement Right-to-left
+ − Unary plus and minus
! ~ Logical NOT and bitwise NOT
(type) Type cast
* Indirection (dereference)
& Address-of
sizeof Size-of
new, new[] Dynamic memory allocation
delete,
Dynamic memory deallocation
delete[]
4 .* ->* Pointer to member Left-to-right
5 * / % Multiplication, division, and remainder
6 + − Addition and subtraction
7 << >> Bitwise left shift and right shift
8
< <= For relational operators < and respectively
= For relational operators and respectively
9 == != For relational = and respectively
10 Bitwise AND
11 ^ Bitwise XOR (exclusive or)
12 | Bitwise OR (inclusive or)
13 Logical AND
14 || Logical OR
15 ?: Ternary conditional Right-to-left](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-64-320.jpg)
![compile time. But now, as the function is declared virtual it is a candidate for run
and the derived class function is being invoked.
C++ Virtual function - Call Mechanism:
VC++
Whenever a program has a C++ virtual function declared, a v
The v-table consists of addresses to the virtual functions for classes and pointers to the functions
from each of the objects of the derived class. Whenever there is a function call mad
virtual function, the v-table is used to resolve to the function address. This is how the Dynamic
binding happens during a virtual function call.
Arrays
An array is a series of elements of the same type placed in contiguous memory locations t
be individually referenced by adding an index to a unique identifier.
That means that, for example, we can store 5 values of type
declare 5 different variables, each one with a different identifier. Instead of tha
we can store 5 different values of the same type,
For example, an array to contain 5 integer values of type
this:
where each blank panel represents
type int. These elements are numbered from
independently of its length.
Like a regular variable, an array must be declared before it is u
array in C++ is:
type name [elements];
where type is a valid type (like int
is always enclosed in square brackets
contain.
VISUAL C++ PROGRAMMING
rmmakaha@gmail.com
v-table is constructed for the class.
int in an array without having to
int for example, with a unique identifier.
int called billy could be represented like
an element of the array, that in this case are integer values of
. 0 to 4 since in arrays the first index is always
used. A typical declaration for an
int, float...), name is a valid identifier and the elements
[]), specifies how many of these elements the array has to
94
run-time linking
made to the c++
that can
that, using an array
0,
sed. field (which
nts](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-94-320.jpg)
![Therefore, in order to declare an array called
simple as:
int billy [5];
billy as the one shown in the above diagram it is as
NOTE: The elements field within brackets
is going to hold, must be a constant
whose size must be determined before execution. In order to create arrays with a variable length
dynamic memory is needed, which is explained later in these tutorials.
Initializing arrays.
[] which represents the number of elements the array
value, since arrays are blocks of non-dynamic memory
When declaring a regular array of local scope (within a function, for example), if we do not
specify otherwise, its elements will not be initialized to any value by default, so their content will
be undetermined until we store some v
value in them. The elements of global and static arrays, on
the other hand, are automatically initialized with their default values, which for all fundamental
types this means they are filled with zeros.
In both cases, local and global, when we declare an
values to each one of its elements by enclosing the values in braces
int billy [5] = { 16, 2, 77, 40, 12071 };
array, we have the possibility to assign initial
This declaration would have created an array like this:
The amount of values between braces
we declare for the array between square brackets
we have declared that it has 5 elements and in the list of initial values within
specified 5 values, one for each element.
When an initialization of values is provided for an array, C++ allows the possibility of leaving
the square brackets empty [ ]. In this case, the compiler will assume a size for the array that
matches the number of values included between braces
int billy [] = { 16, 2, 77, 40, 12071 };
VISUAL C++ PROGRAMMING
rmmakaha@gmail.com
{ }. For example:
{ } must not be larger than the number of elements that
[ ]. For example, in the example of array
braces
. { }:
VC++
95
alue . . billy
{ } we have](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-95-320.jpg)
![After this declaration, array billy
values.
VC++
would be 5 ints long, since we have provided 5 initialization
Accessing the values of an array.
VISUAL C++ PROGRAMMING
In any point of a program in which an array is visible, we can access the value of any of its
elements individually as if it was a normal variable, thus being able to both read and modify its
value. The format is as simple as:
name[index]
Following the previous examples in which
of type int, the name which we can use to refer to each element is the following:
ples billy had 5 elements and each of those elements was
For example, to store the value
statement:
billy[2] = 75;
and, for example, to pass the value of the third element of
write:
a = billy[2];
Therefore, the expression billy[2]
Notice that the third element o
one is billy[1], and therefore, the third one is
billy[4]. Therefore, if we write billy[5], we would be accessing the sixth elemen
therefore exceeding the size of the array.
In C++ it is syntactically correct to exceed the valid range of indices for an array. This can create
problems, since accessing out-of
runtime errors. The reason why this is allowed will be seen further ahead when we begin to use
pointers.
rmmakaha@gmail.com
, 75 in the third element of billy, we could write the following
billy to a variable called
is for all purposes like a variable of type int.
of billy is specified billy[2], since the first one is
, billy[2]. By this same reason, its last element is
. of-range elements do not cause compilation errors but can cause
96
, a, we could
.
, billy[0], the second
. element of billy and](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-96-320.jpg)
![VISUAL C++ PROGRAMMING VC++
At this point it is important to be able to clearly distinguish between the two uses that brackets [
] have related to arrays. They perform two different tasks: one is to specify the size of arrays
when they are declared; and the second one is to specify indices for concrete array elements. Do
not confuse these two possible uses of brackets [ ] with arrays.
int billy[5]; // declaration of a new array
billy[2] = 75; // access to an element of the array.
If you read carefully, you will see that a type specifier always precedes a variable or array
declaration, while it never precedes an access.
Some other valid operations with arrays:
rmmakaha@gmail.com 97
billy[0] = a;
billy[a] = 75;
b = billy [a+2];
billy[billy[a]] = billy[2] + 5;
// arrays example
#include iostream
using namespace std;
int billy [] = {16, 2, 77, 40, 12071};
int n, result=0;
int main ()
{
for ( n=0 ; n5 ; n++ )
{
result += billy[n];
}
cout result;
return 0;
}
12206
Multidimensional arrays
Multidimensional arrays can be described as arrays of arrays. For example, a bidimensional
array can be imagined as a bidimensional table made of elements, all of them of a same uniform
data type.](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-97-320.jpg)
![VISUAL C++ PROGRAMMING
jimmy represents a bidimensional array of 3 per 5 elements of type
array in C++ would be:
int jimmy [3][5];
and, for example, the way to reference the second element vertically and fourth horizontally in
an expression would be:
jimmy[1][3]
(remember that array indices always begin by zero).
Multidimensional arrays are not limited to two indices (i.e., two dimensions). They can contain as
many indices as needed. But be careful! The amount of memory need
increases with each dimension. For example:
char century [100][365][24][60][60];
declares an array with a char element for each second in a century, that is more than 3 billion
chars. So this declaration would consume more than 3 g
Multidimensional arrays are just an abstraction for programmers, since we can obtain the same
results with a simple array just by putting a factor between its indices:
int jimmy [3][5]; // is equivalent to
int jimmy [15]; // (3 * 5 = 15)
With the only difference that with multidimensional arrays the compiler remembers the depth of
each imaginary dimension for us. Take as example these two pieces of code, with both exactly
the same result. One uses a bidimensional array and the
rmmakaha@gmail.com
int. The way to declare this
needed for an array rapidly
gigabytes of memory!
other one uses a simple array:
VC++
98
. ed](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-98-320.jpg)
![multidimensional array
#define WIDTH 5
#define HEIGHT 3
int jimmy [HEIGHT][WIDTH];
int n,m;
int main ()
{
for (n=0;nHEIGHT;n++)
for (m=0;mWIDTH;m++)
{
jimmy[n][m]=(n+1)*(m+1);
}
return 0;
}
VISUAL C++ PROGRAMMING
pseudo-multidimensional array
#define WIDTH 5
#define HEIGHT 3
int jimmy [HEIGHT * WIDTH];
int n,m;
int main ()
{
for (n=0;nHEIGHT;n++)
for (m=0;mWIDTH;m++)
{
jimmy[n*WIDTH+m]=(n+1)*(m+1);
}
return 0;
}
None of the two source codes abo
to the memory block called jimmy in the following way:
above produce any output on the screen, but both assign values
We have used defined constants (
program. For example, in case that we decided
could be done simply by changing the line:
#define HEIGHT 3
to:
#define HEIGHT 4
(#define) to simplify possible future modifications of the
to enlarge the array to a height of 4 instead of 3 it
with no need to make any other modifications to the program.
Arrays as parameters
At some moment we may need to pass
an array to a function as a parameter. In C++ it is not
possible to pass a complete block of memory by value as a parameter to a function, but we are
allowed to pass its address. In practice this has almost the same effect and it is a much faster and
more efficient operation.
rmmakaha@gmail.com
VC++
99
ve )](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-99-320.jpg)
![VISUAL C++ PROGRAMMING VC++
In order to accept arrays as parameters the only thing that we have to do when declaring the
function is to specify in its parameters the element type of the array, an identifier and a pair of
void brackets []. For example, the following function:
rmmakaha@gmail.com 100
void procedure (int arg[])
accepts a parameter of type array of int called arg. In order to pass to this function an array
declared as:
int myarray [40];
it would be enough to write a call like this:
procedure (myarray);
Here you have a complete example:
// arrays as parameters
#include iostream
using namespace std;
void printarray (int arg[], int length) {
for (int n=0; nlength; n++)
cout arg[n] ;
cout n;
}
int main ()
{
int firstarray[] = {5, 10, 15};
int secondarray[] = {2, 4, 6, 8, 10};
printarray (firstarray,3);
printarray (secondarray,5);
return 0;
}
5 10 15
2 4 6 8 10
As you can see, the first parameter (int arg[]) accepts any array whose elements are of type int,
whatever its length. For that reason we have included a second parameter that tells the function
the length of each array that we pass to it as its first parameter. This allows the for loop that
prints out the array to know the range to iterate in the passed array without going out of range.
In a function declaration it is also possible to include multidimensional arrays. The format for a
tridimensional array parameter is:](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-100-320.jpg)
![base_type[][depth][depth]
VISUAL C++ PROGRAMMING
for example, a function with a multidimensional array as argument could be:
void procedure (int myarray[][3][4])
VC++
Notice that the first brackets []
are left blank while the following ones are not. This is so because
the compiler must be able to determine within the function which is the depth of each additional
dimension.
Arrays, both simple or multidimensional, passed as function parameters are a quite common
source of errors for novice programmers. I recommend the reading of the chapter about
Pointers for a better understanding on how arrays operate.
Character Sequences
As you may already know, the C++ Standard Library implements a powerful
is very useful to handle and manipulate strings of characters. However, because strings are in fact
sequences of characters, we can represent them also as plain arrays of
For example, the following array:
char jenny [20];
is an array that can store up to 20 elements of type
Therefore, in this array, in theory, we can store sequ
But we can also store shorter sequences. For example,
program either the sequence Hello
than 20 characters.
rmmakaha@gmail.com
char elements.
char. It can be represented as:
sequences of characters up to 20 characters long.
jenny could store at some point in a
or the sequence Merry christmas, since both are shorter
101
string class, which
. ences ,](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-101-320.jpg)
![Therefore, since the array of characters can store shorter sequences than its total length, a special
character is used to signal the end of the valid sequence: the
can be written as '0' (backslash, zero).
Our array of 20 elements of type
sequences Hello and Merry Christmas
Notice how after the valid content a null character (
the end of the sequence. The panels in gray color represent
values.
Initialization of null-terminated character sequences
Because arrays of characters are ordinary arrays they follow all their same rules. For example, if
we want to initialize an array of characters with some predetermined sequence of characters we
can do it just like any other array:
char myword[] = { 'H', 'e'
In this case we would have declared an array of 6 elements of type
characters that form the word Hello
But arrays of char elements have an additional method to initialize their values: using string
literals.
In the expressions we have used in some examples in previous chapters, con
entire strings of characters have already showed up several times. These are specified enclosing
the text to become a string literal between double quotes (). For example:
the result is:
is a constant string literal that we have
VISUAL C++ PROGRAMMING
rmmakaha@gmail.com
refore, null character, whose literal constant
f char, called jenny, can be represented storing the characters
as:
('0') has been included in order to indicate
he char elements with undetermined
e', 'l', 'l', 'o', '0' };
char initialized with the
plus a null character '0' at the end.
constants that represent
probably used already.
VC++
102
, , ) stants](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-102-320.jpg)
![VISUAL C++ PROGRAMMING VC++
Double quoted strings () are literal constants whose type is in fact a null-terminated array of
characters. So string literals enclosed between double quotes always have a null character ('0')
automatically appended at the end.
Therefore we can initialize the array of char elements called myword with a null-terminated
sequence of characters by either one of these two methods:
char myword [] = { 'H', 'e', 'l', 'l', 'o', '0' };
rmmakaha@gmail.com 103
char myword [] = Hello;
In both cases the array of characters myword is declared with a size of 6 elements of type char:
the 5 characters that compose the word Hello plus a final null character ('0') which specifies
the end of the sequence and that, in the second case, when using double quotes () it is appended
automatically.
Please notice that we are talking about initializing an array of characters in the moment it is being
declared, and not about assigning values to them once they have already been declared. In fact
because this type of null-terminated arrays of characters are regular arrays we have the same
restrictions that we have with any other array, so we are not able to copy blocks of data with an
assignment operation.
Assuming mytext is a char[] variable, expressions within a source code like:
mystext = Hello;
mystext[] = Hello;
would not be valid, like neither would be:
mystext = { 'H', 'e', 'l', 'l', 'o', '0' };
The reason for this may become more comprehensible once you know a bit more about
pointers, since then it will be clarified that an array is in fact a constant pointer pointing to a
block of memory.](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-103-320.jpg)
![VISUAL C++ PROGRAMMING VC++
Using null-terminated sequences of characters
Null-terminated sequences of characters are the natural way of treating strings in C++, so they
can be used as such in many procedures. In fact, regular string literals have this type (char[]) and
can also be used in most cases.
For example, cin and cout support null-terminated sequences as valid containers for sequences
of characters, so they can be used directly to extract strings of characters from cin or to insert
them into cout. For example:
// null-terminated sequences of characters
#include iostream
using namespace std;
int main ()
{
char question[] = Please, enter your first name: ;
char greeting[] = Hello, ;
char yourname [80];
cout question;
cin yourname;
cout greeting yourname !;
return 0;
}
Please, enter your first name: John
Hello, John!
As you can see, we have declared three arrays of char elements. The first two were initialized
with string literal constants, while the third one was left uninitialized. In any case, we have to
speficify the size of the array: in the first two (question and greeting) the size was implicitly
defined by the length of the literal constant they were initialized to. While for yourname we have
explicitly specified that it has a size of 80 chars.
Finally, sequences of characters stored in char arrays can easily be converted into string objects
just by using the assignment operator:
rmmakaha@gmail.com 104
string mystring;
char myntcs[]=some text;
mystring = myntcs;](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-104-320.jpg)
![VISUAL C++ PROGRAMMING VC++
Bubble Sort
The bubble sort gets its name because as elements are sorted they gradually bubble (or rise) to
their proper positions, like bubbles rising in a glass of soda. The bubble sort repeatedly
compares adjacent elements of an array, starting with the first and second elements, and
swapping them if they are out of order. After the first and second elements are compared, the
second and third elements are compared, and swapped if they are out of order. This process
continues until the end of the list is reached.
When the end is reached, the bubble sort returns to elements one and two and starts the process
all over again. So, when does it stop? The bubble sort knows that it is finished when it
examines the entire array and no swaps are needed (thus the list is in proper order).
The bubble sort keeps track of the occurrence of swaps by the use of a flag.
The table below follows an array of numbers before, during, and after a bubble sort for descending
order. A pass is defined as one full trip through the array comparing and if necessary,
swapping, adjacent elements. Several passes have to be made through the array before it is
finally sorted.
Array at beginning: 84 69 76 86 94 91
After Pass #1: 84 76 86 94 91 69
After Pass #2: 84 86 94 91 76 69
After Pass #3: 86 94 91 84 76 69
After Pass #4: 94 91 86 84 76 69
After Pass #5 (done): 94 91 86 84 76 69
The bubble sort is an easy algorithm to program, but it is slower than many other sorts. With a
bubble sort, it is always necessary to make one final pass through the array to check to see that
no swaps are made to ensure that the process is finished. In actuality, the process is finished
before this last pass is made.
// Bubble Sort Function for Descending Order
void bubble_sort(apvector int array)
{
int i, j, flag = 1; // set flag to 1 to begin initial pass
int temp; // holding variable
int arrayLength = array.length( );
for(i = 1; (i = arrayLength) flag; i++)
{
flag = 0;
for (j=0; j (arrayLength -1); j++)
{
if (array[j+1] array[j]) // ascending order simply changes to
{
temp = array[j]; // swap elements
array[j] = array[j+1];
array[j+1] = temp;
flag = 1; // indicates that a swap occurred.
rmmakaha@gmail.com 105
}](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-105-320.jpg)
![VISUAL C++ PROGRAMMING VC++
rmmakaha@gmail.com 106
}
}
return; //arrays are passed to functions by address; nothing is returned
}
Selection Sort
The selection sort is a combination of searching and sorting.
During each pass, the unsorted element with the smallest (or largest) value is moved to
its proper position in the array.
The number of times the sort passes through the array is one less than the number of items in
the array. In the selection sort, the inner loop finds the next smallest (or largest) value and the
outer loop places that value into its proper location.
Let's look at our same table of elements using a selection sort for descending order. Remember,
a pass is defined as one full trip through the array comparing and if necessary, swapping
elements.
Array at beginning: 84 69 76 86 94 91
After Pass #1: 84 91 76 86 94 69
After Pass #2: 84 91 94 86 76 69
After Pass #3: 86 91 94 84 76 69
After Pass #4: 94 91 86 84 76 69
After Pass #5 (done): 94 91 86 84 76 69
While being an easy sort to program, the selection sort is one of the least efficient. The
algorithm offers no way to end the sort early, even if it begins with an already sorted list.
// Selection Sort Function for Descending Order
void selection_sort(apvector int array)
{
int i, j, first, temp;
int array_size = array.length( );
for (i= array_size - 1; i 0; i--)
{
first = 0; // initialize first to the subscript of the first element
for (j=1; j=i; j++) //Find smallest element between the positions 1 and i.
{
if (array[j] array[first])
first = j;
}
temp = array[first]; // Swap smallest element found with one in position i.
array[first] = array[i];
array[i] = temp;
}
return;
}
Sequential Search
(Page 2 of 4 )](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-106-320.jpg)
![VISUAL C++ PROGRAMMING VC++
The sequential search is best used if the array you are searching is unsorted. This method of
searching is usually used on small arrays of less than 16 elements. We start the sequential search
by first declaring a target to be found. The search initiates at the beginning of the array until it
finds the target.
In the following example we will find a target value of 23 within a one dimensional array. At
index 0, 32 is not equal to 23 so we proceed on to the next element.
a[0] a[1] a[2] a[3] a[4]
32 431 -34 23 12
At index 1, 431 is not equal to 23 so we proceed.
a[0] a[1] a[2] a[3] a[4]
32 431 -34 23 12
At index 2, -34 is not equal to 23 so we proceed.
a[0] a[1] a[2] a[3] a[4]
32 431 -34 23 12
Finally at index 3, 23 is equal to 23 and we have found our target.
a[0] a[1] a[2] a[3] a[4]
32 431 -34 23 12
Now we will implement this example of a sequential search into C++ code. The program below
asks the user for a target to be found, then uses a for loop to analyze each element of the array.
If the array element is equal to the target it will display that the target was found. Whenever a
target is found the variable “flag” will be incremented by 1. At the end of the program if the
variable “flag” is less than one, then the target was obviously not found.
#include iostream
using namespace std;
int main()
{
const int arraySize = 5;
double target;
int array[arraySize] = {32, 431, -34, 23, 12};
int flag;
// flag is used to log how many times the target is encountered.
flag = 0;
cout Enter a target to be found: ;
cin target;
for(int cntr = 0; cntr arraySize; cntr++)
{
rmmakaha@gmail.com 107
if(array[cntr] == target)
{
cout Target found in array index cntr .
endl;
flag += 1;
}
}
// Test to see if target was found.
if(flag 1)
{](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-107-320.jpg)
![VISUAL C++ PROGRAMMING VC++
rmmakaha@gmail.com 112
};
Employee::Employee(char *name, int id)
{
_id = id;
_name = new char[strlen(name) + 1];
//Allocates an character array object
strcpy(_name, name);
}
Employee::~Employee()
{
delete[] _name;
}
int main()
{
Employee programmer(John,22);
cout programmer.getName() endl;
return 0;
}
The function strlen returns the length of the string passed into the constructor. Notice that the
Employee name is now stored in a dynamically allocated character array. It is of string
length + 1 to allow for the null terminator used with C-style strings, '0'. The strcpy function
automatically adds the null terminator to the destination string. Also, notice that the
destructor frees the memory used to hold the employee name. This is needed to avoid a
memory leak](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-112-320.jpg)
![VISUAL C++ PROGRAMMING VC++
The usual rules of function call overloading apply, plus it must be remembered that the built-in
operators are prospective candidates for resolving any call, and there are the following
rmmakaha@gmail.com 114
general restrictions:
• The standard C++ precedences and associativities determine how any expression will
be parsed and can't be changed.
• No new operators can be invented, e.g. @ can't be overloaded as an operator.
• The operator must be written as unary or binary depending on its use with the built-in
types, e.g. a binary operator can't be overloaded in a unary way for a class if there's no
built-in type for which it's used in a unary manner.
• For non-member functions overloading operators, at least one function argument must
be an instance or a reference to a class. This ensures no C++ code can change the
built-in behavior of operators with built-in types.
• No default values for the operator arguments are allowed.
• There are also operator-specific restrictions, notations and features which are
enumerated below.
The following operators can't be overloaded: `.', `.*', `::', sizeof, () value construction and
the ternary conditional expression operator (`?:').
The following operators must be implemented as member functions:
= - () []
6.3 Guidelines
The good news is that the programmer has complete control over the return type and
argument types of the function. This flexibility must be used with some sensitivity. For
example, + should be overloaded for a class only when replacing functional notation for an
operation analagous to addition for the class. Unless there is a good reason for doing
otherwise, it should mimic the behavior of the built-in operator as much as possible meaning
(for a function overloading +):
• It shouldn't change the value of either operand, since + doesn't do that when applied to
built-in types.
• It should be commutative.
As another example, it might be a good idea for overloaded = to return a reference to the left
operand.
Each operator has a sort of culture, a way programmers expect it to behave given their
experience with built-in types. The more a new version of the operator fits in with this
experience, the easier it will be for the programmer to utilize it in an effective and error-free
manner. In addition, the class will be more likely to fit into and work properly in templates
that require the operator.
6.4 Operator-specific restrictions, notations and features
Copy assignment operator](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-114-320.jpg)
![VISUAL C++ PROGRAMMING VC++
Unit 9: Working with files (25 HOURS)
rmmakaha@gmail.com 135
THEORY
At the end of the unit the student should be able to:
1. ..employ classes for file stream operators
2. ..open and close a file
3. ..detect an end-of-line
4. ..distinguish between file modes
5. ..know about file pointers and their manipulators
6. ..employ sequential input and output operations
7. ..to update a file: random access
8. ..take care of error handling during file operations
9. ..use command-line arguments
Input/Output with files
C++ provides the following classes to perform output and input of characters to/from files:
• ofstream: Stream class to write on files
• ifstream: Stream class to read from files
• fstream: Stream class to both read and write from/to files.
These classes are derived directly or indirectly from the classes istream, and ostream. We have
already used objects whose types were these classes: cin is an object of class istream and cout is
an object of class ostream. Therfore, we have already been using classes that are related to our
file streams. And in fact, we can use our file streams the same way we are already used to use cin
and cout, with the only difference that we have to associate these streams with physical files.
Let's see an example:
// basic file operations
#include iostream
#include fstream
using namespace std;
int main () {
ofstream myfile;
myfile.open (example.txt);
myfile Writing this to a file.n;
myfile.close();
return 0;
}
[file example.txt]](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-135-320.jpg)
![VISUAL C++ PROGRAMMING VC++
rmmakaha@gmail.com 138
myfile.close();
Once this member function is called, the stream object can be used to open another file, and the
file is available again to be opened by other processes.
In case that an object is destructed while still associated with an open file, the destructor
automatically calls the member function close().
Text file streams are those where we do not include the ios::binary flag in their opening mode.
These files are designed to store text and thus all values that we input or output from/to them
can suffer some formatting transformations, which do not necessarily correspond to their literal
binary value.
Data output operations on text files are performed in the same way we operated with cout:
// writing on a text file
#include iostream
#include fstream
using namespace std;
int main () {
ofstream myfile (example.txt);
if (myfile.is_open())
{
myfile This is a line.n;
myfile This is another line.n;
myfile.close();
}
else cout Unable to open file;
return 0;
}
[file example.txt]
This is a line.
This is another line.
Data input from a file can also be performed in the same way that we did with cin:
// reading a text file
#include iostream
#include fstream
#include string
using namespace std;
int main () {
string line;](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-138-320.jpg)
![VISUAL C++ PROGRAMMING VC++
rmmakaha@gmail.com 142
int main()
{
dout.open(stud.txt, ios::out);
char text[5];
coutEnter textendl;
cintext;
dout.write(text,sizeof(text));
dout.close();
return 0;
}
Binary files
In binary files, to input and output data with the extraction and insertion operators ( and )
and functions like getline is not efficient, since we do not need to format any data, and data may
not use the separation codes used by text files to separate elements (like space, newline, etc...).
File streams include two member functions specifically designed to input and output binary data
sequentially: write and read. The first one (write) is a member function of ostream inherited by
ofstream. And read is a member function of istream that is inherited by ifstream. Objects of class
fstream have both members.;
Where memory_block is of type pointer to char (char*), and represents the address of an array
of bytes where the read data elements are stored or from where the data elements to be written
are taken. The size parameter is an integer value that specifies the number of characters to be
read or written from/to the memory block.
// reading a complete binary file
#include iostream
#include fstream
using namespace std;
ifstream::pos_type size;
char * memblock;](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-142-320.jpg)
![VISUAL C++ PROGRAMMING VC++
int main () {
ifstream file (example.txt, ios::in|ios::binary|ios::ate);
if (file.is_open())
{
size = file.tellg();
memblock = new char [size];
file.seekg (0, ios::beg);
file.read (memblock, size);
file.close();
cout the complete file content is in memory;
delete[] memblock;
}
else cout Unable to open file;
return 0;
} the complete file content is in memory
In this example the entire file is read and stored in a memory block. Let's examine how this is
done:
First, the file is open with the ios::ate flag, which means that the get pointer will be positioned at
the end of the file. This way, when we call to member tellg(), we will directly obtain the size of
the file. Notice the type we have used to declare variable size:
rmmakaha@gmail.com 143
ifstream::pos_type size;
ifstream::pos_type is a specific type used for buffer and file positioning and is the type returned
by file.tellg(). This type is defined as an integer type, therefore we can conduct on it the same
operations we conduct on any other integer value, and can safely be converted to another integer
type large enough to contain the size of the file. For a file with a size under 2GB we could use
int:
int size;
size = (int) file.tellg();
Once we have obtained the size of the file, we request the allocation of a memory block large
enough to hold the entire file:
memblock = new char[size];
Right after that, we proceed to set the get pointer at the beginning of the file (remember that we
opened the file with this pointer at the end), then read the entire file, and finally close it:
file.seekg (0, ios::beg);](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-143-320.jpg)
![VISUAL C++ PROGRAMMING VC++
Depending on the implementation of C++, it may be possible to specify a variety of other mode
options, such as appending to the end of an existing file, triggering an error rather than
overwriting an existing file, or specifying that a file is binary for operating systems (e.g. MS-DOS)
which distinguish binary and ASCII files.
rmmakaha@gmail.com 146
Passing streams to functions
File streams must be passed to functions by reference, not by value.
void myfunction(ifstream fp, ...) // use this
void myfunction(ifstream fp, ...) // not this
If you pass streams by value, the C++ compiler will not complain. However, mysterious bad
things will start happening, often in parts of the code which don't appear to be related to the
offending function.
Item by item input and output
If each input item is surrounded by whitespace (blanks, tabs, newlines), the items can be read
easily using the extraction operator .
int myinteger; // declarations
float myfloat;
char mychar;
char *mystring; // two ways to declare a string
char mystring[64];
fp_in myinteger; // input from file pointer or standard input
cin myfloat;
fp_in mychar;
cin mystring;
The extraction operator works for numbers (ints, floats), characters (char), and strings (declared
as arrays of type char or pointers to type char).
The extraction operator returns a zero value if it encounters a problem (typically, the end of the
file). Therefore, it can be used as the test in an if statement or a while loop.
WARNING: when reading data into a character string, bad things will happen if the input word
is longer than your string. To avoid problems, use the operator setw to force excessively long
input to be broken up. (You must include the iomanip header file.) The input to setw should be
the length of your string (including the null character '0' at the end of the string).](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-146-320.jpg)
![VISUAL C++ PROGRAMMING VC++
cout setw(8); make item occupy 8 characters
cout.width(8); an alternative syntax
Setting the width to k forces the item to occupy at least k characters. If its printed representation
is shorter, blanks will be added so that it occupies k characters. If its printed representation is
longer, it will occupy more than k characters.
When you reset parameters such as the precision, it is not clear whether the new value lasts only
for the next item printed or whether it persists for subsequent values. Apparently this varies with
the parameter and the C++ compiler. Therefore, if you care about the value of some formatting
parameter, explicitly set it to the right thing before you output each item.
rmmakaha@gmail.com 150
Repositioning and error states
In general, it is possible to move to any position in a file stream. This is a capability most
programmers use rarely, if at all. For fairly obvious reasons, don't try to use repositioning
operations on the standard input, output, or error streams.
The most likely reason for using a repositioning command is to rewind a file to the start, so that
you can read its contents again. This is done as follows:
fp_in.clear(); // forget we hit the end of file
fp_in.seekg(0, ios::beg); // move to the start of the file
Data Conversion
The following functions convert data types:
atof() converts an ASCII character to a double (to float in c++)
atoi() converts an ASCII character to an integer
atol() converts an ASCII to a long integer
itoa() converts an integer to a character array
Example:
/* convert a string to an integer */
#include stdio.h
#include stdlib.h
char string[] = “1234”;](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-150-320.jpg)
![VISUAL C++ PROGRAMMING VC++
rmmakaha@gmail.com 151
int main(void)
{
int sum;
sum = atoi( string);
printf(“Sum = %dn”, sum);
return 0;
}
Another example:
/* convert a string to an integer */
#include stdio.h
#include stdlib.h
int main(void) // remember that the default return data is int
{
int sum;
char buff[20];
printf(“Enter in an integer “);
scanf (“%d”, sum);
printf(“As a string it is %sn”, itoa(sum, buff, 10));
return 0;
}
Note that itoa() takes three parameters:
• The integer to be converted
• A character buffer into which the resultant string is stored
• A radix value (10=decimal, 16=hexadecimal);
Built in functions for character handling
The following character handling functions are defined in ctype.h
isalnum tests for alphanumeric character
isalpha tests for alphabetic character
isascii tests for ASCII character
iscntrl tests for control character
isdigit tests for 0 to 9
isgraph tests for printable character
islower tests for lowercase
isprint tests for printable character
ispunct tests for punctuation character
isspace tests for space character
isupper tests for uppercase character
isxdigit tests for hexadecimal
toascii converts character to ASCII code
tolower converts character to lowercase
toupper converts a character to uppercase](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-151-320.jpg)
![VISUAL C++ PROGRAMMING VC++
To convert a string array to uppercase one character at a time using touper()
rmmakaha@gmail.com 152
#include stdio.h
#include ctype.h
int main(void)
{
char name[80];
int loop;
printf(“Enter in a name in lowercasen”);
scanf(“%s”, name);
for (loop = 0; name[loop] !=0; loop++)
name[loop] = toupper(name[loop]);
printf(“The name in uppercase is |%sn”, name);
return 0;
}
Sample program output
Enter in a name in lowercase
samuel
The name In uppercase in SAMUEL
Don't worry if this lesson seems too complicated. Pay attention only to the keywords that are
summarized in the bullet points on top of this page.
A new section is under construction which will list all the standard libraries (as defined by
ASCII) and will sort all keywords alphabetically and by library.
Pointers
What is a Pointer?
So what is a pointer? A pointer is a way to get at another object. Essentially it is a way to grab an
instance of an object and then either pass that instance a message or retreive some data from that
object. A pointer is actually just an address of where an instance is held in memory.
Some piece of your program can either possess an instance of an object, or know where an
instance of an object is. An instance of an object is a chunk of memory that is big enough to
store all the member data of that object. A pointer is an address that explains how to get to
where the instance is actually held in memory. Here's a quick example:
Our Player object has three pieces of data that it owns: strength, agility, and health. These are a
part of the player object. That makes sense in real world terms. The player knows about two](https://image.slidesharecdn.com/vcnotes-141110000415-conversion-gate02/85/C-VISUAL-C-152-320.jpg)
C++ & VISUAL C++
- 1. VISUAL C++ PROGRAMMING VC++ SUBJECT: Visual C++ CODE: 710/04/S05 AIMS OF THE SUBJECT 1. To review Object Oriented Program Design and relate it to specific programming languages. 2. To impart the skills to design applications in the VC++ development environment. 3. To impart the skills and knowledge to develop applications that use the fundamental features of C++, including Classes, Objects, Constructors, Inheritance, encapsulation, overloading etc.. [email protected] 1 DESIGN LENGTH THEORY 160 :Hours LABORATORY 100 :Hours TOTAL 260 :Hours SUBJECT OBJECTIVES The general objective of the units are stated below and the specific objectives of the topic areas are set out in the syllabus content. At the end of the course the student should be able to…. 1. ..design and implement Visual C++ applications using Object Oriented Programming design techniques. 2. ..create a Graphical User Interface in the Visual C++ programming environment. 3. ..add code to existing objects and add user defined classes and objects. 4. .. apply basic and more advanced features of Visual C++ in his/her programs, like objects, classes, inheritance, polymorphism etc.. 5. ..develop applications in Visual C++ that incorporate databases and access files.
- 2. VISUAL C++ PROGRAMMING VC++ Unit 1: Developing applications using the Visual C++ IDE (100HOURS) PRACTICAL This unit is for practical classes. It should complement the other units that are used for theory classes. At the end of the unit the student should be able to: 1. ..use the editor, the menus and the application wizard. 2. ..create projects. 3. ..understand and use controls: - General concepts - Specific controls: e.g. combo box, labels, etc… 4. .. be able to create windows 5. .. understand what events and methods are 6. ..create a simple database application using DAO 7. ..design and implement applications using things like: - Dialog Boxes - Mouse events, keyboard events, timers - Menu’s - Graphics 8. .. employ the above mentioned things in practical assignments [email protected] 2
- 3. VISUAL C++ PROGRAMMING VC++ Unit 2: Principal Concepts of OOP (10 HOURS) [email protected] 3 THEORY At the end of the unit the student should be able to.. 1. .. describe the evolution of software. 2. .. distinguish between procedure-oriented programming and object oriented programming. 3. ..know the Object Oriented Paradigm 3. ..understand basic concepts of Object Oriented Programming. 4. ..mention at least two Object Oriented Programming languages 5. ..mention several applications of Object Oriented Programming. Distinguish between procedure-oriented programming and object oriented programming. Procedure orientated programming is when a programmer creates functions and variables that are separate parts for his or her program. They may work together but they are distinctly separated. Object orientated programming is when a programmer takes a concept (an object) and encapsulates all variables and functions into that concept so that the variables and functions are distinctly connected. In C++, for example, these objects are called classes. C++ is an extension of the language C along with the capability of creating object classes. Procedure Oriented Programming 1.Prime focus is on functions and procedures that operate on data 2.Large programs are divided into smaller program units called functions 3.Data and the functions that act upo it are treated as separate entities. 4. Data move freely around the systems from one function to another. 5. Program design follows “Top Down Approach”. --------------------------------------… Object Oriented Programming 1.Here more emphasis is laid on the data that is being operated and not the functions or procedures 2.Programs are divided into what are called objects. 3. Both data and functions are treated together as an integral entity. 4. Data is hidden and cannot be accessed by external functions. 5. Program design follows “Bottom UP Approach”.
- 4. The OOP Paradigm VISUAL C++ PROGRAMMING VC++ Object oriented programming is a concept that was created because of the need to overcome the problems that were found with using structured pro While structured programming uses an approach which is top down, OOP uses an approach which is bottom up. Traditionally, programming has placed an emphasis on logic and actions. Object oriented programming has taken a completely diffe emphasis on objects and information. broken down into a number of units. These units are called objects. The foundation of OOP is the fact that it will place an emphasis Objects will be defined, and they will interact inside the system in a number of different ways. There are a number of advantages to be found with usin of these are simple maintenance, an advanced analysis of complicated programs, and reusability. There are a number of programming languages that use OOP, and some of these are Java, C++, and Ada. One concept that you will want modeling. Before you can construct an object oriented system, you will first need to find the objects within the system and determine the relationships they have. This process is called data modeling. There are some other OO A class is a unit that holds data and functions which will carry out operations on the data. A class will be comprised of three access modifiers. These three modifiers are protected, private, and public. A member that designated as private cannot be accessed by objects that exist outside the system. In addition to this, it cannot be inherited. While a member who is protected can be inherited, they cannot be accessed by objects which reside outside of the class hierarchy. Another term that you will hear about often in OOP is objects. An object is a state of class. It can receive and send messages to other objects, and it can handle data. The objects which exist w and will behave in the same manner. There are two things that are found with all objects that exist in the real world. These two things are behaviors and states. While behaviors and states are found in real world objects, they can also be found in software objects as well. Another OOP concept that you will need to know is a method. A method is a process that is used to handle an object. A method can be public, protected, or private. The visibility of th will determine how much of it can be seen by outside objects. Inheritance is an aspect of OOP that allows subclasses to inherit the traits and characteristics of its superclass. The subclass will inherit all members except those that designated private. The subclass may have a large number classes from multiple bases, and this concept is called Multiple Inheritance. It is possible for a subclass to use the behavior of the members it has inherited, and it can also add new members as well. that inheritance has, and these are the implementation of abstract data and reusability. Encapsulation is another important concept that you will need to know. In a nutshell, encapsulation is responsible for shielding t only reveal the functional information. However, the implementation will be hidden. [email protected] programming techniques. different direction, and will place an With object oriented programming, a problem will be on objects and classes . using the OOP paradigm, and some to become familiar with is data OOP terms that you will want to know. is public can be accessed and inherited. A member that is sed within software are often based off real world objects, There are two powerful advantages the data within a class from outside objects. It will 4 gramming rent g P ithin the member as being he
- 5. VISUAL C++ PROGRAMMING VC++ Encapsulation is a concept which promotes modularity, and it is also crucial for hiding information. Abstraction is an important concept as well. It allows an application to process objects on a general level. It is not connected to the instance of an object. It is excellent for locating patterns that may have a number of different variations. Abstractions are useful because they can create a common identification that can be found within all their subclasses. This article serves to demonstrate some of the general concepts of OOP. Object oriented programming is a paradigm that has created a revolution within the field of computer programming. If it is used to design programs properly, they will be efficient and easy to expand and maintain. Polymorphism Polymorphism is the ability to apply a standard interface to an object. For example, just about every electronic device has an on/off switch which performs the same task. The interface to us is simple and straight forward. What happens inside the electronic device will vary greatly between devices. What this means to a C++ programmer is that objects have functions assigned to them, and similar objects (or derived objects) will implement the same function name to accomplish the same task, while the mechanics of how it is done may be greatly different. For example, imaging two objects, one handles input from a keyboard, and the other handles input from a modem. Both objects implement a function called GetLine. This GetLine function does the same job from the application viewpoint, it gets a line from an input device. Our application doesn't have to care if the user is at the keyboard, or across the country via modem. Chances are that our keyboard object and modem object are both derived from a standard 'Input' object or base object. All similar objects derived from the Input object would have to provide the GetLine function. [email protected] 5
- 6. VISUAL C++ PROGRAMMING VC++ Unit 3: Visual C++ basics: Data types, expressions, control structur (30 HOURS) [email protected] 6 THEORY At the end of the unit the student should be able to: 1. ..describe what Visual C++ is. 2. ..give examples of applications of C++ 3. ..write a simple Visual C++ program 4. ..describe the structure of a Visual C++ program 5. ..use pre-processor directives 6. ..know how to compile and link a source program 7. ..describe what tokens, keywords and identifiers are 8. ..list the basic data types and their characteristics 9. ..create user defined data types and derived data types 10. ..know about compatibility of data types 11. ..be able to declare variables 12. ..employ dynamic initialisation of variables 13. ..know what reference variables are 14. ..list the possible operations in Visual C++ 15. ..know the following about operators: - Scope Resolution Operator - Member Differencing Operator - Memory Management Operator - Manipulators - operator overloading - operator precedence - Type Cast Operator 16. ..know expressions and implicit conversions 17. ..know the control structures of C++ e.g. while, for, if etc.. Your first C++ program You now know almost enough of the basics to create and compile a program. The program will use the Standard C++ iostream classes. These read from and write to files and “standard” input and output (which normally comes from and goes to the console, but may be redirected to files or devices). In this very simple program, a stream object will be used to print a message on the screen. Using the iostreams class To declare the functions and external data in the iostreams class, include the header file with the statement #include <iostream>
- 7. VISUAL C++ PROGRAMMING VC++ The first program uses the concept of standard output, which means “a general-purpose place to send output.” You will see other examples using standard output in different ways, but here it will just go to the console. The iostream package automatically defines a variable (an object) called cout that accepts all data bound for standard output. To send data to standard output, you use the operator <<. C programmers know this operator as the “bitwise left shift,” which will be described in the next chapter. Suffice it to say that a bitwise left shift has nothing to do with output. However, C++ allows operators to be overloaded. When you overload an operator, you give it a new meaning when that operator is used with an object of a particular type. With iostream objects, the operator << means “send to.” For example: cout << "howdy!"; sends the string “howdy!” to the object called cout (which is short for “console output”). That’s enough operator overloading to get you started. Chapter XX covers operator overloading in detail. Namespaces As mentioned in the previous chapter, one of the problems encountered in the C language is that you “run out of names” for functions and identifiers when your programs reach a certain size. Of course, you don’t really run out of names – however, it becomes harder to think of new ones after awhile. More importantly, when a program reaches a certain size it’s typically broken up into pieces, each of which is built and maintained by a different person or group. Since C effectively has a single arena where all the identifier and function names live, this means that all the developers must be careful not to accidentally use the same names in situations where they can conflict. This rapidly becomes tedious, time-wasting and, ultimately, expensive. Standard C++ has a mechanism to prevent this collison: the namespace keyword. Each set of C++ definitions in a library or program is “wrapped” in a namespace, and if some other definition has an identical name, but is in a different namespace, then there is no collision. Namespaces are a convenient and helpful tool, but their presence means you must be aware of them before you can write any programs at all. If you simply include a header file and use some functions or objects from that header, you’ll probably get strange-sounding errors when you try to compile the program, to the effect that the compiler cannot find any of the declarations for the items that you just included in the header file! After you see this message a few times you’ll become familiar with its meaning (which is: “you included the header file but all the declarations are within a namespace and you didn’t tell the compiler that you wanted to use the declarations in that namespace”). There’s a keyword that allows you to say “I want to use the declarations and/or definitions in this namespace.” This keyword, appropriately enough, is using. All of the Standard C++ libraries are wrapped in a single namespace, which is std (for “standard”). As this book uses the standard libraries almost exclusively, you’ll see the following using directive in almost every program: using namespace std; This means that you want to expose all the elements from the namespace called std. After this statement, you don’t have to worry that your particular library component is inside a namespace, since the using directive makes that namespace available throughout the file where the using directive was written. [email protected] 7
- 8. VISUAL C++ PROGRAMMING VC++ Exposing all the elements from a namespace after someone has gone to the trouble to hide them may seem a bit counterproductive, and in fact you should be careful about thoughtlessly doing this (as you’ll learn later in the book). However, the using directive only exposes those names for the current file, so it is not quite so drastic as it first sounds (but think twice about doing it in a header file – that is reckless). There’s a relationship between namespaces and the way header files are included. Before the current header file inclusion style of <iostream> (that is, no trailing ‘ .h’) was standardized, the typical way to include a header file was with the ‘ .h’, such as <iostream.h>. At that time, namespaces were not part of the language, either. So to provide backwards compatibility with existing code, if you say [email protected] 8 #include <iostream.h> It means #include <iostream> using namespace std; However, in this book the standard include format will be used (without the ‘ .h’) and so the using directive must be explicit. For now, that’s all you need to know about namespaces, but in Chapter XX the subject is covered much more thoroughly. Fundamentals of program structure A C or C++ program is a collection of variables, function definitions and function calls. When the program starts, it executes initialization code and calls a special function, “ main( ).” You put the primary code for the program here. As mentioned earlier, a function definition consists of a return type (which must be specified in C++), a function name, an argument list in parentheses, and the function code contained in braces. Here is a sample function definition: int function() { // Function code here (this is a comment) } The above function has an empty argument list, and a body that contains only a comment. There can be many sets of braces within a function definition, but there must always be at least one set surrounding the function body. Since main( ) is a function, it must follow these rules. In C++, main( ) always has return type of int. C and C++ are free form languages. With few exceptions, the compiler ignores newlines and white space, so it must have some way to determine the end of a statement. Statements are delimited by semicolons. C comments start with /* and end with */. They can include newlines. C++ uses C-style comments and has an additional type of comment: //. The // starts a comment that terminates with a newline. It is more convenient than /* */ for one-line comments, and is used extensively in this book.
- 9. VISUAL C++ PROGRAMMING VC++ [email protected] 9 "Hello, world!" And now, finally, the first program: //: C02:Hello.cpp // Saying Hello with C++ #include <iostream> // Stream declarations using namespace std; int main() { cout << "Hello, World! I am " << 8 << " Today!" << endl; } ///:~ The cout object is handed a series of arguments via the ‘ <<’ operators. It prints out these arguments in left-to-right order. The special iostream function endl outputs the line and a newline. With iostreams, you can string together a series of arguments like this, which makes the class easy to use. In C, text inside double quotes is traditionally called a “string.” However, the Standard C++ library has a powerful class called string for manipulating text, and so I shall use the more precise term character array for text inside double quotes. The compiler creates storage for character arrays and stores the ASCII equivalent for each character in this storage. The compiler automatically terminates this array of characters with an extra piece of storage containing the value 0, to indicate the end of the character array. Inside a character array, you can insert special characters by using escape sequences. These consist of a backslash ( ) followed by a special code. For example n means newline. Your compiler manual or local C guide gives a complete set of escape sequences; others include t (tab), (backslash) and b (backspace). Notice that the entire statement terminates with a semicolon. Character array arguments and constant numbers are mixed together in the above cout statement. Because the operator << is overloaded with a variety of meanings when used with cout, you can send cout a variety of different arguments, and it will “figure out what to do with the message.” Throughout this book you’ll notice that the first line of each file will be a comment that starts with the characters that start a comment (typically //), followed by a colon. // my first program in C++ #include <iostream> using namespace std; int main () { cout << "Hello World!"; return 0; } // my first program in C++
- 10. VISUAL C++ PROGRAMMING VC++ This is a comment line. All lines beginning with two slash signs (//) are considered comments and do not have any effect on the behavior of the program. The programmer can use them to include short explanations or observations within the source code itself. In this case, the line is a brief description of what our program is. [email protected] 10 #include <iostream> Lines beginning with a pound sign (#) are directives for the preprocessor. They are not regular code lines with expressions but indications for the compiler's preprocessor. In this case the directive #include <iostream> tells the preprocessor to include the iostream standard file. This specific file (iostream) includes the declarations of the basic standard input-output library in C++, and it is included because its functionality is going to be used later in the program. using namespace std; All the elements of the standard C++ library are declared within what is called a namespace, the namespace with the name std. So in order to access its functionality we declare with this expression that we will be using these entities. This line is very frequent in C++ programs that use the standard library, and in fact it will be included in most of the source codes included in these tutorials. int main () This line corresponds to the beginning of the definition of the main function. The main function is the point by where all C++ programs start their execution, independently of its location within the source code. It does not matter whether there are other functions with other names defined before of after it - the instructions contained within this function's definition will always be the first ones to be executed in any C++ program. For that same reason, it is essential that all C++ programs have a main function. The word main is followed in the code by a pair of parentheses (()). That is because it is a function declaration: In C++, what differentiates a function declaration from other types of expressions are these parentheses that follow its name. Optionally, these parentheses may enclose a list of parameters within them. Right after these parentheses we can find the body of the main function enclosed in braces ({}). What is contained within these braces is what the function does when it is executed. cout << "Hello World"; This line is a C++ statement. A statement is a simple or compound expression that can actually produce some effect. In fact, this statement performs the only action that generates a visible effect in our first program. cout represents the standard output stream in C++, and the meaning of the entire statement is to insert a sequence of characters (in this case the Hello World sequence of characters) into the standard output stream (which usually is the screen). cout is declared in the iostream standard file within the std namespace, so that's why we needed to include that specific file and to declare that we were going to use this specific namespace earlier in our code.
- 11. VISUAL C++ PROGRAMMING VC++ Notice that the statement ends with a semicolon character (;). This character is used to mark the end of the statement and in fact it must be included at the end of all expression statements in all C++ programs (one of the most common syntax errors is indeed to forget to include some semicolon after a statement). [email protected] 11 return 0; The return statement causes the main function to finish. return may be followed by a return code (in our example is followed by the return code 0). A return code of 0 for the main function is generally interpreted as the program worked as expected without any errors during its execution. This is the most usual way to end a C++ program. You may have noticed that not all the lines of this program perform actions when the code is executed. There were lines containing only comments (those beginning by //). There were lines with directives for the compiler's preprocessor (those beginning by #). Then there were lines that began the declaration of a function (in this case, the main function) and, finally lines with statements (like the insertion into cout), which were all included within the block delimited by the braces ({}) of the main function. Preprocessor directives (those that begin by #) are out of this general rule since they are not statements. They are lines read and discarded by the preprocessor and do not produce any code by themselves. Preprocessor directives must be specified in their own line and do not have to end with a semicolon (;). Comments Comments are parts of the source code disregarded by the compiler. They simply do nothing. Their purpose is only to allow the programmer to insert notes or descriptions embedded within the source code. C++ supports two ways to insert comments: // line comment /* block comment */ The first of them, known as line comment, discards everything from where the pair of slash signs (//) is found up to the end of that same line. The second one, known as block comment, discards everything between the /* characters and the first appearance of the */ characters, with the possibility of including more than one line.
- 12. VISUAL C++ PROGRAMMING VC++ If you include comments within the source code of your programs without using the comment characters combinations //, /* or */, the compiler will take them as if they were C++ expressions, most likely causing one or several error messages when you compile it. [email protected] 12 Variables. Data Types. Each variable needs an identifier that distinguishes it from the others, for example, in the previous code the variable identifiers were a, b and result, but we could have called the variables any names we wanted to invent, as long as they were valid identifiers. Identifiers A valid identifier is a sequence of one or more letters, digits or underline characters (_). Neither spaces nor punctuation marks or symbols can be part of an identifier. Only letters, digits and underline characters are valid. In addition, variable identifiers always have to begin with a letter. They can also begin with an underline character (_ ), but this is usually reserved for compiler specific keywords or external identifiers. In no case they can begin with a digit. Another rule that you have to consider when inventing your own identifiers is that they cannot match any keyword of the C++ language or your compiler's specific ones since they could be confused with these. The standard reserved keywords are: asm, auto, bool, break, case, catch, char, class, const, const_cast, continue, default, delete, do, double, dynamic_cast, else, enum, explicit, export, extern, false, float, for, friend, goto, if, inline, int, long, mutable, namespace, new, operator, private, protected, public, register, reinterpret_cast, return, short, signed, sizeof, static, static_cast, struct, switch, template, this, throw, true, try, typedef, typeid, typename, union, unsigned, using, virtual, void, volatile, wchar_t, while Additionally, alternative representations for some operators cannot be used as identifiers since they are reserved words under some circumstances: and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq Your compiler may also include some additional specific reserved keywords. Very important: The C++ language is a "case sensitive" language. That means that an identifier
- 13. VISUAL C++ PROGRAMMING VC++ written in capital letters is not equivalent to another one with the same name but written in small letters. Thus, for example, the RESULT variable is not the same as the result variable or the Result variable. These are three different variable identifiers. [email protected] 13 Fundamental data types When programming, we store the variables in our computer's memory, but the computer has to know what we want to store in them, since it is not going to occupy the same amount of memory to store a simple number than to store a single letter or a large number, and they are not going to be interpreted the same way. The memory in our computers is organized in bytes. A byte is the minimum amount of memory that we can manage in C++. A byte can store a relatively small amount of data: one single character or a small integer (generally an integer between 0 and 255). In addition, the computer can manipulate more complex data types that come from grouping several bytes, such as long numbers or non-integer numbers. Next you have a summary of the basic fundamental data types in C++, as well as the range of values that can be represented with each one: Name Description Size* Range* char Character or small integer. 1byte signed: -128 to 127 unsigned: 0 to 255 short int (short) Short Integer. 2bytes signed: -32768 to 32767 unsigned: 0 to 65535 int Integer. 4bytes signed: -2147483648 to 2147483647 unsigned: 0 to 4294967295 long int (long) Long integer. 4bytes signed: -2147483648 to 2147483647 unsigned: 0 to 4294967295 bool Boolean value. It can take one of two values: true or false. 1byte true or false float Floating point number. 4bytes 3.4e +/- 38 (7 digits) double Double precision floating point number. 8bytes 1.7e +/- 308 (15 digits) long double Long double precision floating point number. 8bytes 1.7e +/- 308 (15 digits) wchar_t Wide character. 2bytes 1 wide character * The values of the columns Size and Range depend on the architecture of the system where the program is compiled and executed. The values shown above are those found on most 32bit systems. But for other systems, the general specification is that int has the natural size suggested by the system architecture (one word) and the four integer types char, short, int and long must
- 14. VISUAL C++ PROGRAMMING VC++ each one be at least as large as the one preceding it. The same applies to the floating point types float, double and long double, where each one must provide at least as much precision as the preceding one. Introducing strings While a character array can be fairly useful, it is quite limited. It’s simply a group of characters in memory, but if you want to do anything with it you must manage all the little details. For example, the size of a quoted character array is fixed at compile time. If you have a character array and you want to add some more characters to it, you’ll need to understand quite a lot (inluding dynamic memory management, character array copying and concatenation) before you can get your wish. This is exactly the kind of thing we’d like to have an object do for us. The Standard C++ string class is designed to take care of (and hide) all the low-level manipulations of character arrays that were previously required of the C++ programmer. These manipulations have been a constant source of time-wasting and errors since the inception of the C language. So, although an entire chapter is devoted to the string class later in the book, the string is so important and it makes life so much easier that it will be introduced here and used in much of the early part of the book. To use strings you include the C++ header file <string>. The string class is in the namespace std so a using directive is necessary. Because of operator overloading, the syntax for using strings is quite intuitive: //: C02:HelloStrings.cpp // The basics of the Standard C++ string class #include <string> #include <iostream> using namespace std; int main() { string s1, s2; // Empty strings string s3 = "Hello, World."; // Initialized string s4("I am"); // Also initialized s2 = "Today"; // Assigning to a string s1 = s3 + " " + s4; // Combining strings s1 += " 8 "; // Appending to a string cout << s1 + s2 + "!" << endl; } ///:~ The first two strings, s1 and s2, start out empty, while s3 and s4 show two equivalent ways to initialize string objects from character arrays (you can as easily initialize string objects from other string objects). You can assign to any string object using ‘ =’. This replaces the previous contents of the string with whatever is on the right-hand side, and you don’t have to worry about what happens to the previous contents – that’s handled automatically for you. To combine strings you simply use the ‘ +’ operator, which also allows you to combine character arrays with strings. If you want to append either a string or a character array to another string, you can use the operator ‘ +=’. Finally, note that iostreams already know what to do with strings, so you can just send a string (or an expression that produces a string, which happens with s1 + s2 + "!" ) directly to cout in order to print it. [email protected] 14
- 15. VISUAL C++ PROGRAMMING VC++ Variables that can store non-numerical values that are longer than one single character are known as strings. The C++ language library provides support for strings through the standard not a fundamental type, but it behaves in a similar way as fundamental types do in its most basic usage. A first difference with fundamental data types is that in order to declare and use objects (variables) of this type we need to include an addit and have access to the std namespace (which we already had in all our previous programs thanks to the using namespace statement). // my first string #include <iostream> #include <string> using namespace std; int main () { string mystring = "This is a string" cout << mystring; return 0; } Scope of variables All the variables that we intend to use in a program must have been declared with its type specifier in an earlier point in the code, like we did i body of the function main when we declared that A variable can be either of global or local scope. A global variable is a variable declared in the main body of the source code, the body of a function or a block. [email protected] string additional header file in our source code: string"; in the previous code at the beginning of the a, b, and result were of type outside all functions, while a local variable is one declared within 15 class. This is ional <string> n int.
- 16. VISUAL C++ PROGRAMMING VC++ Global variables can be referred from anywhere in the code, even inside functions, whenever it is after its declaration. The scope of local variables is limited to the block enclosed in braces ({}) where they are declared. For example, if they are declared at the beginning of the body of a function (like in function main) their scope is between its declaration point and the end of that function. In the example above, this means that if another function existed in addition to main, the local variables declared in main could not be accessed from the other function and vice versa. [email protected] 16 Initialization of variables When declaring a regular local variable, its value is by default undetermined. But you may want a variable to store a concrete value at the same moment that it is declared. In order to do that, you can initialize the variable. There are two ways to do this in C++: The first one, known as c-like, is done by appending an equal sign followed by the value to which the variable will be initialized: type identifier = initial_value ; For example, if we want to declare an int variable called a initialized with a value of 0 at the moment in which it is declared, we could write: int a = 0; The other way to initialize variables, known as constructor initialization, is done by enclosing the initial value between parentheses (()): type identifier (initial_value) ; For example: int a (0); Both ways of initializing variables are valid and equivalent in C++. // initialization of variables #include <iostream>
- 17. VISUAL C++ PROGRAMMING VC++ using namespace std; int main () { int a=5; // initial value = 5 int b(2); // initial value = 2 int result; // initial value undetermined a = a + 3; result = a - b; cout << result; return 0; } 6 Specifying storage allocation When creating a variable, you have a number of options to specify the lifetime of the variable, how the storage is allocated for that variable, and how the variable is treated by the compiler. [email protected] 17 Global variables Global variables are defined outside all function bodies and are available to all parts of the program (even code in other files). Global variables are unaffected by scopes and are always available (i.e., the lifetime of a global variable lasts until the program ends). If the existence of a global variable in one file is declared using the extern keyword in another file, the data is available for use by the second file. Here’s an example of the use of global variables: //: C03:Global.cpp //{L} Global2 // Demonstration of global variables #include <iostream> using namespace std; int globe; void func(); int main() { globe = 12; cout << globe << endl; func(); // Modifies globe cout << globe << endl; } ///:~ Here’s a file that accesses globe as an extern: //: C03:Global2.cpp {O} // Accessing external global variables extern int globe; // (The linker resolves the reference) void func() { globe = 47; } ///:~
- 18. VISUAL C++ PROGRAMMING VC++ Storage for the variable globe is created by the definition in Global.cpp, and that same variable is accessed by the code in Global2.cpp. Since the code in Global2.cpp is compiled separately from the code in Global.cpp, the compiler must be informed that the variable exists elsewhere by the declaration extern int globe; When you run the program, you’ll see that the call to func( ) does indeed affect the single global instance of globe. In Global.cpp, you can see the special comment tag (which is my own design): //{L} Global2 This says that to create the final program, the object file with the name Global2 must be linked in (there is no extension because the extension names of object files differ from one system to the next). In Global2.cpp, the first line has another special comment tag {O} which says “don’t try to create an executable out of this file, it’s being compiled so that it can be linked into some other executable.” The ExtractCode.cpp program at the end of this book reads these tags and creates the appropriate makefile so everything compiles properly (you’ll learn about makefiles at the end of this chapter). [email protected] 18 Local variables Local variables occur within a scope; they are “local” to a function. They are often called automatic variables because they automatically come into being when the scope is entered, and automatically go away when the scope closes. The keyword auto makes this explicit, but local variables default to auto so it is never necessary to declare something as an auto. Register variables A register variable is a type of local variable. The register keyword tells the compiler “make accesses to this variable as fast as possible.” Increasing the access speed is implementation dependent but, as the name suggests, it is often done by placing the variable in a register. There is no guarantee that the variable will be placed in a register or even that the access speed will increase. It is a hint to the compiler. There are restrictions to the use of register variables. You cannot take or compute the address of a register variable. A register variable can only be declared within a block (you cannot have global or static register variables). You can, however, use a register variable as a formal argument in a function (i.e., in the argument list). Generally, you shouldn’t try to second-guess the compiler’s optimizer, since it will probably do a better job than you can. Thus, the register keyword is best avoided. Static The static keyword has several distinct meanings. Normally, variables defined local to a function disappear at the end of the function scope. When you call the function again, storage for the variables is created anew and the values are re-initialized. If you want a value to be extant
- 19. VISUAL C++ PROGRAMMING VC++ throughout the life of a program, you can define a function’s local variable to be static and give it an initial value. The initialization is only performed the first time the function is called, and the data retains its value between function calls. This way, a function can “remember” some piece of information between function calls. You may wonder why a global variable isn’t used instead. The beauty of a static variable is that it is unavailable outside the scope of the function, so it can’t be inadvertently changed. This localizes errors. Here’s an example of the use of static variables: //: C03:Static.cpp // Using a static variable in a function #include <iostream> using namespace std; [email protected] 19 void func() { static int i = 0; cout << "i = " << ++i << endl; } int main() { for(int x = 0; x < 10; x++) func(); } ///:~ Each time func( ) is called in the for loop, it prints a different value. If the keyword static is not used, the value printed will always be ‘1’. The second meaning of static is related to the first in the “unavailable outside a certain scope” sense. When static is applied to a function name or to a variable that is outside of all functions, it means “this name is unavailable outside of this file.” The function name or variable is local to the file; we say it has file scope. As a demonstration, compiling and linking the following two files will cause a linker error: //: C03:FileStatic.cpp // File scope demonstration. Compiling and // linking this file with FileStatic2.cpp // will cause a linker error // File scope means only available in this file: static int fs; int main() { fs = 1; } ///:~ Even though the variable fs is claimed to exist as an extern in the following file, the linker won’t find it because it has been declared static in FileStatic.cpp. //: C03:FileStatic2.cpp {O} // Trying to reference fs extern int fs;
- 20. VISUAL C++ PROGRAMMING VC++ [email protected] 20 void func() { fs = 100; } ///:~ The static specifier may also be used inside a class. This explanation will be delayed until you learn to create classes, later in the book. Extern The extern keyword has already been briefly described and demonstrated. It tells the compiler that a variable or a function exists, even if the compiler hasn’t yet seen it in the file currently being compiled. This variable or function may be defined in another file or further down in the current file. As an example of the latter: //: C03:Forward.cpp // Forward function & data declarations #include <iostream> using namespace std; // This is not actually external, but the // compiler must be told it exists somewhere: extern int i; extern void func(); int main() { i = 0; func(); } int i; // The data definition void func() { i++; cout << i; } ///:~ When the compiler encounters the declaration ‘ extern int i ’ it knows that the definition for i must exist somewhere as a global variable. When the compiler reaches the definition of i, no other declaration is visible so it knows it has found the same i declared earlier in the file. If you were to define i as static, you would be telling the compiler that i is defined globally (via the extern), but it also has file scope (via the static), so the compiler will generate an error. Linkage To understand the behavior of C and C++ programs, you need to know about linkage. In an executing program, an identifier is represented by storage in memory that holds a variable or a compiled function body. Linkage describes this storage it is seen by the linker. There are two types of linkage: internal linkage and external linkage. Internal linkage means that storage is created to represent the identifier only for the file being compiled. Other files may use the same identifier name with internal linkage, or for a global variable, and no conflicts will be found by the linker – separate storage is created for each identifier. Internal linkage is specified by the keyword static in C and C++.
- 21. VISUAL C++ PROGRAMMING VC++ External linkage means that a single piece of storage is created to represent the identifier for all files being compiled. The storage is created once, and the linker must resolve all other references to that storage. Global variables and function names have external linkage. These are accessed from other files by declaring them with the keyword extern. Variables defined outside all functions (with the exception of const in C++) and function definitions default to external linkage. You can specifically force them to have internal linkage using the static keyword. You can explicitly state that an identifier has external linkage by defining it with the extern keyword. Defining a variable or function with extern is not necessary in C, but it is sometimes necessary for const in C++. Automatic (local) variables exist only temporarily, on the stack, while a function is being called. The linker doesn’t know about automatic variables, and they have no linkage. [email protected] 21 Constants In old (pre-Standard) C, if you wanted to make a constant, you had to use the preprocessor: #define PI 3.14159 Everywhere you used PI, the value 3.14159 was substituted by the preprocessor (you can still use this method in C and C++). When you use the preprocessor to create constants, you place control of those constants outside the scope of the compiler. No type checking is performed on the name PI and you can’t take the address of PI (so you can’t pass a pointer or a reference to PI). PI cannot be a variable of a user-defined type. The meaning of PI lasts from the point it is defined to the end of the file; the preprocessor doesn’t recognize scoping. C++ introduces the concept of a named constant that is just like a variable, except its value cannot be changed. The modifier const tells the compiler that a name represents a constant. Any data type, built-in or user-defined, may be defined as const. If you define something as const and then attempt to modify it, the compiler will generate an error. You must specify the type of a const, like this: const int x = 10; In Standard C and C++, you can use a named constant in an argument list, even if the argument it fills is a pointer or a reference (i.e., you can take the address of a const). A const has a scope, just like a regular variable, so you can “hide” a const inside a function and be sure that the name will not affect the rest of the program. The const was taken from C++ and incorporated into Standard C, albeit quite differently. In C, the compiler treats a const just like a variable that has a special tag attached that says “don’t change me.” When you define a const in C, the compiler creates storage for it, so if you define more than one const with the same name in two different files (or put the definition in a header file), the linker will generate error messages about conflicts. The intended use of const in C is quite different from its intended use in C++ (in short, it’s nicer in C++).
- 22. VISUAL C++ PROGRAMMING VC++ Constant values In C++, a const must always have an initialization value (in C, this is not true). Constant values for built-in types are expressed as decimal, octal, hexadecimal, or floating-point numbers (sadly, binary numbers were not considered important), or as characters. In the absence of any other clues, the compiler assumes a constant value is a decimal number. The numbers 47, 0 and 1101 are all treated as decimal numbers. A constant value with a leading 0 is treated as an octal number (base 8). Base 8 numbers can only contain digits 0-7; the compiler flags other digits as an error. A legitimate octal number is 017 (15 in base 10). A constant value with a leading 0x is treated as a hexadecimal number (base 16). Base 16 numbers contain the digits 0-9 and a-f or A-F. A legitimate hexadecimal number is 0x1fe (510 in base 10). Floating point numbers can contain decimal points and exponential powers (represented by e, which means “10 to the power”). Both the decimal point and the e are optional. If you assign a constant to a floating-point variable, the compiler will take the constant value and convert it to a floating-point number (this process is one form of what’s called implicit type conversion). However, it is a good idea to use either a decimal point or an e to remind the reader you are using a floating-point number; some older compilers also need the hint. Legitimate floating-point constant values are: 1e4, 1.0001, 47.0, 0.0 and -1.159e-77. You can add suffixes to force the type of floating-point number: f or F forces a float, L or l forces a long double, otherwise the number will be a double. Character constants are characters surrounded by single quotes, as: ‘ A’, ‘ 0’, ‘ ‘. Notice there is a big difference between the character ‘ 0’ (ASCII 96) and the value 0. Special characters are represented with the “backslash escape”: ‘ n’ (newline), ‘ t’ (tab), ‘ ’ (backslash), ‘ r’ (carriage return), ‘ "’ (double quotes), ‘ '’ (single quote), etc. You can also express char constants in octal: ‘ 17’ or hexadecimal: ‘ xff’. [email protected] 22 Volatile Whereas the qualifier const tells the compiler “this never changes” (which allows the compiler to perform extra optimizations) the qualifier volatile tells the compiler “you never know when this will change,” and prevents the compiler from performing any optimizations. Use this keyword when you read some value outside the control of your code, such as a register in a piece of communication hardware. A volatile variable is always read whenever its value is required, even if it was just read the line before. A special case of some storage being “outside the control of your code” is in a multithreaded program. If you’re watching a particular flag that is modified by another thread or process, that flag should be volatile so the compiler doesn’t make the assumption that it can optimize away multiple reads of the flag.
- 23. VISUAL C++ PROGRAMMING VC++ [email protected] 23 Constants Constants are expressions with a fixed value. Literals Literals are used to express particular values within the source code of a program. We have already used these previously to give concrete values to variables or to express messages we wanted our programs to print out, for example, when we wrote: a = 5; the 5 in this piece of code was a literal constant. Literal constants can be divided in Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean Values. Integer Numerals 1776 707 -273 They are numerical constants that identify integer decimal values. Notice that to express a numerical constant we do not have to write quotes (") nor any special character. There is no doubt that it is a constant: whenever we write 1776 in a program, we will be referring to the value 1776. In addition to decimal numbers (those that all of us are used to use every day) C++ allows the use as literal constants of octal numbers (base 8) and hexadecimal numbers (base 16). If we want to express an octal number we have to precede it with a 0 (zero character). And in order to express a hexadecimal number we have to precede it with the characters 0x (zero, x). For example, the following literal constants are all equivalent to each other: 75 // decimal 0113 // octal 0x4b // hexadecimal
- 24. VISUAL C++ PROGRAMMING VC++ All of these represent the same number: 75 (seventy-five) expressed as a base-10 numeral, octal numeral and hexadecimal numeral, respectively. Literal constants, like variables, are considered to have a specific data type. By default, integer literals are of type int. However, we can force them to either be unsigned by appending the u character to it, or long by appending l: [email protected] 24 75 // int 75u // unsigned int 75l // long 75ul // unsigned long In both cases, the suffix can be specified using either upper or lowercase letters. Floating Point Numbers They express numbers with decimals and/or exponents. They can include either a decimal point, an e character (that expresses "by ten at the Xth height", where X is an integer value that follows the e character), or both a decimal point and an e character: 3.14159 // 3.14159 6.02e23 // 6.02 x 1023 1.6e-19 // 1.6 x 10-19 3.0 // 3.0 These are four valid numbers with decimals expressed in C++. The first number is PI, the second one is the number of Avogadro, the third is the electric charge of an electron (an extremely small number) -all of them approximated- and the last one is the number three expressed as a floating-point numeric literal. The default type for floating point literals is double. If you explicitly want to express a float or long double numerical literal, you can use the f or l suffixes respectively: 3.14159L // long double 6.02e23f // float Any of the letters than can be part of a floating-point numerical constant (e, f, l) can be written using either lower or uppercase letters without any difference in their meanings. Character and string literals There also exist non-numerical constants, like: 'z'
- 25. VISUAL C++ PROGRAMMING VC++ [email protected] 25 'p' "Hello world" "How do you do?" The first two expressions represent single character constants, and the following two represent string literals composed of several characters. Notice that to represent a single character we enclose it between single quotes (') and to express a string (which generally consists of more than one character) we enclose it between double quotes ("). When writing both single character and string literals, it is necessary to put the quotation marks surrounding them to distinguish them from possible variable identifiers or reserved keywords. Notice the difference between these two expressions: x 'x' x alone would refer to a variable whose identifier is x, whereas 'x' (enclosed within single quotation marks) would refer to the character constant 'x'. Character and string literals have certain peculiarities, like the escape codes. These are special characters that are difficult or impossible to express otherwise in the source code of a program, like newline (n) or tab (t). All of them are preceded by a backslash (). Here you have a list of some of such escape codes: n newline r carriage return t tab v vertical tab b backspace f form feed (page feed) a alert (beep) ' single quote (') " double quote (") ? question mark (?) backslash ()
- 26. VISUAL C++ PROGRAMMING VC++ [email protected] 26 For example: 'n' 't' "Left t Right" "onentwonthree" Additionally, you can express any character by its numerical ASCII code by writing a backslash character () followed by the ASCII code expressed as an octal (base-8) or hexadecimal (base-16) number. In the first case (octal) the digits must immediately follow the backslash (for example 23 or 40), in the second case (hexadecimal), an x character must be written before the digits themselves (for example x20 or x4A). String literals can extend to more than a single line of code by putting a backslash sign () at the end of each unfinished line. "string expressed in two lines" You can also concatenate several string constants separating them by one or several blank spaces, tabulators, newline or any other valid blank character: "this forms" "a single" "string" "of characters" Finally, if we want the string literal to be explicitly made of wide characters (wchar_t), instead of narrow characters (char), we can precede the constant with the L prefix: L"This is a wide character string" Wide characters are used mainly to represent non-English or exotic character sets. Boolean literals There are only two valid Boolean values: true and false. These can be expressed in C++ as values of type bool by using the Boolean literals true and false. Defined constants (#define)
- 27. VISUAL C++ PROGRAMMING VC++ You can define your own names for constants that you use very often without having to resort to memory-consuming variables, simply by using the #define preprocessor directive. Its format is: [email protected] 27 #define identifier value For example: #define PI 3.14159265 #define NEWLINE 'n' This defines two new constants: PI and NEWLINE. Once they are defined, you can use them in the rest of the code as if they were any other regular constant, for example: // defined constants: calculate circumference #include <iostream> using namespace std; #define PI 3.14159 #define NEWLINE 'n'; int main () { double r=5.0; // radius double circle; circle = 2 * PI * r; cout << circle; cout << NEWLINE; return 0; } 31.4159 In fact the only thing that the compiler preprocessor does when it encounters #define directives is to literally replace any occurrence of their identifier (in the previous example, these were PI and NEWLINE) by the code to which they have been defined (3.14159265 and 'n' respectively). The #define directive is not a C++ statement but a directive for the preprocessor; therefore it assumes the entire line as the directive and does not require a semicolon (;) at its end. If you append a semicolon character (;) at the end, it will also be appended in all occurrences within the body of the program that the preprocessor replaces.
- 28. VISUAL C++ PROGRAMMING VC++ type typical size description short 2 bytes stores a short (i.e., small) integer int 4 bytes stores an integer long 4 bytes stores a long (i.e., large) integer float 4 bytes stores a floating-point number double 8 bytes stores a "double-precision" floating-point number [email protected] 28 Declared constants (const) With the const prefix you can declare constants with a specific type in the same way as you would do with a variable: const int pathwidth = 100; const char tabulator = 't'; const zipcode =12440; In case that no type is explicitly specified (as in the last example) the compiler assumes that it is of type int. String Stream The standard header file <sstream> defines a class called stringstream that allows a string-based object to be treated as a stream. This way we can perform extraction or insertion operations from/to strings, which is especially useful to convert strings to numerical values and vice versa. For example, if we want to extract an integer from a string we can write: string mystr ("1204"); int myint; stringstream(mystr) >> myint; This declares a string object with a value of "1204", and an int object. Then we use stringstream's constructor to construct an object of this type from the string object. Because we can use stringstream objects as if they were streams, we can extract an integer from it as we would have done on cin by applying the extractor operator (>>) on it followed by a variable of type int. After this piece of code, the variable myint will contain the numerical value 1204. // stringstreams #include <iostream> #include <string> #include <sstream> using namespace std;
- 29. VISUAL C++ PROGRAMMING VC++ int main () { string mystr; float price=0; int quantity=0; cout << "Enter price: "; getline (cin,mystr); stringstream(mystr) >> price; cout << "Enter quantity: "; getline (cin,mystr); stringstream(mystr) >> quantity; cout << "Total price: " << price*quantity << endl; return 0; } Enter price: 22.25 Enter quantity: 7 Total price: 155.75 In this example, we acquire numeric values from the standard input indirectly. Instead of extracting numeric values directly from the standard input, we get lines from the standard input (cin) into a string object (mystr), and then we extract the integer values from this string into a variable of type int (myint). Using this method, instead of direct extractions of integer values, we have more control over what happens with the input of numeric values from the user, since we are separating the process of obtaining input from the user (we now simply ask for lines) with the interpretation of that input. Therefore, this method is usually preferred to get numerical values from the user in all programs that are intensive in user input. [email protected] 29 Example #include "stdafx.h" #include <iostream> #include <string> using namespace std; int main () { string mystr;
- 30. cout << "What's your name? "; getline (cin, mystr); cout << "Hello " << mystr << ". ".n"; cout << "What is your favorite team? "; getline (cin, mystr); cout << "I like " << mystr << " too! return 0; } Operators < too!n"; Booleans: True and False VC++ Before talking about operators, we'll take a quick aside into what a boolean is before discussing operators. A boolean value is one that can be either true or false. No other values are allowed. Booleans and boolean operations are at the heart of programming. Many times in a program, you'll want to do one thing if a certain and a different thing if the condition is false. For example, when processing a series of checkboxes, you may want to take an action only if a box is checked, and do not That's when you'll want to use a boolean. Most programming languages have a type for booleans, usually called "boolean" or "bool". Some C++ compilers recognize the type supports the bool type. We'll discuss what to do if your compiler doesn't, in a moment. In order to use boolean logic to your advantage, you need to learn about the three basic boolean operations. They are called and inputs, and returns a boolean output. They shown below. and VISUAL C++ PROGRAMMING [email protected] booleans, since we'll need to know bool, others do not. For now, assume that your compiler and, or, and not. Each operation takes either one or two boolean are often represented by symbols known as "gates", The "and" operation takes two inputs and produces one output. If both inputs are true, the output is true; in all other cases, the output is false. It can be interpreted as follows: "I will return true if input 1 and 30 , condition is true, nothing otherwise. , . input 2 are true."
- 31. or not Boolean operators in C++ VISUAL C++ PROGRAMMING VC++ The "or" operation takes two inputs and produces one output. If either of the inputs are true, the output is true; otherwise (i.e., if neither input is true), the output is false. It can be interpreted as follows: "I will return true if either input 1 or input 2 is true." The "not" operation takes one input and produces one output. If the input is true, the output is false. If the input is false, the output is true. In other words, the "not" takes the input and returns its opposite. There are operators in C++ which behave just as the boolean gates shown above! We'll show you an example of how to use each one. and: && The "and" operator is used by placing //suppose that Fran is tired bool franIsTired = true; the "and" symbol, &&, in between two boolean values. //but Fran doesn't have to wake up early bool franMustWakeUpEarly = false; //will Fran go to sleep now? bool bedTime = franIsTired & && franMustWakeUpEarly; What does this chunk of code do? It initializes two variables, franMustWakeUpEarly, to true including comments!), we determine that Fran will go to sleep true -- that is, if both inputs to the "and" operation are true. In this case, the first input is true and the second input is false. Since "and" requires both inputs to be true in order for the output to be true, but one of the inputs is false, the output will be false. So, the variable store the value false. Also, take note that the variable names used here are lengthy. How you decide to program is up to you, but often times it's better to have lengthie short, obfuscated variable names like "i" or "zz". (The names in this example may have gone overboard, though.) [email protected] franIsTired and and false, respectively. Then, in the third line of code (not if and only if the "and" operation is lengthier variable names that are readable, rather than 31 operation , , bedTime will r
- 32. VISUAL C++ PROGRAMMING VC++ [email protected] 32 or: || The "or" operator is used by placing the "or" symbol, ||, in between two boolean values. //suppose that Graham is tired bool grahamIsTired = true; //but Graham doesn't have to wake up early bool grahamMustWakeUpEarly = false; //will Graham go to sleep now? bool bedTime = grahamIsTired || grahamMustWakeUpEarly; This example is very similar to the example involving Fran, except notice the key difference: whether or not Graham goes to sleep is determined differently. Graham will go to sleep if he is tired or if he needs to wake up early. Whereas Fran would go to sleep only if both conditions were true, Graham will go to sleep if either condition (or both) is true. Therefore, the value of bedTime is true. not: ! The "not" operator is used by placing the "not" symbol, !, before a boolean value. //suppose that Julian stayed up late bool julianStayedUpLate = true; //will Julian be peppy tomorrow? bool julianIsPeppy = !julianStayedUpLate; This example illustrates the "not" operator. At the end of this block of code, the variable julianIsPeppy will take on the opposite value of julianStayedUpLate. If julianStayedUpLate were false, then julianIsPeppy would be true. In this case, the opposite is true, so julianIsPeppy gets a value of false. It is perfectly legal in C++ to use boolean operators on variables which are not booleans. In C++, "0" is false and any non-zero value is true. Let's look at a contrived example. int hours = 4; int minutes = 21; int seconds = 0; bool timeIsTrue = hours && minutes && seconds; Since hours evaluates to true, and since minutes evaluates to true, and since seconds evaluates to false, the entire expression hours && minutes && seconds evaluates to false.
- 33. VISUAL C++ PROGRAMMING VC++ [email protected] 33 Arithmetic operators in C++ In addition to the boolean operators, C++ has a number of arithmetic operators. Here they are: Arithmetic operators name symbol sample usage addition + int sum = 4 + 7 subtraction - float difference = 18.55 - 14.21 multiplication * float product = 5 * 3.5 division / int quotient = 14 / 3 modulo ("mod") % int remainder = 10 % 6 They all probably look familiar with the exception of mod (%). The mod is simply the remainder produced by dividing two integers. In the example shown in the table above, if we treat 10 / 6 as an integer divison, the quotient is 1 (rather than 1.666) and the remainder is 4. Hence, the variable remainder will get the value 4. Equality operators in C++ You are undoubtedly familiar with equality operators, even if you don't know it. An equality operator is one that tests a condition such as "is less than", "is greater than", and "is equal to". You will find it useful to be able to compare two numbers using expressions like "x is less than y". Let's say you are writing software for a bank ATM (automated teller machine). A customer makes a request for a certain amount of cash, and your responsibility is to determine if they should be allowed to withdraw that amount. You could decide to use the following algorithm: "if the amount requested is less than the account balance, that amount should be withdrawn; otherwise, the customer should be notified and no money should be withdrawn." Makes sense, right? So, the next step is coming up with some pseudo-code. Once you have pseudo-code, writing the C++ code will be easy. Pseudo-code for the ATM problem might look like this: if the amount requested < account balance then withdraw the amount requested otherwise withdraw nothing and notify the customer Now that we have pseudo-code, writing the C++ code is as simple as "translating" your pseudo-code into C++. In this case, it's easy: if (amountRequested < accountBalance) {
- 34. VISUAL C++ PROGRAMMING VC++ [email protected] 34 withdraw(amountRequested); } else { withdraw(0); notifyCustomer(); } You'll notice some new syntax in this example, but don't worry about it too much. Pay close attention to the very first line, which checks to make sure that the amount requested is less than the account balance. The way it works is, if the expression between parentheses (()) evaluates to true, then the first block of code will be read. That is, the code inside the first set of curly braces ({}) will be executed. If the expression in parentheses evaluates to false, on the other hand, then the second block of code (the code following the word else) will be read. In this case, the first block of code withdraws the amount requested by the customer, while the second block of code withdraws nothing, and notifies the customer. That wasn't so hard! All we did was take the original English description of how we would solve the problem, write some pseudo-code for the English description, and translate the pseudo-code into C++. Once you know how to use one equality operator, you know how to use all of them. They all work the same way: they take the expressions on either side of them, and either return true or false. Here they are: Equality operators name symbol sample usage result is less than < bool result = (4 < 7) true is greater than > bool result = (3.1 > 3.1) false is equal to == bool result = (11 == 8) false is less than or equal to <= bool result = (41.1 <= 42) true is greater than or equal to >= bool result = (41.1 >= 42) false is not equal to != bool result = (12 != 12) false Assignment operators in C++ Believe it or not, you've already been using assignment operators! Probably the most common assignment operator is the equals sign (=). It is called "assignment" because you are "assigning" a variable to a value. This operator takes the expression on its right-hand-side and places it into the
- 35. VISUAL C++ PROGRAMMING VC++ variable on its left-hand-side. So, when you write x = 5, the operator takes the expression on the right, 5, and stores it in the variable on the left, x. Remember how the equality operators, like < and !=, returned a value that indicated the result? In that case, the return value was either true or false. In fact, almost every expression in C++ returns something! You don't always have to use the return value, though -- it's completely up to you. In the case of the assignment operators, the return value is simply the value that it stored in the variable on the left-hand-side. Sometimes your code will use the return value to do something useful. In the ATM example, one line of code was executed if the condition was true (that is, if the equality operator returned true). Two different lines were executed if the condition was false. Other times, you'll completely ignore the return value, because you're not interested in it. Take a look at the following code: int x; int y; x = 5; y = 9; cout << "The value of x is " << x << endl; cout << "The value of y is " << y << endl; int sum; sum = x + y; cout << "The sum of x and y is " << sum << endl; This chunk of code shows why you might want to throw away the return value of an operator. Look at the third line, x = 5. We're using the assignment operator here to place the value 5 in the variable x. Since the expression x = 5 returns a value, and we're not using it, then you could say we are ignoring the return value. However, note that a few of lines down, we are very interested in the return value of an operator. The addition operator in the expression x + y returns the sum of its left-hand-side and right-hand-side. That's how we are able to assign a value to sum. You can think of it as sum = (x + y), since that's what it's really doing. The other assignment operators are all based on the equals sign, so make sure you understand that before going on. Here's another assignment operator: +=. How does it work? You might guess that it has something to do with addition, and something to do with assignment. You'd be absolutely right! The += operator takes the variable on its left-hand-side and adds the expression on its right-hand-side. Whenever you see a statement that looks like the following: [email protected] 35
- 36. VISUAL C++ PROGRAMMING VC++ [email protected] 36 myVar += something; it is identical to saying the following: myVar = myVar + something; That's exactly what it's doing! It's simply a shortcut. The other common assignment operators are -=, *=, /=, and %=. They all function just like the += operator, except instead of adding the value on the right-hand-side, they subtract, or multiply, or divide, or "mod" it. Just as the simple assignment operator = returns the value that it stored, all of the assignment operators return the value stored in the variable on the left-hand-side. Here's an example of how you might take advantage of this return value. It's not used terribly often, but it can sometimes be useful. //these four variables represent the sides of a rectangle int left; int top; int right; int bottom; //make it a square whose sides are 4 left = top = right = bottom = 4; All this code does is store the value in each of the four variables left, top, right, and bottom. How does it work? It starts on the far right-hand side. It sees bottom = 4. So it places the value 4 in the variable bottom, and returns the value it stored in bottom (which is 4). Since bottom = 4 evaluates to 4, the variable right will also get the value 4, which means top will also get 4, which means left will also get 4. Phew! Of course, this code could have just as easily been written //these four variables represent the sides of a rectangle int left; int top; int right; int bottom; //make it a square whose sides are 4 left = 4; top = 4; right = 4; bottom = 4; Operator Precedence So far, we've seen a number of different operators. Here's a summary of the operators we've covered so far: Boolean operators &&, ||, !
- 37. VISUAL C++ PROGRAMMING VC++ Arithmetic operators +, -, *, /, % Equality operators <, >, ==, <=, >=, != Assignment operators =, +=, -=, *=, /=, %= [email protected] 37 What is operator precedence? Operator precedence refers to the order in which operators get used. An operator with high precedence will get used before an operator with lower precedence. Here's an example: int result = 4 + 5 * 6 + 2; What will be the value of result? The answer depends on the precedence of the operators. In C++, the multiplication operator (*) has higher precedence than the addition operator (+). What that means is, the multiplication 5 * 6 will take place before either of the additions, so your expression will resolve to 4 + 30 + 2 , so result will store the value 36. Since C++ doesn't really care about whitespace, the same thing would be true if you had written: int result = 4+5 * 6+2; The result would still be 36. Maybe you wanted to take the sum 4 + 5 and multiply it by the sum 6 + 2 for a result of 72? Just as in math class, add parentheses. You can write: int result = (4 + 5) * (6 + 2); Operator precedence in C++ Operator precedence in C++ is incredibly easy! Don't let anyone tell you otherwise! Here's the trick: if you don't know the order of precedence, or you're not sure, add parentheses! Don't even bother looking it up. We can guarantee that it will be faster for you to add parentheses than to look it up in this tutorial or in a C++ book. Adding parentheses has another obvious benefit - it makes your code much easier to read. Chances are, if you are uncertain about the order of precedence, anyone reading your code will have the same uncertainty. That having been said, here's the order of operator precedence. In general, the order is what you would think it is - that is, you can safely say int x = 4 + 3; and it will correctly add 4 and 3 before assigning to x. Our advice is to read this table once and then never refer to it again. Operator precedence operators have the same precedence as other operators in their group, and higher precedence than operators in lower groups
- 38. VISUAL C++ PROGRAMMING VC++ operator name ! boolean not * multiplication / division % mod + addition - subtraction < is less than <= is less than or equal to > is greater than >= is greater than or equal to == is equal to != is not equal to && boolean and || boolean or = assignment *= multiply and assign /= divide and assign %= mod and assign += add and assign -= subtract and assign Branching Statements (if, else, switch) [email protected] 38 The if statement The first type of branching statement we will look at is the if statement. An if statement has the form: if (condition) { // code to execute if condition is true } else { // code to execute if condition is false } In an if statement, condition is a value or an expression that is used to determine which code block is executed, and the curly braces act as "begin" and "end" markers. Here is a full C++ program as an example: //include this file for cout #include <iostream.h>
- 39. VISUAL C++ PROGRAMMING VC++ [email protected] 39 int main() { // define two integers int x = 3; int y = 4; //print out a message telling which is bigger if (x > y) { cout << "x is bigger than y" << endl; } else { cout << "x is smaller than y" << endl; } return 0; } In this case condition is equal to "(x > y)" which is equal to "(3 > 4)" which is a false statement. So the code within the else clause will be executed. The output of this program will be: x is smaller than y If instead the value for x was 6 and the value for y was 2, then condition would be "(6 > 2)" which is a true statement and the output of the program would be: x is bigger than y the switch statement The next branching statement is called a switch statement. A switch statement is used in place of many if statements. Let's consider the following case: Joel is writing a program that figures interest on money that is held in a bank. The amount of interest that money earns in this bank depends on which type of account the money is in. There are 6 different types of accounts and they earn interest as follows: account type interest earned personal financial 2.3% personal homeowner 2.6% personal gold 2.9% small business 3.3% big business 3.5% gold business 3.8%
- 40. VISUAL C++ PROGRAMMING VC++ One way for Joel to write this program is as follows: (assuming also that Joel has assigned numbers to the account types starting with personal financial and ending with gold business.) // declare a variable to keep track of the interest float interest = 0.0; // decide which interest rate to use. if (account_type == 1){ interest = 2.3; } else { [email protected] 40 if (account_type == 2) { interest = 2.6; } else { if (account_type == 3){ interest = 2.9; } else { if (account_type == 4){ interest = 3.3; } else { if (account_type == 5){ interest = 3.5; } else { // account type must be 6 interest = 3.8; } } } } } That code is hard to read and hard to understand. There is an easier way to write this, using the switch statement. The preceding chunk of code could be written as follows: switch (account_value){ case 1: interest = 2.3; break; case 2: interest = 2.6; break; case 3: interest = 2.9;
- 41. VISUAL C++ PROGRAMMING VC++ [email protected] 41 break; case 4: interest = 3.3; break; case 5: interest = 3.5; break; case 6: interest = 3.8; break; default: interest = 0.0; } The switch statement allows a programmer to compound a group of if statements, provided that the condition being tested is an integer. The switch statement has the form: switch(integer_val){ case val_1: // code to execute if integer_val is val_1 break; ... case val_n: // code to execute if integer_val is val_n break; default: // code to execute if integer_val is none of the above } The default clause is optional, but it is good programming practice to use it. The default clause is executed if none of the other clauses have been executed. For example, if my code looked like: switch (place) { case 1: cout << "we're first" << endl; break; case 2: cout << "we're second" << endl; break; default: cout << "we're not first or second" << endl; }
- 42. VISUAL C++ PROGRAMMING VC++ This switch statement will write "we're first" if the variable place is equal to 1, it will write "we're second" if place is equal to 2, and will write "we're not first or second" if place is any other value. The break keyword means "jump out of the switch statement, and do not execute any more code." To show how this works, examine the following piece of code: [email protected] 42 int value = 0; switch(input){ case 1: value+=4; case 2: value+=3; case 3: value+=2; default: value++; } If input is 1 then 4 will be added to value. Since there is no break statement, the program will go on to the next line of code which adds 3, then the line of code that adds 2, and then the line of code that adds 1. So value will be set to 10! The code that was intended was probably: int value = 0; switch(input){ case 1: value+=4; break; case 2: value+=3; break; case 3: value+=2; break; default: value++; } This feature of switch statements can sometimes be used to a programmers' advantage. In the example with the different types of bank accounts, say that the interest earned was a follows: account type interest earned personal financial 2.3% personal homeowner 2.6% personal gold 2.9%
- 43. VISUAL C++ PROGRAMMING VC++ small business 2.6% big business 2.9% gold business 3.0% Now, the code for this could be written as: [email protected] 43 switch (account_value){ case 1: interest = 2.3; break; case 2: case 4: interest = 2.6; break; case 3: case 5: interest = 2.9; break; case 6: interest = 3.8; break; default: interest = 0.0; } #include <iostream> using namespace std; int main(int argc, char *argv[]) { switch( tolower( *argv[1] ) ) { // Error. Unreachable declaration. char szChEntered[] = "Character entered was: "; case 'a' : { // Declaration of szChEntered OK. Local scope.
- 44. VISUAL C++ PROGRAMMING VC++ char szChEntered[] = "Character entered was: "; cout << szChEntered << "an"; } break; case 'b' : [email protected] 44 // Value of szChEntered undefined. cout << szChEntered << "bn"; break; default: // Value of szChEntered undefined. cout << szChEntered << "neither a nor bn"; break; } } Control Structures A program is usually not limited to a linear sequence of instructions. During its process it may bifurcate, repeat code or take decisions. For that purpose, C++ provides control structures that serve to specify what has to be done by our program, when and under which circumstances. With the introduction of control structures we are going to have to introduce a new concept: the compound-statement or block. A block is a group of statements which are separated by semicolons (;) like all C++ statements, but grouped together in a block enclosed in braces: { }: { statement1; statement2; statement3; } Most of the control structures that we will see in this section require a generic statement as part of its syntax. A statement can be either a simple statement (a simple instruction ending with a semicolon) or a compund statement (several instructions grouped in a block), like the one just described. In the case that we want the statement to be a simple statement, we do not need to enclose it in braces ({}). But in the case that we want the statement to be a compund statement it must be enclosed between braces ({}), forming a block. Conditional structure: if and else The if keyword is used to execute a statement or block only if a condition is fulfilled. Its form is: if (condition) statement
- 45. VISUAL C++ PROGRAMMING VC++ Where condition is the expression that is being evaluated. If this condition is true, statement is executed. If it is false, statement is ignored (not executed) and the program continues right after this conditional structure. For example, the following code fragment prints x is 100 only if the value stored in the x variable is indeed 100: [email protected] 45 if (x == 100) cout << "x is 100"; If we want more than a single statement to be executed in case that the condition is true we can specify a block using braces { }: if (x == 100) { cout << "x is "; cout << x; } We can additionally specify what we want to happen if the condition is not fulfilled by using the keyword else. Its form used in conjunction with if is: if (condition) statement1 else statement2 For example: if (x == 100) cout << "x is 100"; else cout << "x is not 100"; prints on the screen x is 100 if indeed x has a value of 100, but if it has not -and only if not- it prints out x is not 100. The if + else structures can be concatenated with the intention of verifying a range of values. The following example shows its use telling if the value currently stored in x is positive, negative or none of them (i.e. zero):
- 46. VISUAL C++ PROGRAMMING VC++ [email protected] 46 if (x > 0) cout << "x is positive"; else if (x < 0) cout << "x is negative"; else cout << "x is 0"; Remember that in case that we want more than a single statement to be executed, we must group them in a block by enclosing them in braces { }. Loops (for, while, do) Iteration structures (loops) Loops have as purpose to repeat a statement a certain number of times or while a condition is fulfilled. The while loop Its format is: while (expression) statement and its functionality is simply to repeat statement while the condition set in expression is true. For example, we are going to make a program to countdown using a while-loop: // custom countdown using while #include <iostream> using namespace std; int main () { int n; cout << "Enter the starting number > "; cin >> n; while (n>0) { cout << n << ", "; --n; } cout << "FIRE!n"; return 0; } Enter the starting number > 8 8, 7, 6, 5, 4, 3, 2, 1, FIRE!
- 47. VISUAL C++ PROGRAMMING VC++ When the program starts the user is prompted to insert a starting number for the countdown. Then the while loop begins, if the value entered by the user fulfills the condition n>0 (that n is greater than zero) the block that follows the condition will be executed and repeated while the condition (n>0) remains being true. The whole process of the previous program can be interpreted according to the following script (beginning in main): 1. User assigns a value to n 2. The while condition is checked (n>0). At this point there are two posibilities: * condition is true: statement is executed (to step 3) * condition is false: ignore statement and continue after it (to step 5) [email protected] 47 3. Execute statement: cout << n << ", "; --n; (prints the value of n on the screen and decreases n by 1) 4. End of block. Return automatically to step 2 5. Continue the program right after the block: print FIRE! and end program. When creating a while-loop, we must always consider that it has to end at some point, therefore we must provide within the block some method to force the condition to become false at some point, otherwise the loop will continue looping forever. In this case we have included --n; that decreases the value of the variable that is being evaluated in the condition (n) by one - this will eventually make the condition (n>0) to become false after a certain number of loop iterations: to be more specific, when n becomes 0, that is where our while-loop and our countdown end. Of course this is such a simple action for our computer that the whole countdown is performed instantly without any practical delay between numbers. The do-while loop Its format is: do statement while (condition); Its functionality is exactly the same as the while loop, except that condition in the do-while loop is evaluated after the execution of statement instead of before, granting at least one execution of statement even if condition is never fulfilled. For example, the following example program echoes any number you enter until you enter 0. // number echoer #include <iostream> using namespace std;
- 48. VISUAL C++ PROGRAMMING VC++ [email protected] 48 int main () { unsigned long n; do { cout << "Enter number (0 to end): "; cin >> n; cout << "You entered: " << n << "n"; } while (n != 0); return 0; } Enter number (0 to end): 12345 You entered: 12345 Enter number (0 to end): 160277 You entered: 160277 Enter number (0 to end): 0 You entered: 0 The do-while loop is usually used when the condition that has to determine the end of the loop is determined within the loop statement itself, like in the previous case, where the user input within the block is what is used to determine if the loop has to end. In fact if you never enter the value 0 in the previous example you can be prompted for more numbers forever. The for loop Its format is: for (initialization; condition; increase) statement; and its main function is to repeat statement while condition remains true, like the while loop. But in addition, the for loop provides specific locations to contain an initialization statement and an increase statement. So this loop is specially designed to perform a repetitive action with a counter which is initialized and increased on each iteration. It works in the following way: 1. initialization is executed. Generally it is an initial value setting for a counter variable. This is executed only once. 2. condition is checked. If it is true the loop continues, otherwise the loop ends and statement is skipped (not executed). 3. statement is executed. As usual, it can be either a single statement or a block enclosed in braces { }. 4. finally, whatever is specified in the increase field is executed and the loop gets back to step 2.
- 49. Here is an example of countdown using a for loop: // countdown using a for loop #include <iostream> using namespace std; int main () { for (int n=10; n>0; n--) { cout << n << ", "; } cout << "FIRE!n"; return 0; } The initialization and increase fields are optional. They can remain empty, but in all cases the semicolon signs between them must be written. For example we could write: wanted to specify no initialization and no increase; or an increase field but no initialization (maybe because the variable for (;n<10;n++) if we wanted to include Optionally, using the comma operator ( fields included in a for loop, like in expression separator, it serves to separate more than one expression where only one is generally expected. For example, suppose that we wanted to initialize more than one variable in our loop: for ( n=0, i=100 ; n!=i ; n++, i { // whatever here... } This loop will execute for 50 times if neither n starts with a value of 0, and i is increased by one and i decreased by one, the loop's condition will become false after t loop, when both n and i will be equal to Jump statements. VISUAL C++ PROGRAMMING [email protected] was already initialized before). (,) we can specify more than one expression in any of the initialization, for example. The comma operator ( ves i-- ) r n or i are modified within the loop: with 100, the condition is n!=i (that n is not equal to 50. VC++ 49 for (;n<10;) if we ) , (,) is an i). Because n the 50th
- 50. VISUAL C++ PROGRAMMING VC++ [email protected] 50 The break statement Using break we can leave a loop even if the condition for its end is not fulfilled. It can be used to end an infinite loop, or to force it to end before its natural end. For example, we are going to stop the count down before its natural end (maybe because of an engine check failure?): // break loop example #include <iostream> using namespace std; int main () { int n; for (n=10; n>0; n--) { cout << n << ", "; if (n==3) { cout << "countdown aborted!"; break; } } return 0; } The continue statement The continue statement causes the program to skip the rest of the loop in the current iteration as if the end of the statement block had been reached, causing it to jump to the start of the following iteration. For example, we are going to skip the number 5 in our countdown: // continue loop example #include <iostream> using namespace std; int main () { for (int n=10; n>0; n--) { if (n==5) continue; cout << n << ", "; } cout << "FIRE!n"; return 0; } The goto statement goto allows to make an absolute jump to another point in the program. You should use this feature with caution since its execution causes an unconditional jump ignoring any type of nesting limitations.
- 51. VISUAL C++ PROGRAMMING VC++ The destination point is identified by a label, which is then used as an argument for the goto statement. A label is made of a valid identifier followed by a colon (:). Generally speaking, this instruction has no concrete use in structured or object oriented programming aside from those that low-level programming fans may find for it. For example, here is our countdown loop using goto: [email protected] 51 // goto loop example #include <iostream> using namespace std; int main () { int n=10; loop: cout << n << ", "; n--; if (n>0) goto loop; cout << "FIRE!n"; return 0; } The exit function exit is a function defined in the cstdlib library. The purpose of exit is to terminate the current program with a specific exit code. Its prototype is: void exit (int exitcode); The exitcode is used by some operating systems and may be used by calling programs. By convention, an exit code of 0 means that the program finished normally and any other value means that some error or unexpected results happened. The selective structure: switch. The syntax of the switch statement is a bit peculiar. Its objective is to check several possible constant values for an expression. Something similar to what we did at the beginning of this section with the concatenation of several if and else if instructions. Its form is the following: switch (expression) { case constant1: group of statements 1; break; case constant2: group of statements 2; break;
- 52. VISUAL C++ PROGRAMMING VC++ [email protected] 52 . . . default: default group of statements } It works in the following way: switch evaluates expression and checks if it is equivalent to constant1, if it is, it executes group of statements 1 until it finds the break statement. When it finds this break statement the program jumps to the end of the switch selective structure. If expression was not equal to constant1 it will be checked against constant2. If it is equal to this, it will execute group of statements 2 until a break keyword is found, and then will jump to the end of the switch selective structure. Finally, if the value of expression did not match any of the previously specified constants (you can include as many case labels as values you want to check), the program will execute the statements included after the default: label, if it exists (since it is optional). Both of the following code fragments have the same behavior: switch example if-else equivalent switch (x) { case 1: cout << "x is 1"; break; case 2: cout << "x is 2"; break; default: cout << "value of x unknown"; } if (x == 1) { cout << "x is 1"; } else if (x == 2) { cout << "x is 2"; } else { cout << "value of x unknown"; } The switch statement is a bit peculiar within the C++ language because it uses labels instead of blocks. This forces us to put break statements after the group of statements that we want to be executed for a specific condition. Otherwise the remainder statements -including those corresponding to other labels- will also be executed until the end of the switch selective block or a break statement is reached.
- 53. VISUAL C++ PROGRAMMING VC++ For example, if we did not include a break statement after the first group for case one, the program will not automatically jump to the end of the switch selective block and it would continue executing the rest of statements until it reaches either a break instruction or the end of the switch selective block. This makes unnecessary to include braces { } surrounding the statements for each of the cases, and it can also be useful to execute the same block of instructions for different possible values for the expression being evaluated. For example: [email protected] 53 switch (x) { case 1: case 2: case 3: cout << "x is 1, 2 or 3"; break; default: cout << "x is not 1, 2 nor 3"; } Notice that switch can only be used to compare an expression against constants. Therefore we cannot put variables as labels (for example case n: where n is a variable) or ranges (case (1..3):) because they are not valid C++ constants. If you need to check ranges or values that are not constants, use a concatenation of if and else if statements. The for statement the for statement has the form: for(initial_value,test_condition,step){ // code to execute inside loop }; • initial_value sets up the initial value of the loop counter. • test_condition this is the condition that is tested to see if the loop is executed again. • step this describes how the counter is changed on each execution of the loop. Here is an example: // The following code adds together the numbers 1 through 10 // this variable keeps the running total int total=0; // this loop adds the numbers 1 through 10 to the variable total for (int i=1; i < 11; i++){ total = total + i; }
- 54. VISUAL C++ PROGRAMMING VC++ So in the preceding chunk of code we have: [email protected] 54 • initial_condition is int i=0; • test_condition is i < 11; • step is i++; So, upon initial execution of the loop, the integer variable i is set to 1. The statement total = total + i; is executed and the value of the variable total becomes 1. The step code is now executed and i is incremented by 1, so its new value is 2. The test_condition is then checked, and since i is less than 11, the loop code is executed and the variable total gets the value 3 (since total was 1, and i was 2. i is then incremented by 1 again. The loop continues to execute until the condition i<11 fails. At that point total will have the value 1+2+3+4+5+6+7+8+9+10 = 55. The while statement The while statement has the form: while(condition) { // code to execute }; • condition is a boolean statement that is checked each time after the final "}" of the while statement executes. If the condition is true then the while statement executes again. If the condition is false, the while statement does not execute again. As an example, let's say that we wanted to write all the even numbers between 11 and 23 to the screen. The following is a full C++ program that does that. // include this file for cout #include <iostream.h> int main(){ // this variable holds the present number int current_number = 12; // while loop that prints all even numbers between // 11 and 23 to the screen while (current_number < 23){ cerr << current_number << endl; current_number += 2; } cerr << "all done" << endl; }
- 55. VISUAL C++ PROGRAMMING VC++ The preceding example prints the value of current_number to the screen and then adds 2 to its value. As soon as the value of the variable current_number goes above 23, the while loop exits and the next line is executed. The output of the preceding program would be: [email protected] 55 12 14 16 18 20 22 All done The program to display Fibonacci series until i=14 // fibo.cpp : Defines the entry point for the console application. // #include "stdafx.h" # include "iostream.h" int main() { int curr,prev,next,i=0; prev=0; curr=1; cout<<prev<<" "; for(i=0;i<15;i++) { next=curr+prev; curr=prev; prev=next; cout<<next<<" "; } cout<<endl; return 0; }
- 56. VISUAL C++ PROGRAMMING VC++ C++ Scope Resolution Operator :: The :: (scope resolution) operator is used to qualify hidden names so that you can still use them. You can use the unary scope operator if a namespace scope or global scope name is hidden by an explicit declaration of the same name in a block or class. For example: [email protected] 56 int count = 0; int main(void) { int count = 0; ::count = 1; // set global count to 1 count = 2; // set local count to 2 return 0; } The declaration of count declared in the main() function hides the integer named count declared in global namespace scope. The statement ::count = 1 accesses the variable named count declared in global namespace scope. You can also use the class scope operator to qualify class names or class member names. If a class member name is hidden, you can use it by qualifying it with its class name and the class scope operator. In the following example, the declaration of the variable X hides the class type X, but you can still use the static class member count by qualifying it with the class type X and the scope resolution operator. #include <iostream> using namespace std; class X { public: static int count; };
- 57. int X::count = 10; // define sta int main () { static data member int X = 0; // hides class type X cout << X::count << endl; // use static member of class X } C++ Memory Management Operators Need for Memory Management operators The concept of arrays has a block of memory reserved. The disadvantage with the concept of arrays is that the programmer must know, while programming, the size of memory to be allocated in addition to the array size remaining constant. In programming there may be scenarios where programmers may not know the memory needed until run time. In this case, the programmer can opt to reserve as much memory as possible, assigning the maximum memory space needed to tackl result in wastage of unused memory spaces. Memory handle this situation in C++ programming language. What are memory management operators? There are two types of memory • new • delete These two memory management operators are used for allocating and freeing memory blocks in efficient and convenient ways. New operator: The new operator in C++ is used for dynamic storage allocation. This operator can be used to create object of any type. General syntax of new operator in C++: The general syntax of new operator in C++ is as follows: pointer variable = new datatype; In the above statement, new is a keyword and the pointer variable is a variable of type datatype. For example: VISUAL C++ PROGRAMMING [email protected] tackle this situation. This would management operators are used to management operators in C++: or ement, VC++ 57 tic e ators
- 58. VISUAL C++ PROGRAMMING VC++ [email protected] 58 int *a=new int; In the above example, the new operator allocates sufficient memory to hold the object of datatype int and returns a pointer to its starting point. The pointer variable a holds the address of memory space allocated. Dynamic variables are never initialized by the compiler. Therefore, the programmer should make it a practice to first assign them a value. The assignment can be made in either of the two ways: Sample Code 1. int *a = new int; 2. *a = 20; Copyright exforsys.com or Sample Code 1. int *a = new int(20); Copyright exforsys.com delete operator: The delete operator in C++ is used for releasing memory space when the object is no longer needed. Once a new operator is used, it is efficient to use the corresponding delete operator for release of memory. General syntax of delete operator in C++: The general syntax of delete operator in C++ is as follows: delete pointer variable; In the above example, delete is a keyword and the pointer variable is the pointer that points to the objects already created in the new operator. Some of the important points the programmer must note while using memory management operators are described below: • The programmer must take care not to free or delete a pointer variable that has already been deleted. • Overloading of new and delete operator is possible (to be discussed in detail in later section on overloading).
- 59. VISUAL C++ PROGRAMMING VC++ • We know that sizeof operator is used for computing the size of the object. Using memory management operator, the size of the object is automatically computed. • The programmer must take care not to free or delete pointer variables that have not been allocated using a new operator. • Null pointer is returned by the new operator when there is insufficient memory [email protected] 59 available for allocation. Example: To understand the concept of new and delete memory management operator in C++: Sample Code 1. #include <iostream> 2. using namespace std; 3. void main() 4. { 5. //Allocates using new operator memory space in memory for storing a integer datatype 6. int *a= new int; 7. *a=100; 8. cout << " The Output is:a= " << *a; 9. //Memory Released using delete operator 10. delete a; 11. 12. } Copyright exforsys.com The output of the above program is In the above program, the statement: int *a= new a; Holds memory space in memory for storing a integer datatype. The statement: *a=100 This denotes that the value present in address location pointed by the pointer variable a is 100 and this value of a is printed in the output statement giving the output shown in the example
- 60. VISUAL C++ PROGRAMMING VC++ above. The memory allocated by the new operator for storing the integer variable pointed by a is released using the delete operator as: [email protected] 60 Ads delete a; Manipulators In C++ Manipulators Manipulators are operators used with the insertion operator << to modify or manipulate the way data is displayed. It just changes certain characteristics of the input and output. The following are the manipulators used in c++:- • Endl It is a manipulator that causes a line feed to be inserted into the stream. It I similar to ‘n’ character.E.g. Cout<<”a=”<<a<<endl; // Program to display a message on two new lines using the endl manipulator #include<iostream.h> #include<conio.h> Void main () { Cout<<”hello”<<endl; Cout<<”many greetings”<<endl; getch (); } • Setw() This Setw () stands for the set width. The Setw () manipulator is used to set the width of the word to be displayed on screen. The general form of Setw () is:- Setw (int w) Where, the integer inside the bracket indicates the total field width. E.g. // demonstrate Setw () manipulator #include<iostream.h> #include<iomanip.h> // for Setw () #include<conio.h>
- 61. VISUAL C++ PROGRAMMING VC++ Void main () { Cout<<Setw (10) <<”name”<<Setw (10) <<”marks”<<endl; Cout<<Setw (10) <<”ram”<<Setw (10) <<”95”<<endl; Cout<<Setw (10) <<”hari”<<Setw (10) <<”85”<<endl; getch (); } Operator Overloading in C++ In C++ the overloading principle applies not only to functions, but to operators too. That is, of operators can be extended to work not just with built-in types but also classes. A programmer can provide his or her own operator to a class by overloading the built-in operator to perform some specific computation when the operator is used on objects of that class. Is operator overloading really useful in real world implementations? It certainlly can be, making it very easy to write code that feels natural (we'll see some examples soon). On the other hand, operator overloading, like any advanced C++ feature, makes the language more complicated. In addition, operators tend to have very specific meaning, and most programmers don't expect operators to do a lot of work, so overloading operators can be abused to make code unreadable. But we won't do that. An Example of Operator Overloading Complex a(1.2,1.3); //this class is used to represent complex numbers Complex b(2.1,3); //notice the construction taking 2 parameters for the real and imaginary part Complex c = a+b; //for this to work the addition operator must be overloaded The addition without having overloaded operator + could look like this: Complex c = a.Add(b); This piece of code is not as readable as the first example though--we're dealing with numbers, so doing addition should be natural. (In contrast to cases when programmers abuse this technique, when the concept represented by the class is not related to the operator--ike using + and - to add and remove elements from a data structure. In this cases operator overloading is a bad idea, creating confusion.) In order to allow operations like Complex c = a+b, in above code we overload the "+" operator. The overloading syntax is quite simple, similar to function overloading, the keyword operator must be followed by the operator we want to overload: class Complex { public: Complex(double re,double im) :real(re),imag(im) {}; Complex operator+(const Complex& other); Complex operator=(const Complex& other); [email protected] 61
- 62. VISUAL C++ PROGRAMMING VC++ [email protected] 62 private: double real; double imag; }; Complex Complex::operator+(const Complex& other) { double result_real = real + other.real; double result_imaginary = imag + other.imag; return Complex( result_real, result_imaginary ); } The assignment operator can be overloaded similarly. Notice that we did not have to call any accessor functions in order to get the real and imaginary parts from the parameter other since the overloaded operator is a member of the class and has full access to all private data. Alternatively, we could have defined the addition operator globally and called a member to do the actual work. In that case, we'd also have to make the method a friend of the class, or use an accessor method to get at the private data: friend Complex operator+(Complex); Complex operator+(const Complex &num1, const Complex &num2) { double result_real = num1.real + num2.real; double result_imaginary = num1.imag + num2.imag; return Complex( result_real, result_imaginary ); } Why would you do this? when the operator is a class member, the first object in the expression must be of that particular type. It's as if you were writing: Complex a( 1, 2 ); Complex a( 2, 2 ); Complex c = a.operator=( b ); when it's a global function, the implicit or user-defined conversion can allow the operator to act even if the first operand is not exactly of the same type: Complex c = 2+b; //if the integer 2 can be converted by the Complex class, this expression is valid By the way, the number of operands to a function is fixed; that is, a binary operator takes two operands, a unary only one, and you can't change it. The same is true for the precedence of operators too; for example the multiplication operator is called before addition. There are some operators that need the first operand to be assignable, such as : operator=, operator(), operator[] and operator->, so their use is restricted just as member functions(non-static), they can't be overloaded globally. The operator=, operator& and operator, (sequencing) have already defined meanings by default for all objects, but their meanings can be changed by overloading or erased by making them private. Another intuitive meaning of the "+" operator from the STL string class which is overloaded to do concatenation: string prefix("de"); string word("composed"); string composed = prefix+word; Using "+" to concatenate is also allowed in Java, but note that this is not extensible to other classes, and it's not a user defined behavior. Almost all operators can be overloaded in C++: + - * / % ^ & | ~ ! , = = ++ -- << >> == != && || += -= /= %= ^= & = |= *= <<= >>= [ ] ( ) -> ->* new delete
- 63. VISUAL C++ PROGRAMMING VC++ The only operators that can't be overloaded are the operators for scope resolution (::), member selection (.), and member selection through a pointer to a function(.*). Overloading assumes you specify a behavior for an operator that acts on a user defined type and it can't be used just with general pointers. The standard behavior of operators for built-in (primitive) types cannot be changed by overloading, that is, you can't overload operator+(int,int). The logic(boolean) operators have by the default a short-circuiting way of acting in expressions with multiple boolean operations. This means that the expression: if(a && b && c) will not evaluate all three operations and will stop after a false one is found. This behavior does not apply to operators that are overloaded by the programmer. Even the simplest C++ application, like a "hello world" program, is using overloaded operators. This is due to the use of this technique almost everywhere in the standard library (STL). Actually the most basic operations in C++ are done with overloaded operators, the IO(input/output) operators are overloaded versions of shift operators(<<, >>). Their use comes naturally to many beginning programmers, but their implementation is not straightforward. However a general format for overloading the input/output operators must be known by any C++ developer. We will apply this general form to manage the input/output for our Complex class: friend ostream &operator<<(ostream &out, Complex c) //output { out<<"real part: "<<real<<"n"; out<<"imag part: "<<imag<<"n"; return out; } friend istream &operator>>(istream &in, Complex &c) //input { cout<<"enter real part:n"; in>>c.real; cout<<"enter imag part: n"; in>>c.imag; return in; } Notice the use of the friend keyword in order to access the private members in the above implementations. The main distinction between them is that the operator>> may encounter unexpected errors for incorrect input, which will make it fail sometimes because we haven't handled the errors correctly. A important trick that can be seen in this general way of overloading IO is the returning reference for istream/ostream which is needed in order to use them in a recursive manner: Complex a(2,3); Complex b(5.3,6); cout<<a<<b; C++ Operator Precedence It is important to understand that operators have a precedence, but it is not essential to memorize the precedence. [email protected] 63
- 64. VISUAL C++ PROGRAMMING VC++ Definition: Precedence is the order in which a program performs the operations in a formula. If one operator has precedence over another operator, it is evaluated first. Higher precedence operators "bind tighter" than lower precedence operators; thus, higher precedence operators are evaluated first. The lower the rank in the following chart, the higher the precedence. The following table lists the precedence and associativity of C++ operators. Operators are listed top to bottom, in descending precedence. Precedence Operator Description Associativity 1 :: Scope resolution Left-to-right [email protected] 64 2 ++ -- Suffix/postfix increment and decrement () Function call [] Array subscripting . Element selection by reference −> Element selection through pointer 3 ++ -- Prefix increment and decrement Right-to-left + − Unary plus and minus ! ~ Logical NOT and bitwise NOT (type) Type cast * Indirection (dereference) & Address-of sizeof Size-of new, new[] Dynamic memory allocation delete, Dynamic memory deallocation delete[] 4 .* ->* Pointer to member Left-to-right 5 * / % Multiplication, division, and remainder 6 + − Addition and subtraction 7 << >> Bitwise left shift and right shift 8 < <= For relational operators < and respectively = For relational operators and respectively 9 == != For relational = and respectively 10 Bitwise AND 11 ^ Bitwise XOR (exclusive or) 12 | Bitwise OR (inclusive or) 13 Logical AND 14 || Logical OR 15 ?: Ternary conditional Right-to-left
- 65. VISUAL C++ PROGRAMMING VC++ [email protected] 65 = Direct assignment (provided by default for C++ classes) += −= Assignment by sum and difference *= /= %= Assignment by product, quotient, and remainder = = Assignment by bitwise left shift and right shift = ^= |= Assignment by bitwise AND, XOR, and OR 16 throw Throw operator (for exceptions) 17 , Comma Left-to-right When parsing an expression, an operator which is listed on some row will be bound tighter (as if by parentheses) to its arguments than any operator that is listed on a row further below it. For example, the expressions std::coutab and *p++ are parsed as (std::couta)b and *(p++), and not as std::cout(ab) or (*p)++. Operators that are in the same cell (there may be several rows of operators listed in a cell) are evaluated with the same precedence, in the given direction. For example, the expression a=b=c is parsed as a=(b=c), and not as (a=b)=c because of right-to-left associativity. An operator's precedence is unaffected by overloading. Type Cast Operator: (type) cast-expression : unary-expression ( type-name ) cast-expression A type cast provides a method for explicit conversion of the type of an object in a specific situation. The compiler treats cast-expression as type type-name after a type cast has been made. Casts can be used to convert objects of any scalar type to or from any other scalar type. Explicit type casts are constrained by the same rules that determine the effects of implicit conversions. Additional restraints on casts may result from the actual sizes or representation of specific types. Example In the following example, the type cast operator converts the float value of 3.1 to an integer value of 3. // Example of the type cast operator float x = 3.1; int i; i = (int)x; // the value of i is now 3
- 66. VISUAL C++ PROGRAMMING VC++ Unit 4: Functions, classes and objects(25 HOURS) [email protected] 66 THEORY At the end of the unit the student should be able to: 1. ..know the main function 2. ..know and employ function prototyping 3. ..call a function by reference 4. ..return a function by reference 5. ..know what inline functions are 6. ..know what default arguments and constant arguments are 7. ..give examples of function overloading 8. ..explain what friend and virtual functions are 9. ..describe what structures in C are 10. ..specify a class 11. ..define a member function 12. ..make an outside function inline 13. ..nest member functions 14. ..employ private member functions 15. ..use arrays within a class 16. ..allocate memory for objects 17. ..employ static member functions 18. ..make arrays of objects 19. ..use objects as function arguments 20. ..use friendly functions 21. ..make functions return objects 22. ..create const member functions 23. ..know what pointers to members are Functions Introduction to User-defined functions in C++ A function is a subprogram that acts on data and often returns a value. You are already familiar with the one function that every C++ program possesses: int main(void). Good C++ programmers write programs that consist of many of these small functions. These programmers know that a program written with numerous functions is easier to maintain, update and debug than one very long program. By programming in a modular (functional) fashion, several programmers can work independently on separate functions which can be assembled at a later date to create the entire project. Ideally, your main( ) function should be very short and should consist primarily of function calls. Each function has its own name. When that name is encountered in a program, the execution of the program branches to the body of that function. When the function is finished, execution
- 67. VISUAL C++ PROGRAMMING VC++ returns to the area of the program code from which it was called, and the program continues on to the next line of code. Local Variable ..a variable that is restricted to use within a function of a program. void functionName(void) Our first style of function will simply perform an independent task. It will not send or receive any parameters and it will not return any values. The word void appears as the return type and the parameters. //Example program //Screen display shown at the right //Prototyping, defining and calling a function #includeiostream.h #includestdlib.h void astericks(void); //function prototype int main(void) { system(CLS); coutHeads up, function!n; astericks( ); //function call coutAgain, function!n; astericks( ); //function call coutJob well done!n; return 0; //main( ) is over - ALL STOP!! [email protected] 67 } //function definition void astericks(void) { int count; // declaring LOCAL variable for(count = 1; count=10; count++) cout*; coutendl; return; //return value is VOID, no return } SCREEN DISPLAY
- 68. VISUAL C++ PROGRAMMING VC++ Heads up, function! ********** Again, function! ********** Job well done! *Note: One of the important features of functions is their ability to be reused. Notice how the function astericks() was called twice from main(). The programmer did not have to duplicate the code. C++ allows programmers to define their own functions. For example the following is a definition of a function which given the co-ordinates of a point (x,y) will return its distance from the origin. [email protected] 68 float distance(float x, float y) // Returns the distance of (x, y) from origin { float dist; //local variable dist = sqrt(x * x + y * y); return dist; } This function has two input parameters, real values x and y, and returns the distance of the point (x,y) from the origin. In the function a local variable dist is used to temporarily hold the calculated value inside the function. The general form of a function definition in C++ is as follows: function-type function-name( parameter-list ) { local-definitions; function-implementation; } • If the function returns a value then the type of that value must be specified in function-type. For the moment this could be int, float or char. If the function does not return a value then the function-type must be void. • The function-name follows the same rules of composition as identifiers. • The parameter-list lists the formal parameters of the function together with their types. • The local-definitions are definitions of variables that are used in the function-implementation. These variables have no meaning outside the function. • The function-implementation consists of C++ executable statements that implement the effect of the function.
- 69. VISUAL C++ PROGRAMMING VC++ [email protected] 69 Functions with no parameters Functions with no parameters are of limited use. Usually they will not return a value but carry out some operation. For example consider the following function which skips three lines on output. void skipthree(void) // skips three lines on output { cout endl endl endl; } Note that the function-type has been given as void, this tells the compiler that this function does not return any value. Because the function does not take any parameters the parameter-list is empty, this is indicated by the void parameter-list. No local variables are required by this function and the function implementation only requires the sending of three successive end of line characters to the output stream cout. Note the introductory comment that describes what the function does. All functions should include this information as minimal comment. Since this function does not return a value it cannot be used in an expression and is called by treating it as a statement as follows: skipthree(); Even though there are no parameters the empty parameter list () must be inserted. When a function is called the C++ compiler must insert appropriate instructions into the object code to arrange to pass the actual parameter values to the function code and to obtain any values returned by the function. To do this correctly the compiler must know the types of all parameters and the type of any return value. Thus before processing the call of a function it must already know how the function is defined. This can be done by defining any functions that are used in the main program before the main program, for example the function skipthree could be incorporated in a program as follows: #include iostream.h void skipthree(void) // Function to skip three lines { cout endl endl endl; } void main() { int ....; float ....; cout Title Line 1;
- 70. VISUAL C++ PROGRAMMING VC++ [email protected] 70 skipthree(); cout Title Line 2; . . } However this has disadvantages, namely: • The main program tends to convey much more information of use in understanding the program than do individual functions. So it is better if the main program comes first. However this means that the compiler meets the call of a function before it meets the definition of the function. • If using functions from a library of functions then the main program is linked with the pre-compiled object code of the functions. Thus while compiling the main program on its own the compiler has no knowledge of the function definitions. The way round both the problems above is to use Function prototypes. A function prototype supplies information about the return type of a function and the types of its parameters. This function prototype is then placed before the main program that uses the function. The full function definition is then placed after the main program or may be contained in a separate file that is compiled separately and linked to the main program later. The function prototype is merely a copy of the function heading. Thus the function prototype for the function skipthree is: void skipthree(void); which would be included in the program file as follows: #include iostream.h void skipthree(void); // function prototype void main() { int ....; float ....; cout Title Line 1; skipthree(); cout Title Line 2; . . } // Now the function definition void skipthree(void) // Function to skip three lines { cout endl endl endl; }
- 71. VISUAL C++ PROGRAMMING VC++ In fact when using functions from the stream libraries and the mathematical libraries prototypes are required for these functions. This is handled by including the files iostream.h and math.h which, among other things, contain the function prototypes. Using functions we can structure our programs in a more modular way, accessing all the potential that structured programming can offer to us in C++. A function is a group of statements that is executed when it is called from some point of the program. The following is its format: type name ( parameter1, parameter2, ...) { statement } [email protected] 71 where: • type is the data type specifier of the data returned by the function. • name is the identifier by which it will be possible to call the function. • parameters (as many as needed): Each parameter consists of a data type specifier followed by an identifier, like any regular variable declaration (for example: int x) and which acts within the function as a regular local variable. They allow to pass arguments to the function when it is called. The different parameters are separated by commas. • statements is the function's body. It is a block of statements surrounded by braces { }. Here you have the first function example: // function example #include iostream using namespace std; int addition (int a, int b) { int r; r=a+b; return (r); } int main () { int z; z = addition (5,3); cout The result is z; return 0; }The result is 8
- 72. VISUAL C++ PROGRAMMING In order to examine this code, first of all remember something said at the beginning of this tutorial: a C++ program always begins its execution by the We can see how the main function begins by declaring the variable we see a call to a function called between the structure of the call to the function and the declaration of the function itself some code lines above: The parameters and arguments have a clear correspondence. Within the to addition passing two values: declared for function addition. At the point at which the function is called passed to function addition. The value of both arguments passed in the call ( to the local variables int a and int b Function addition declares another local variabl it assigns to r the result of a plus respectively, the result is 8. The following line of code: return (r); finalizes function addition, and returns the control back to the function that called it in the first place (in this case, main). At this moment the program follows it regular course from the same point at which it was interrupted by the call to statement in function addition that moment had a value of 8. This value becomes the value of evaluating the function call. [email protected] r main function. So we will begin there. z of type int addition. Paying attention we will be able to see the similarity ameters main 5 and 3, that correspond to the int a and int b from within main, the control is lost by . 5 within the function. variable (int r), and by means of the expression b. Because the actual parameters passed for a d ). addition. But additionally, because the specified a value: the content of variable r (return (r); . VC++ 72 int. Right after that, . function we called parameters , main and and 3) are copied ), r=a+b, and b are 5 and 3 . return r);), which at
- 73. So being the value returned by a function the value giv evaluated, the variable z will be set to the value returned by another way, you can imagine that the call to a function ( value it returns (8). The following line of code in main is: cout The result is z; given to the function call itself when it is That, as you may already expect, produces the printing of the result on the screen. Scope of variables The scope of variables declared within a function or any other their own block and cannot be used outside of them. For example, in the previous example it would have been impossible to use the variables function addition. Also, it would have been impossible to use the variable addition, since this was a variable local to the function Therefore, the scope of local variables is limited to the same block level in which they a Nevertheless, we also have the possibility to declare global variables; These are visible from any point of the code, inside and outside all functions. In order to declare global variables you simply have to declare the variable outside any function or block; that means, directly in the body of the program. And here is another example about functions: // function example #include iostream using namespace std; VISUAL C++ PROGRAMMING [email protected] addition (5, 3), that is addition (5,3)) is literally replaced by the inner block is only their own function or a, b or r directly in function main since they were variables local to . z directly within function , main. nction VC++ 73 en , 8. To explain it ) are declared.
- 74. VISUAL C++ PROGRAMMING VC++ [email protected] 74 int subtraction (int a, int b) { int r; r=a-b; return (r); } int main () { int x=5, y=3, z; z = subtraction (7,2); cout The first result is z 'n'; cout The second result is subtraction (7,2) 'n'; cout The third result is subtraction (x,y) 'n'; z= 4 + subtraction (x,y); cout The fourth result is z 'n'; return 0; } The first result is 5 The second result is 5 The third result is 2 The fourth result is 6 In this case we have created a function called subtraction. The only thing that this function does is to subtract both passed parameters and to return the result. Nevertheless, if we examine function main we will see that we have made several calls to function subtraction. We have used some different calling methods so that you see other ways or moments when a function can be called. In order to fully understand these examples you must consider once again that a call to a function could be replaced by the value that the function call itself is going to return. For example, the first case (that you should already know because it is the same pattern that we have used in previous examples): z = subtraction (7,2); cout The first result is z; If we replace the function call by the value it returns (i.e., 5), we would have: z = 5; cout The first result is z; As well as
- 75. VISUAL C++ PROGRAMMING VC++ cout The second result is subtraction (7,2); has the same result as the previous call, but in this case we made the call to subtraction directly as an insertion parameter for cout. Simply consider that the result is the same as if we had written: cout The second result is 5; since 5 is the value returned by subtraction (7,2). [email protected] 75 In the case of: cout The third result is subtraction (x,y); The only new thing that we introduced is that the parameters of subtraction are variables instead of constants. That is perfectly valid. In this case the values passed to function subtraction are the values of x and y, that are 5 and 3 respectively, giving 2 as result. The fourth case is more of the same. Simply note that instead of: z = 4 + subtraction (x,y); we could have written: z = subtraction (x,y) + 4; with exactly the same result. I have switched places so you can see that the semicolon sign (;) goes at the end of the whole statement. It does not necessarily have to go right after the function call. The explanation might be once again that you imagine that a function can be replaced by its returned value: z = 4 + 2; z = 2 + 4; Functions with no type. The use of void. If you remember the syntax of a function declaration: type name ( argument1, argument2 ...) statement you will see that the declaration begins with a type, that is the type of the function itself (i.e., the type of the datum that will be returned by the function with the return statement). But what if we want to return no value?
- 76. VISUAL C++ PROGRAMMING VC++ Imagine that we want to make a function just to show a message on the screen. We do not need it to return any value. In this case we should use the void type specifier for the function. This is a special specifier that indicates absence of type. [email protected] 76 // void function example #include iostream using namespace std; void printmessage () { cout I'm a function!; } int main () { printmessage (); return 0; } void can also be used in the function's parameter list to explicitly specify that we want the function to take no actual parameters when it is called. For example, function printmessage could have been declared as: void printmessage (void) { cout I'm a function!; } Although it is optional to specify void in the parameter list. In C++, a parameter list can simply be left blank if we want a function with no parameters. What you must always remember is that the format for calling a function includes specifying its name and enclosing its parameters between parentheses. The non-existence of parameters does not exempt us from the obligation to write the parentheses. For that reason the call to printmessage is: printmessage (); The parentheses clearly indicate that this is a call to a function and not the name of a variable or some other C++ statement. The following call would have been incorrect: printmessage;
- 77. VISUAL C++ PROGRAMMING VC++ Functions with parameters and no return value The function of the previous section is not very useful, what if four lines were to be skipped, or two lines? It would be much more useful if it was possible to tell the function how many lines to skip. That is the function should have an input parameter which indicates how many lines should be skipped. The function skipthree() is now changed to the function skip which has a parameter n indicating how many lines have to be skipped as follows: [email protected] 77 void skip(int n) // Function skips n lines on output { int i; // a local variable to this function // now loop n times for (i = 0; i n; i++) cout endl; } As before this function does not return a value hence it is declared as having type void. It now takes an integer parameter n which indicates the number of lines to be skipped. The parameter list then consists of a type and a name for this formal parameter. Inside the body of the function (enclosed in {}) a loop control variable i is declared. This variable is a local variable to the function. A local variable defined within the body of the function has no meaning, or value, except within the body of the function. It can use an identifier name that is used elsewhere in the program without there being any confusion with that variable. Thus changing the value of the local variable i in the function skip will not affect the value of any other variable i used elsewhere in the program. Similarly changing the value of a variable i used elsewhere in the program will not affect the value of the local variable i in skip. The function is called in the same manner as skipthree() above, but a value must be given for the parameter n. Thus all the following calls are acceptable: void main() { int m = 6, n = 3; ...............; skip(m); .......; skip(m + n); ............; skip(4); .......; }
- 78. VISUAL C++ PROGRAMMING VC++ however the call: skip (4.0); would not be acceptable because the actual parameter type must match the formal parameter type given in the definition of the function. In writing the function prototype for a function with parameters it is not necessary to detail the formal names given to the parameters of the function, only their types. Thus a suitable function prototype for the parameterised version of skip would be: void skip(int); // function prototype [email protected] 78 Functions that return values One of the most useful forms of function is one that returns a value that is a function of its parameters. In this case the type given to the function is that of the value to be returned. Thus consider the function, previously considered, which given the co-ordinates of a point (x,y) will return its distance from the origin: float distance(float x, float y) // Returns the distance of (x, y) from origin { float dist; //local variable dist = sqrt(x * x + y * y); return dist; } The function prototype for this function is: float distance(float, float); // function prototype This function introduces several new features. Note the following: • The function has been given the type float because it is going to return a float value. • The parameter-list now has two parameters, namely, x and y. Each parameter is declared by giving its type and name and successive parameter declarations are separated by a comma. • A local variable dist has been declared to temporarily hold the calculated distance. • Because this function returns a value it includes a return statement which returns the value. In a statement return value the value may be a constant, a variable or an expression. Hence the use of the local variable dist was not essential since the return statement could have been written: • return sqrt(x*x + y*y);
- 79. VISUAL C++ PROGRAMMING VC++ When the function is called the formal parameters x and y are replaced by actual parameters of type float and in the same order, i.e. the x co-ordinate first. Since the function returns a value it can only be used in an expression. Hence the following examples of the use of the above function in a program in which it is declared: [email protected] 79 float a, b, c, d, x, y; a = 3.0; b = 4.4; c = 5.1; d = 2.6; x = distance(a, b); y = distance(c, d); if (distance(4.1, 6.7) distance(x, y)) cout Message 1 endl; A function may have several return statements. This is illustrated in the following function which implements the algorithm for evaluating the square root previously considered. float mysqrt(float x) // Function returns square root of x. // If x is negative it returns zero. { const float tol = 1.0e-7; // 7 significant figures float xold, xnew; // local variables if (x = 0.0) return 0.0; // covers -ve and zero case else { xold = x; // x as first approx xnew = 0.5 * (xold + x / xold); // better approx while (fabs((xold-xnew)/xnew) tol) { xold = xnew; xnew = 0.5 * (xold + x / xold); } return xnew; // must return float value } } // end mysqrt If the function has type void then it must not return a value. If a void function does return a value then most compilers will issue some form of warning message that a return value is not expected. Arguments passed by value and by reference.
- 80. Until now, in all the functions we have seen, the arguments passed to the functions have been passed by value. This means that when calling a function with parameters, what we have passed to the function were copies of their value suppose that we called our first function int x=5, y=3, z; z = addition ( x , y ); What we did in this case was to call to function addition passing the values of 3 respectively, but not the variables This way, when the function addition is called, the value of its local variables and 3 respectively, but any modification to either have any effect in the values of passed to the function, but only copies of their values at the moment the function was called. But there might be some cases where you need an external variable. For that purpose we can use arguments passed by reference, as in the function duplicate of the following example: // passing parameters by reference #include iostream using namespace std; void duplicate (int a, int b, int c) { a*=2; b*=2; c*=2; } int main () { int x=1, y=3, z=7; duplicate (x, y, z); cout x= x , y= return 0; } VISUAL C++ PROGRAMMING [email protected] . values but never the variables themselves. For example, addition using the following code: x and y themselves. a a or b within the function ad x and y outside it, because variables x and y were not themselves to manipulate from inside a function the value of y , z= z; x=2, y=6, z=14 VC++ 80 s x and y, i.e. 5 and and b become 5 addition will not
- 81. The first thing that should call your a each parameter was followed by an ampersand sign ( their corresponding arguments are to be passed attention is that in the declaration of duplicate VC++ (). This ampersand is what specifies that by reference instead of by value. When a variable is passed by reference we are not passing a copy of its value, but we are somehow passing the variable itself to the function and any modification that we do to the local variables will have an effect in their counterpart variables passed as arguments in the ca function. To explain it in another way, we associate call (x, y and z) and any change that we do on outside it. Any change that we do on That is why our program's output, that shows the values stored in duplicate, shows the values of all the three variables of If when declaring the following function: void duplicate (int a, int b, we had declared it this way: void duplicate (int a, int b, int i.e., without the ampersand signs ( copy of their values instead, and therefore, the ou been the values of x, y and z without having been modified. Passing by reference is also an effective way to allow a function to return more than one value. For example, here is a function that returns the previou passed. // more than one returning value #include iostream VISUAL C++ PROGRAMMING [email protected] ssed a, b and c with the arguments passed on the function ) a within the function will affect the value of b will affect y, and the same with c and z x, y and z after the call to , main doubled. int c) c) (), we would have not passed the variables by reference, but a output on screen of our program would have previous and next numbers of the first parameter 81 the type of ). call to the x z. ), tput s
- 82. VISUAL C++ PROGRAMMING VC++ [email protected] 82 using namespace std; void prevnext (int x, int prev, int next) { prev = x-1; next = x+1; } int main () { int x=100, y, z; prevnext (x, y, z); cout Previous= y , Next= z; return 0; } Previous=99, Next=101 Default values in parameters. When declaring a function we can specify a default value for each parameter. This value will be used if the corresponding argument is left blank when calling to the function. To do that, we simply have to use the assignment operator and a value for the arguments in the function declaration. If a value for that parameter is not passed when the function is called, the default value is used, but if a value is specified this default value is ignored and the passed value is used instead. For example: // default values in functions #include iostream using namespace std; int divide (int a, int b=2) { int r; r=a/b; return (r); } int main () { cout divide (12); cout endl; cout divide (20,4); return 0; } As we can see in the body of the program there are two calls to function divide. In the first one: divide (12) we have only specified one argument, but the function divide allows up to two. So the function divide has assumed that the second parameter is 2 since that is what we have specified to happen
- 83. VISUAL C++ PROGRAMMING VC++ if this parameter was not passed (notice the function declaration, which finishes with int b=2, not just int b). Therefore the result of this function call is 6 (12/2). [email protected] 83 In the second call: divide (20,4) there are two parameters, so the default value for b (int b=2) is ignored and b takes the value passed as argument, that is 4, making the result returned equal to 5 (20/4). Example function: sum of squares of integers The following function returns the sum of the squares of the first n integers when it is called with parameter n. // This function returns the sum of squares of the // first n integers int sumsq(int n) { int sum = 0; int i; for (i = 1; i = n; i++) sum += i * i; return sum; } // End of sumsq A typical use of sumsq is: float sumsquare; int number; cout Enter number (= 0): ; cin number; sumsquare = sumsq(number); Example Function: Raising to the power This function returns the value of its first parameter raised to the power of its second parameter. The second parameter is an integer, but may be 0 or negative. float power(float x, int n) { float product = 1.0; int absn; int i; if ( n == 0) return 1.0; else { absn = int(fabs(n)); for (i = 1; i = absn; i++) product *= x; if (n 0) return 1.0 / product; else return product; } } // end of power A typical use of the power function is shown below
- 84. VISUAL C++ PROGRAMMING VC++ [email protected] 84 float x, y; int p; cout Enter a float and an integer: ; cin x p; y = power(x, p); y = power(x + y, 3); Call-by-value parameters Suppose the function power above is now amended to include the statement n++; just before the final closing } and the following statements are executed: p = 4; y = power(x, p); cout p; What would be printed out for the value of p? In fact instead of the value 5 that you might expect p would still have the value 4. This is because the parameter has been passed by value. This means that when the function is called a copy of the value of the actual parameter used in the call is passed across to the memory space of the function. Anything that happens inside the function to this copy of the value of the parameter cannot affect the original actual parameter. All the examples that have been considered have used call-by-value parameters. This is because all the parameters used have been input parameters. To make a parameter call-by-value it is specified in the parameter list by giving its type followed by its name. Thus if a parameter is only to be used for passing information into a function and does not have to be returned or passed back from the function then the formal parameter representing that parameter should be call-by-value. Note also that since the function cannot change the value of a call-by-value parameter in the calling program strange side effects of calling a function are avoided. Further User-defined functions in C++ In Lesson 21 all function parameters were input-only parameters and thus were implemented as call-by value parameters. The only method used to return information to the calling program was by the function returning a single value. Frequently it is necessary to write functions that return more than one value. For example a function that took a sum of money in pence might have to return the equivalent sum in pounds and pence.
- 85. VISUAL C++ PROGRAMMING VC++ To allow information to be returned to the calling program C++ allows information to be returned by parameters. As explained in Lesson 21 this cannot be done by the use of call-by-value parameters. To allow a parameter to return a value it must be declared to be a call-by-reference [email protected] 85 parameter. Call-by-reference parameters Values cannot be returned to the calling program via call-by-value parameters because the function only operates on a copy of the value of the parameters, not on the actual parameter itself. If it is required to return a value by a parameter then the address of the actual parameter used in the function call must be passed to the function. The function can then use this address to access the actual parameter in its own space in the calling program and change it if required. Thus what we are passing is a reference to the parameter. Hence call-by-reference parameters. To indicate that a parameter is called by reference an is placed after the type in the parameter list. Any change that is made to that parameter in the function body will then be reflected in its value in the calling program. For example consider the following function to evaluate the solution of a quadratic equation: // solves the quadratic equation a*x*x+b*x+c = 0. // If the roots are real then the roots are // returned in two parameters root1 and root2 and // the function returns true, if they are complex // then the function returns false. bool quadsolve(float a, // IN coefficient float b, // IN coefficient float c, // IN coefficient float root1, // OUT root float root2) // OUT root { float disc; // local variable disc = b * b - 4 * a * c; if (disc 0.0) return false; else { root1 = (-b + sqrt(disc))/(2 * a); root2 = (-b - sqrt(disc))/(2 * a); return true; } } Note that the roots, which are output parameters, have been declared to be reference parameters, while the coefficients are input parameters and hence are declared to be value parameters. The function prototype would have the following form:
- 86. VISUAL C++ PROGRAMMING VC++ int quadsolve(float, float, float, float, float); This might be called in a program as follows: [email protected] 86 float c1, c2, c3; float r1, r2; . . if (quadsolve(c1, c2, c3, r1, r2)) cout Roots are r1 and r2 endl; else cout Complex Roots endl; Note how the return value has been used to discriminate between the situation where the roots are real and are found in the two output parameters and the case where the roots are complex and no values for the roots are returned. Recursivity. Recursivity is the property that functions have to be called by themselves. It is useful for many tasks, like sorting or calculate the factorial of numbers. For example, to obtain the factorial of a number (n!) the mathematical formula would be: n! = n * (n-1) * (n-2) * (n-3) ... * 1 more concretely, 5! (factorial of 5) would be: 5! = 5 * 4 * 3 * 2 * 1 = 120 and a recursive function to calculate this in C++ could be: // factorial calculator #include iostream using namespace std; long factorial (long a) { if (a 1) return (a * factorial (a-1)); else return (1); } int main () { long number;
- 87. VISUAL C++ PROGRAMMING VC++ cout Please type a number: ; cin number; cout number ! = factorial (number); return 0; } [email protected] 87 Please type a number: 9 9! = 362880 Notice how in function factorial we included a call to itself, but only if the argument passed was greater than 1, since otherwise the function would perform an infinite recursive loop in which once it arrived to 0 it would continue multiplying by all the negative numbers (probably provoking a stack overflow error on runtime). This function has a limitation because of the data type we used in its design (long) for more simplicity. The results given will not be valid for values much greater than 10! or 15!, depending on the system you compile it. Declaring functions. Until now, we have defined all of the functions before the first appearance of calls to them in the source code. These calls were generally in function main which we have always left at the end of the source code. But if you try to repeat some of the examples of functions described so far, but placing the function main before any of the other functions that were called from within it, you will most likely obtain compiling errors. The reason is that to be able to call a function this must have been declared in some earlier point of the code, like we have done in all our examples. But there is an alternative way to avoid writing the whole code of a function before it can be used in main or in some other function. This can be achieved by declaring just a prototype of the function before it is used, instead of the entire definition. This declaration is shorter than the entire definition, but significant enough for the compiler to determine its return type and the types of its parameters. Its form is: type name ( argument_type1, argument_type2, ...); It is identical to a function definition, except that it does not include the body of the function itself (i.e., the function statements that in normal definitions are enclosed in braces { }) and instead of that we end the prototype declaration with a mandatory semicolon (;). The parameter enumeration does not need to include the identifiers, but only the type specifiers. The inclusion of a name for each parameter as in the function definition is optional in the prototype declaration. For example, we can declare a function called protofunction with two int parameters with any of the following declarations:
- 88. VISUAL C++ PROGRAMMING VC++ int protofunction (int first, int second); [email protected] 88 int protofunction (int, int); Anyway, including a name for each variable makes the prototype more legible. // declaring functions prototypes #include iostream using namespace std; void odd (int a); void even (int a); int main () { int i; do { cout Type a number: (0 to exit) ; cin i; odd (i); } while (i!=0); return 0; } void odd (int a) { if ((a%2)!=0) cout Number is odd.n; else even (a); } void even (int a) { if ((a%2)==0) cout Number is even.n; else odd (a); } Type a number (0 to exit): 9 Number is odd. Type a number (0 to exit): 6 Number is even. Type a number (0 to exit): 1030 Number is even. Type a number (0 to exit): 0 Number is even. This example is indeed not an example of efficiency. I am sure that at this point you can already make a program with the same result, but using only half of the code lines that have been used in this example. Anyway this example illustrates how prototyping works. Moreover, in this concrete example the prototyping of at least one of the two functions is necessary in order to compile the code without errors.
- 89. VISUAL C++ PROGRAMMING VC++ The first things that we see are the declaration of functions odd and even: [email protected] 89 void odd (int a); void even (int a); This allows these functions to be used before they are defined, for example, in main, which now is located where some people find it to be a more logical place for the start of a program: the beginning of the source code. Anyway, the reason why this program needs at least one of the functions to be declared before it is defined is because in odd there is a call to even and in even there is a call to odd. If none of the two functions had been previously declared, a compilarion error would happen, since either odd would not not be visible from even (because it has still not been declared), or even would not be visible from odd (for the same reason). Having the prototype of all functions together in the same place within the source code is found practical by some programmers, and this can be easily achieved by declaring all functions prototypes at the beginning of a program. Review Questions 1. How is information supplied as input to a function? How can information be conveyed back to the calling program? 2. What would the following function do? 3. void example(int n) 4. { 5. int i; 6. for (i=0; in; i++) 7. cout '*'; 8. cout endl; 9. } How would you call this function in a program? How would you use this function in producing the following output on the screen? * ** *** **** 10. What would be the output from the following programs? a) void change(void) { int x; x = 1; } void main()
- 90. VISUAL C++ PROGRAMMING VC++ [email protected] 90 { int x; x = 0; change(); cout x endl; } b) void change(int x) { x = 1; } void main() { int x; x = 0; change(x); cout x endl; } 11. Write a function prototype for a function that takes two parameters of type float and returns true (1) if the first parameter is greater than the second and otherwise returns false (0). 12. Write a function prototype for a function that takes two parameters of type int and returns true if these two integers are a valid value for a sum of money in pounds and pence. If not valid then false should be returned. 13. A function named ex1 has a local variable named i and another function ex2 has a local variable named i. These two functions are used together with a main program which has a variable named i. Assuming that there are no other errors in the program will this program compile correctly? Will it execute correctly without any run-time errors? Exercises 1. Write a function which draws a line of n asterisks, n being passed as a parameter to the function. Write a driver program (a program that calls and tests the function) which uses the function to output an m n block of asterisks, m and n entered by the user. 2. Extend the function of the previous exercise so that it prints a line of n asterisks starting in column m. It should take two parameters m and n. If the values of m and n are such that the line of asterisks would extend beyond column 80 then the function should return false and print nothing, otherwise it should output true and print the line of asterisks. Amend your driver program so that it uses the function return value to terminate execution with an error message if m and n are such that there would be line overflow. Think carefully about the test data you would use to test the function. 3. Write a function which converts a sum of money given as an integer number of pence into a floating point value representing the equivalent number of pounds. For example 365 pence would be 3.65 pounds.
- 91. VISUAL C++ PROGRAMMING VC++ [email protected] 91 Overloaded functions. In C++ two different functions can have the same name if their parameter types or number are different. That means that you can give the same name to more than one function if they have either a different number of parameters or different types in their parameters. For example: // overloaded function #include iostream using namespace std; int operate (int a, int b) { return (a*b); } float operate (float a, float b) { return (a/b); } int main () { int x=5,y=2; float n=5.0,m=2.0; cout operate (x,y); cout n; cout operate (n,m); cout n; return 0; } In this case we have defined two functions with the same name, operate, but one of them accepts two parameters of type int and the other one accepts them of type float. The compiler knows which one to call in each case by examining the types passed as arguments when the function is called. If it is called with two ints as its arguments it calls to the function that has two int parameters in its prototype and if it is called with two floats it will call to the one which has two float parameters in its prototype. In the first call to operate the two arguments passed are of type int, therefore, the function with the first prototype is called; This function returns the result of multiplying both parameters. While the second call passes two arguments of type float, so the function with the second prototype is called. This one has a different behavior: it divides one parameter by the other. So the behavior of a call to operate depends on the type of the arguments passed because the function has been overloaded.
- 92. VISUAL C++ PROGRAMMING VC++ Notice that a function cannot be overloaded only by its return type. At least one of its parameters must have a different type. [email protected] 92 inline functions. The inline specifier is an optimization specifier. It does not change the behavior of a function, but serves to indicate to the compiler that the code the function body generates shall be inserted at the point of each call to the function, instead of doing a regular call that generally involves stacking variables and jumping to other memory locations. This is equivalent to declaring a macro. It only represents an overhead advantage for very short functions, in which the resulting code from compiling the program may be faster when the overhead required to call a function is avoided. The format for its declaration is: inline type name ( arguments ... ) { instructions ... } and the call is just like the call to any other function. You do not have to include the inline keyword when calling the function, only in its declaration. Most compilers already optimize code to generate inline functions when it is more convenient. This specifier only indicates to the compiler that inline is preferred for this function. C++ Virtual Function - Properties: C++ virtual function is, • A member function of a class • Declared with virtual keyword • Usually has a different functionality in the derived class • A function call is resolved at run-time The difference between a non-virtual c++ member function and a virtual member function is, the non-virtual member functions are resolved at compile time. This mechanism is called static binding. Where as the c++ virtual member functions are resolved during run-time. This mechanism is known as dynamic binding. C++ Virtual Function - Reasons: The most prominent reason why a C++ virtual function will be used is to have a different functionality in the derived class.
- 93. VISUAL C++ PROGRAMMING VC++ For example a Create function in a class Window may have to create a window with white background. But a class called CommandButton derived or inherited from Window, may have to use a gray background and write a caption on the center. The Create function for CommandButton now should have a functionality different from the one at the class called Window. C++ Virtual function - Example: This article assumes a base class named Window with a virtual member function named Create. The derived class name will be CommandButton, with our over ridden function Create. class Window // Base class for C++ virtual function example { public: virtual void Create() // virtual function for C++ virtual function example { cout Base class Windowendl; [email protected] 93 } }; class CommandButton : public Window { public: void Create() { coutDerived class Command Button - Overridden C++ virtual functionendl; } }; void main() { Window *x, *y; x = new Window(); x-Create(); y = new CommandButton(); y-Create(); } The output of the above program will be, Base class Window Derived class Command Button If the function had not been declared virtual , then the base class function would have been called all the times. Because, the function address would have been statically bound during
- 94. compile time. But now, as the function is declared virtual it is a candidate for run and the derived class function is being invoked. C++ Virtual function - Call Mechanism: VC++ Whenever a program has a C++ virtual function declared, a v The v-table consists of addresses to the virtual functions for classes and pointers to the functions from each of the objects of the derived class. Whenever there is a function call mad virtual function, the v-table is used to resolve to the function address. This is how the Dynamic binding happens during a virtual function call. Arrays An array is a series of elements of the same type placed in contiguous memory locations t be individually referenced by adding an index to a unique identifier. That means that, for example, we can store 5 values of type declare 5 different variables, each one with a different identifier. Instead of tha we can store 5 different values of the same type, For example, an array to contain 5 integer values of type this: where each blank panel represents type int. These elements are numbered from independently of its length. Like a regular variable, an array must be declared before it is u array in C++ is: type name [elements]; where type is a valid type (like int is always enclosed in square brackets contain. VISUAL C++ PROGRAMMING [email protected] v-table is constructed for the class. int in an array without having to int for example, with a unique identifier. int called billy could be represented like an element of the array, that in this case are integer values of . 0 to 4 since in arrays the first index is always used. A typical declaration for an int, float...), name is a valid identifier and the elements []), specifies how many of these elements the array has to 94 run-time linking made to the c++ that can that, using an array 0, sed. field (which nts
- 95. Therefore, in order to declare an array called simple as: int billy [5]; billy as the one shown in the above diagram it is as NOTE: The elements field within brackets is going to hold, must be a constant whose size must be determined before execution. In order to create arrays with a variable length dynamic memory is needed, which is explained later in these tutorials. Initializing arrays. [] which represents the number of elements the array value, since arrays are blocks of non-dynamic memory When declaring a regular array of local scope (within a function, for example), if we do not specify otherwise, its elements will not be initialized to any value by default, so their content will be undetermined until we store some v value in them. The elements of global and static arrays, on the other hand, are automatically initialized with their default values, which for all fundamental types this means they are filled with zeros. In both cases, local and global, when we declare an values to each one of its elements by enclosing the values in braces int billy [5] = { 16, 2, 77, 40, 12071 }; array, we have the possibility to assign initial This declaration would have created an array like this: The amount of values between braces we declare for the array between square brackets we have declared that it has 5 elements and in the list of initial values within specified 5 values, one for each element. When an initialization of values is provided for an array, C++ allows the possibility of leaving the square brackets empty [ ]. In this case, the compiler will assume a size for the array that matches the number of values included between braces int billy [] = { 16, 2, 77, 40, 12071 }; VISUAL C++ PROGRAMMING [email protected] { }. For example: { } must not be larger than the number of elements that [ ]. For example, in the example of array braces . { }: VC++ 95 alue . . billy { } we have
- 96. After this declaration, array billy values. VC++ would be 5 ints long, since we have provided 5 initialization Accessing the values of an array. VISUAL C++ PROGRAMMING In any point of a program in which an array is visible, we can access the value of any of its elements individually as if it was a normal variable, thus being able to both read and modify its value. The format is as simple as: name[index] Following the previous examples in which of type int, the name which we can use to refer to each element is the following: ples billy had 5 elements and each of those elements was For example, to store the value statement: billy[2] = 75; and, for example, to pass the value of the third element of write: a = billy[2]; Therefore, the expression billy[2] Notice that the third element o one is billy[1], and therefore, the third one is billy[4]. Therefore, if we write billy[5], we would be accessing the sixth elemen therefore exceeding the size of the array. In C++ it is syntactically correct to exceed the valid range of indices for an array. This can create problems, since accessing out-of runtime errors. The reason why this is allowed will be seen further ahead when we begin to use pointers. [email protected] , 75 in the third element of billy, we could write the following billy to a variable called is for all purposes like a variable of type int. of billy is specified billy[2], since the first one is , billy[2]. By this same reason, its last element is . of-range elements do not cause compilation errors but can cause 96 , a, we could . , billy[0], the second . element of billy and
- 97. VISUAL C++ PROGRAMMING VC++ At this point it is important to be able to clearly distinguish between the two uses that brackets [ ] have related to arrays. They perform two different tasks: one is to specify the size of arrays when they are declared; and the second one is to specify indices for concrete array elements. Do not confuse these two possible uses of brackets [ ] with arrays. int billy[5]; // declaration of a new array billy[2] = 75; // access to an element of the array. If you read carefully, you will see that a type specifier always precedes a variable or array declaration, while it never precedes an access. Some other valid operations with arrays: [email protected] 97 billy[0] = a; billy[a] = 75; b = billy [a+2]; billy[billy[a]] = billy[2] + 5; // arrays example #include iostream using namespace std; int billy [] = {16, 2, 77, 40, 12071}; int n, result=0; int main () { for ( n=0 ; n5 ; n++ ) { result += billy[n]; } cout result; return 0; } 12206 Multidimensional arrays Multidimensional arrays can be described as arrays of arrays. For example, a bidimensional array can be imagined as a bidimensional table made of elements, all of them of a same uniform data type.
- 98. VISUAL C++ PROGRAMMING jimmy represents a bidimensional array of 3 per 5 elements of type array in C++ would be: int jimmy [3][5]; and, for example, the way to reference the second element vertically and fourth horizontally in an expression would be: jimmy[1][3] (remember that array indices always begin by zero). Multidimensional arrays are not limited to two indices (i.e., two dimensions). They can contain as many indices as needed. But be careful! The amount of memory need increases with each dimension. For example: char century [100][365][24][60][60]; declares an array with a char element for each second in a century, that is more than 3 billion chars. So this declaration would consume more than 3 g Multidimensional arrays are just an abstraction for programmers, since we can obtain the same results with a simple array just by putting a factor between its indices: int jimmy [3][5]; // is equivalent to int jimmy [15]; // (3 * 5 = 15) With the only difference that with multidimensional arrays the compiler remembers the depth of each imaginary dimension for us. Take as example these two pieces of code, with both exactly the same result. One uses a bidimensional array and the [email protected] int. The way to declare this needed for an array rapidly gigabytes of memory! other one uses a simple array: VC++ 98 . ed
- 99. multidimensional array #define WIDTH 5 #define HEIGHT 3 int jimmy [HEIGHT][WIDTH]; int n,m; int main () { for (n=0;nHEIGHT;n++) for (m=0;mWIDTH;m++) { jimmy[n][m]=(n+1)*(m+1); } return 0; } VISUAL C++ PROGRAMMING pseudo-multidimensional array #define WIDTH 5 #define HEIGHT 3 int jimmy [HEIGHT * WIDTH]; int n,m; int main () { for (n=0;nHEIGHT;n++) for (m=0;mWIDTH;m++) { jimmy[n*WIDTH+m]=(n+1)*(m+1); } return 0; } None of the two source codes abo to the memory block called jimmy in the following way: above produce any output on the screen, but both assign values We have used defined constants ( program. For example, in case that we decided could be done simply by changing the line: #define HEIGHT 3 to: #define HEIGHT 4 (#define) to simplify possible future modifications of the to enlarge the array to a height of 4 instead of 3 it with no need to make any other modifications to the program. Arrays as parameters At some moment we may need to pass an array to a function as a parameter. In C++ it is not possible to pass a complete block of memory by value as a parameter to a function, but we are allowed to pass its address. In practice this has almost the same effect and it is a much faster and more efficient operation. [email protected] VC++ 99 ve )
- 100. VISUAL C++ PROGRAMMING VC++ In order to accept arrays as parameters the only thing that we have to do when declaring the function is to specify in its parameters the element type of the array, an identifier and a pair of void brackets []. For example, the following function: [email protected] 100 void procedure (int arg[]) accepts a parameter of type array of int called arg. In order to pass to this function an array declared as: int myarray [40]; it would be enough to write a call like this: procedure (myarray); Here you have a complete example: // arrays as parameters #include iostream using namespace std; void printarray (int arg[], int length) { for (int n=0; nlength; n++) cout arg[n] ; cout n; } int main () { int firstarray[] = {5, 10, 15}; int secondarray[] = {2, 4, 6, 8, 10}; printarray (firstarray,3); printarray (secondarray,5); return 0; } 5 10 15 2 4 6 8 10 As you can see, the first parameter (int arg[]) accepts any array whose elements are of type int, whatever its length. For that reason we have included a second parameter that tells the function the length of each array that we pass to it as its first parameter. This allows the for loop that prints out the array to know the range to iterate in the passed array without going out of range. In a function declaration it is also possible to include multidimensional arrays. The format for a tridimensional array parameter is:
- 101. base_type[][depth][depth] VISUAL C++ PROGRAMMING for example, a function with a multidimensional array as argument could be: void procedure (int myarray[][3][4]) VC++ Notice that the first brackets [] are left blank while the following ones are not. This is so because the compiler must be able to determine within the function which is the depth of each additional dimension. Arrays, both simple or multidimensional, passed as function parameters are a quite common source of errors for novice programmers. I recommend the reading of the chapter about Pointers for a better understanding on how arrays operate. Character Sequences As you may already know, the C++ Standard Library implements a powerful is very useful to handle and manipulate strings of characters. However, because strings are in fact sequences of characters, we can represent them also as plain arrays of For example, the following array: char jenny [20]; is an array that can store up to 20 elements of type Therefore, in this array, in theory, we can store sequ But we can also store shorter sequences. For example, program either the sequence Hello than 20 characters. [email protected] char elements. char. It can be represented as: sequences of characters up to 20 characters long. jenny could store at some point in a or the sequence Merry christmas, since both are shorter 101 string class, which . ences ,
- 102. Therefore, since the array of characters can store shorter sequences than its total length, a special character is used to signal the end of the valid sequence: the can be written as '0' (backslash, zero). Our array of 20 elements of type sequences Hello and Merry Christmas Notice how after the valid content a null character ( the end of the sequence. The panels in gray color represent values. Initialization of null-terminated character sequences Because arrays of characters are ordinary arrays they follow all their same rules. For example, if we want to initialize an array of characters with some predetermined sequence of characters we can do it just like any other array: char myword[] = { 'H', 'e' In this case we would have declared an array of 6 elements of type characters that form the word Hello But arrays of char elements have an additional method to initialize their values: using string literals. In the expressions we have used in some examples in previous chapters, con entire strings of characters have already showed up several times. These are specified enclosing the text to become a string literal between double quotes (). For example: the result is: is a constant string literal that we have VISUAL C++ PROGRAMMING [email protected] refore, null character, whose literal constant f char, called jenny, can be represented storing the characters as: ('0') has been included in order to indicate he char elements with undetermined e', 'l', 'l', 'o', '0' }; char initialized with the plus a null character '0' at the end. constants that represent probably used already. VC++ 102 , , ) stants
- 103. VISUAL C++ PROGRAMMING VC++ Double quoted strings () are literal constants whose type is in fact a null-terminated array of characters. So string literals enclosed between double quotes always have a null character ('0') automatically appended at the end. Therefore we can initialize the array of char elements called myword with a null-terminated sequence of characters by either one of these two methods: char myword [] = { 'H', 'e', 'l', 'l', 'o', '0' }; [email protected] 103 char myword [] = Hello; In both cases the array of characters myword is declared with a size of 6 elements of type char: the 5 characters that compose the word Hello plus a final null character ('0') which specifies the end of the sequence and that, in the second case, when using double quotes () it is appended automatically. Please notice that we are talking about initializing an array of characters in the moment it is being declared, and not about assigning values to them once they have already been declared. In fact because this type of null-terminated arrays of characters are regular arrays we have the same restrictions that we have with any other array, so we are not able to copy blocks of data with an assignment operation. Assuming mytext is a char[] variable, expressions within a source code like: mystext = Hello; mystext[] = Hello; would not be valid, like neither would be: mystext = { 'H', 'e', 'l', 'l', 'o', '0' }; The reason for this may become more comprehensible once you know a bit more about pointers, since then it will be clarified that an array is in fact a constant pointer pointing to a block of memory.
- 104. VISUAL C++ PROGRAMMING VC++ Using null-terminated sequences of characters Null-terminated sequences of characters are the natural way of treating strings in C++, so they can be used as such in many procedures. In fact, regular string literals have this type (char[]) and can also be used in most cases. For example, cin and cout support null-terminated sequences as valid containers for sequences of characters, so they can be used directly to extract strings of characters from cin or to insert them into cout. For example: // null-terminated sequences of characters #include iostream using namespace std; int main () { char question[] = Please, enter your first name: ; char greeting[] = Hello, ; char yourname [80]; cout question; cin yourname; cout greeting yourname !; return 0; } Please, enter your first name: John Hello, John! As you can see, we have declared three arrays of char elements. The first two were initialized with string literal constants, while the third one was left uninitialized. In any case, we have to speficify the size of the array: in the first two (question and greeting) the size was implicitly defined by the length of the literal constant they were initialized to. While for yourname we have explicitly specified that it has a size of 80 chars. Finally, sequences of characters stored in char arrays can easily be converted into string objects just by using the assignment operator: [email protected] 104 string mystring; char myntcs[]=some text; mystring = myntcs;
- 105. VISUAL C++ PROGRAMMING VC++ Bubble Sort The bubble sort gets its name because as elements are sorted they gradually bubble (or rise) to their proper positions, like bubbles rising in a glass of soda. The bubble sort repeatedly compares adjacent elements of an array, starting with the first and second elements, and swapping them if they are out of order. After the first and second elements are compared, the second and third elements are compared, and swapped if they are out of order. This process continues until the end of the list is reached. When the end is reached, the bubble sort returns to elements one and two and starts the process all over again. So, when does it stop? The bubble sort knows that it is finished when it examines the entire array and no swaps are needed (thus the list is in proper order). The bubble sort keeps track of the occurrence of swaps by the use of a flag. The table below follows an array of numbers before, during, and after a bubble sort for descending order. A pass is defined as one full trip through the array comparing and if necessary, swapping, adjacent elements. Several passes have to be made through the array before it is finally sorted. Array at beginning: 84 69 76 86 94 91 After Pass #1: 84 76 86 94 91 69 After Pass #2: 84 86 94 91 76 69 After Pass #3: 86 94 91 84 76 69 After Pass #4: 94 91 86 84 76 69 After Pass #5 (done): 94 91 86 84 76 69 The bubble sort is an easy algorithm to program, but it is slower than many other sorts. With a bubble sort, it is always necessary to make one final pass through the array to check to see that no swaps are made to ensure that the process is finished. In actuality, the process is finished before this last pass is made. // Bubble Sort Function for Descending Order void bubble_sort(apvector int array) { int i, j, flag = 1; // set flag to 1 to begin initial pass int temp; // holding variable int arrayLength = array.length( ); for(i = 1; (i = arrayLength) flag; i++) { flag = 0; for (j=0; j (arrayLength -1); j++) { if (array[j+1] array[j]) // ascending order simply changes to { temp = array[j]; // swap elements array[j] = array[j+1]; array[j+1] = temp; flag = 1; // indicates that a swap occurred. [email protected] 105 }
- 106. VISUAL C++ PROGRAMMING VC++ [email protected] 106 } } return; //arrays are passed to functions by address; nothing is returned } Selection Sort The selection sort is a combination of searching and sorting. During each pass, the unsorted element with the smallest (or largest) value is moved to its proper position in the array. The number of times the sort passes through the array is one less than the number of items in the array. In the selection sort, the inner loop finds the next smallest (or largest) value and the outer loop places that value into its proper location. Let's look at our same table of elements using a selection sort for descending order. Remember, a pass is defined as one full trip through the array comparing and if necessary, swapping elements. Array at beginning: 84 69 76 86 94 91 After Pass #1: 84 91 76 86 94 69 After Pass #2: 84 91 94 86 76 69 After Pass #3: 86 91 94 84 76 69 After Pass #4: 94 91 86 84 76 69 After Pass #5 (done): 94 91 86 84 76 69 While being an easy sort to program, the selection sort is one of the least efficient. The algorithm offers no way to end the sort early, even if it begins with an already sorted list. // Selection Sort Function for Descending Order void selection_sort(apvector int array) { int i, j, first, temp; int array_size = array.length( ); for (i= array_size - 1; i 0; i--) { first = 0; // initialize first to the subscript of the first element for (j=1; j=i; j++) //Find smallest element between the positions 1 and i. { if (array[j] array[first]) first = j; } temp = array[first]; // Swap smallest element found with one in position i. array[first] = array[i]; array[i] = temp; } return; } Sequential Search (Page 2 of 4 )
- 107. VISUAL C++ PROGRAMMING VC++ The sequential search is best used if the array you are searching is unsorted. This method of searching is usually used on small arrays of less than 16 elements. We start the sequential search by first declaring a target to be found. The search initiates at the beginning of the array until it finds the target. In the following example we will find a target value of 23 within a one dimensional array. At index 0, 32 is not equal to 23 so we proceed on to the next element. a[0] a[1] a[2] a[3] a[4] 32 431 -34 23 12 At index 1, 431 is not equal to 23 so we proceed. a[0] a[1] a[2] a[3] a[4] 32 431 -34 23 12 At index 2, -34 is not equal to 23 so we proceed. a[0] a[1] a[2] a[3] a[4] 32 431 -34 23 12 Finally at index 3, 23 is equal to 23 and we have found our target. a[0] a[1] a[2] a[3] a[4] 32 431 -34 23 12 Now we will implement this example of a sequential search into C++ code. The program below asks the user for a target to be found, then uses a for loop to analyze each element of the array. If the array element is equal to the target it will display that the target was found. Whenever a target is found the variable “flag” will be incremented by 1. At the end of the program if the variable “flag” is less than one, then the target was obviously not found. #include iostream using namespace std; int main() { const int arraySize = 5; double target; int array[arraySize] = {32, 431, -34, 23, 12}; int flag; // flag is used to log how many times the target is encountered. flag = 0; cout Enter a target to be found: ; cin target; for(int cntr = 0; cntr arraySize; cntr++) { [email protected] 107 if(array[cntr] == target) { cout Target found in array index cntr . endl; flag += 1; } } // Test to see if target was found. if(flag 1) {
- 108. VISUAL C++ PROGRAMMING VC++ cout Target not found. endl; } return 0; } The sequential search does have a pitfall. It is very slow and its performance rating is low. If a person had an array of one million elements, that would mean there could be up to one million comparisons, and that takes time! The sequential search method would be advisable to use only if the array you were searching was unsorted and small. [email protected] 108
- 109. VISUAL C++ PROGRAMMING VC++ Unit 5: Constructors and distractors (10 HOURS) [email protected] 109 THEORY At the end of the unit the student should be able to: 1. ..use constructors 2. ..use parameterised constructors 3. ..use multiple constructors in a class 4. ..use constructors with default arguments 5. ..use dynamic initialisation of objects 6. ..copy constructor 7. ..use dynamic constructors 8. ..construct a two-dimensional array 9. ..use distracters The definition and use of constructors and destructors is fairly simple. First, I'll outline some basic rules on constructors and destructors, and then provide the details using some simple examples. • A constructor is a method that has the same name as its class. • A destructor is a method that has as its name the class name prefixed by a tilde, ~. • Neither constructors nor destructors return values. They have no return type specified. • Constructors can have arguments. • Constructors can be overloaded. • If any constructor is written for the class, the compiler will not generate a default constructor. • The default constructor is a constructor with no arguments, or a constructor that provides defaults for all arguments. • The container classes such as vector require default constructors to be available for the classes they hold. Dynamically allocated class arrays also require a default constructor. If any constructors are defined, you should always define a default constructor as well. • Destructors have no arguments and thus cannot be overloaded. Constructors and Destructors Overview
- 110. VISUAL C++ PROGRAMMING VC++ Because classes have complicated internal structures, including data and functions, object initialization and cleanup for classes is much more complicated than it is for simple data structures. Constructors and destructors are special member functions of classes that are used to construct and destroy class objects. Construction may involve memory allocation and initialization for objects. Destruction may involve cleanup and deallocation of memory for objects. Like other member functions, constructors and destructors are declared within a class declaration. They can be defined inline or external to the class declaration. Constructors can have default arguments. Unlike other member functions, constructors can have member initialization lists. The following restrictions apply to constructors and destructors: • Constructors and destructors do not have return types nor can they return values. • References and pointers cannot be used on constructors and destructors because their addresses cannot be taken. • Constructors cannot be declared with the keyword virtual. • Constructors and destructors cannot be declared static, const, or volatile. • Unions cannot contain class objects that have constructors or destructors. Constructors and destructors obey the same access rules as member functions. For example, if you declare a constructor with protected access, only derived classes and friends can use it to create class objects. The compiler automatically calls constructors when defining class objects and calls destructors when class objects go out of scope. A constructor does not allocate memory for the class object its this pointer refers to, but may allocate storage for more objects than its class object refers to. If memory allocation is required for objects, constructors can explicitly call the new operator. During cleanup, a destructor may release objects allocated by the corresponding constructor. To release objects, use the delete operator. Derived classes do not inherit constructors or destructors from their base classes, but they do call the constructor and destructor of base classes. Destructors can be declared with the keyword virtual. Constructors are also called when local or temporary class objects are created, and destructors are called when local or temporary objects go out of scope. You can call member functions from constructors or destructors. You can call a virtual function, either directly or indirectly, from a constructor or destructor of a class A. In this case, the function called is the one defined in A or a base class of A, but not a function overridden in any class derived from A. This avoids the possibility of accessing an unconstructed object from a constructor or destructor. The following example demonstrates this: [email protected] 110 #include iostream using namespace std; struct A { virtual void f() { cout void A::f() endl; } virtual void g() { cout void A::g() endl; } virtual void h() { cout void A::h() endl; }
- 111. VISUAL C++ PROGRAMMING VC++ [email protected] 111 }; struct B : A { virtual void f() { cout void B::f() endl; } B() { f(); g(); h(); } }; struct C : B { virtual void f() { cout void C::f() endl; } virtual void g() { cout void C::g() endl; } virtual void h() { cout void C::h() endl; } }; int main() { C obj; } The following is the output of the above example: void B::f() void A::g() void A::h() The constructor of B does not call any of the functions overridden in C because C has been derived from B, although the example creates an object of type C named obj. Copy Constructors A copy constructor is a special constructor that takes as its argument a reference to an object of the same class and creates a new object that is a copy. By default, the compiler provides a copy constructor that performs a member-by-member copy from the original object to the one being created. This is called a member wise or shallow copy. Although it may seem to be the desired behavior, in many cases a shallow copy is not satisfactory. To see why let's look at the Employee class introduced in an earlier lesson with one change. We will store the name in a C-style character string rather than store the employee name using the string class from the standard C++ library. Here is a simple program with a bare bones version of the Employee class. #include iostream using namespace std; class Employee { public: Employee(char *name, int id); ~Employee(); char *getName(){return _name;} //Other Accessor methods private: int _id; char *_name;
- 112. VISUAL C++ PROGRAMMING VC++ [email protected] 112 }; Employee::Employee(char *name, int id) { _id = id; _name = new char[strlen(name) + 1]; //Allocates an character array object strcpy(_name, name); } Employee::~Employee() { delete[] _name; } int main() { Employee programmer(John,22); cout programmer.getName() endl; return 0; } The function strlen returns the length of the string passed into the constructor. Notice that the Employee name is now stored in a dynamically allocated character array. It is of string length + 1 to allow for the null terminator used with C-style strings, '0'. The strcpy function automatically adds the null terminator to the destination string. Also, notice that the destructor frees the memory used to hold the employee name. This is needed to avoid a memory leak
- 113. VISUAL C++ PROGRAMMING VC++ Unit 6: Operator overloading and type conversions (10 HOURS) [email protected] 113 THEORY At the end of the unit the student should be able to: 1. ..define operator overloading 2. ..overload: - Unary operators - Binary operators - Binary operators using friends 3. ..manipulate strings using operators 4. ..list the rules for overloading operators 5. ..make type conversions Operator Overloading Operator overloading completes the data abstraction facilities. data abstraction means programmer-defined types which can be used with the same flexibility as built-in types. The class concept was the first step toward this capability. Operator overloading completes the implementation. This facility allows the programmer to define how meaningful standard C++ operators can be used with a class. 6.1 It's All Notation Really all that's happening is a notational trick. By giving a member or non-member function a special name, we indicate to the compiler that it should consider it when it runs into the use of a particular operator involving a class. When the compiler is looking at a+b it's as if it saw a.operator+(b) or operator+(a,b). +a can be interpreted as a.operator+() or operator+(a). A non-member function implementation of a binary operation has the additional flexibility of being able to apply conversions to the first argument. References are commonly used as arguments. This is where the reference syntax becomes valuable since it allows the usual operator notation while providing the efficiency that comes with passing pointers to classes instead of (possibly constructed) copies. Be careful when returning references though. Don't use references to local objects which are destroyed when the function exits. 6.2 General Restrictions
- 114. VISUAL C++ PROGRAMMING VC++ The usual rules of function call overloading apply, plus it must be remembered that the built-in operators are prospective candidates for resolving any call, and there are the following [email protected] 114 general restrictions: • The standard C++ precedences and associativities determine how any expression will be parsed and can't be changed. • No new operators can be invented, e.g. @ can't be overloaded as an operator. • The operator must be written as unary or binary depending on its use with the built-in types, e.g. a binary operator can't be overloaded in a unary way for a class if there's no built-in type for which it's used in a unary manner. • For non-member functions overloading operators, at least one function argument must be an instance or a reference to a class. This ensures no C++ code can change the built-in behavior of operators with built-in types. • No default values for the operator arguments are allowed. • There are also operator-specific restrictions, notations and features which are enumerated below. The following operators can't be overloaded: `.', `.*', `::', sizeof, () value construction and the ternary conditional expression operator (`?:'). The following operators must be implemented as member functions: = - () [] 6.3 Guidelines The good news is that the programmer has complete control over the return type and argument types of the function. This flexibility must be used with some sensitivity. For example, + should be overloaded for a class only when replacing functional notation for an operation analagous to addition for the class. Unless there is a good reason for doing otherwise, it should mimic the behavior of the built-in operator as much as possible meaning (for a function overloading +): • It shouldn't change the value of either operand, since + doesn't do that when applied to built-in types. • It should be commutative. As another example, it might be a good idea for overloaded = to return a reference to the left operand. Each operator has a sort of culture, a way programmers expect it to behave given their experience with built-in types. The more a new version of the operator fits in with this experience, the easier it will be for the programmer to utilize it in an effective and error-free manner. In addition, the class will be more likely to fit into and work properly in templates that require the operator. 6.4 Operator-specific restrictions, notations and features Copy assignment operator
- 115. VISUAL C++ PROGRAMMING VC++ One operator which frequently needs to be explicitly defined for a safe class interface is the copy assignment operator. This is because, if one is not defined explicitly, the compiler generates a default version which does a member-wise copy. The copy assignment operator has a reference or value of the same type as its argument. This feature is for backward compatibility with C. It can have the same terrible problems in conjunction with dynamically allocated members as the copy constructor does. In designing assignment operation, the effects of assigning an object to itself must be taken into account. This may seem silly at first glance, but it is a very real possibility when indirection or references are being used. Other assignment operators Behavior of overloaded += is independent (and must be defined separately and explicitly to be used) from the behavior of overloaded + and =. This applies to the rest of the assignment operators as well. It often makes sense to define binary + using the (usually simpler) += operation. += will usually be faster since it doesn't require construction of a temporary to hold the final result since the final result is stored in an existing variable. operator++ and operator-- These two unary operators have prefix and postfix versions. The prefix version is implemented as with the other unary operators. The postfix version must be implemented as a.operator++(int) or operator++(a, int). The int argument is a dummy which is not used for anything. Incidentally, in C++, the assignment operators and prefix ++ and -- result in l-values contrary to their behavior in C. Again, overloaded versions of these operators should behave as the built-in types if not overly inconvenient. Here inefficiency can be a big concern as the built-in behavior for the postfix version of these operators is to return by value a snap shot preserving the state of the object before it is operated on. For a large object where copy construction is expensive, this could be prohibitive. operator() function call Overloading this enables instances of the class to be treated as if they were function identifiers. It can be overloaded several times for varying argument lists. Can be very useful. operator- Overloaded versions of this operator could well be termed pass the buck operators. This is implemented as a unary member function and allows - to be applied to an instance of a class, whereas the built-in version always requires a pointer to a class or struct. A name of a data or function member must appear on the right side of the operator. If the operator returns a pointer to an object, that data or function access must make sense for the object. If the operator returns a reference or instance of a class, that class must in turn have [email protected] 115
- 116. VISUAL C++ PROGRAMMING VC++ the - operator overloaded. Thus, one usage of this operator could potentially result in many functions being called if the buck is passed extensively, though in practice it is usually just one layer deep. This is useful for smart pointers. A program can defer construction of an object or loading of data until access makes it necessary. new and delete The sort of overloading which can be done here is restricted to determining where run-time memory is obtained when an object is created via these operators. Typically they are used to optimize heap memory allocation for a class. For example, if a Tree class created many instances of a Node class, it may be more economical to allocate them in batches, or to check a list of used nodes before trying to allocate a new one. The format of the delete member function is void operator delete(void *, std::size_t) The second argument may be omitted. The format of the new member function is void *operator new(std::size_t) Additional arguments of arbitrary type may be added after the initial std::size_t type. If these arguments appear, they are supplied after the new invocation in function call format, i.e. new(...args...) MyClass. It always returns a void * type. Even so, the expression where it is applied has the type of a pointer to the type which is being allocated as is the case with the built-in operator. Conversion functions casts (and conversions in general via assignment, C-style cast, C++ style cast) can be overloaded as well, but it is conceptually distinct since the return type is fixed (can't be programmer-defined as in general case) and they can only be defined as member functions, not friends. It looks similar though. An int conversion function is defined as the member function operator int(). It has no return value and no arguments. If a conversion function is to be written for a complex type, a typedef must be used so it can be manipulated as a single word. The array of needed implementations of an operator can be reduced by defining conversions for the classes involved. Conversions to a class are defined implicitly with single-argument constructors. Conversions from the class are defined using conversion functions and exist by virtue of polymorphism. [email protected] 116
- 117. VISUAL C++ PROGRAMMING VC++ Unit 7: Inheritance: Extending class (25 HOURS) [email protected] 117 THEORY At the end of the unit the student should be able to: 1. ..define derived classes 2. ..describe single inheritance 3. ..make a private member inheritable 4. ..describe multilevel inheritance 5. ..describe multiple inheritance 6. ..describe hierarchical inheritance 7. ..describe hybrid inheritance 8. ..describe virtual base classes 9. ..describe abstract classes 10. ..use constructors in derived classes 11. ..use member classes: Nesting of classes Inheritance Inheritance is the concept to inherit the properties of one class to another class. This has also known as class structure again. For example, classes A contains two-member function ads and subtracts and class b contain two another functions multiply and divide. We want to use all these function with one object then we need to use inheritance where class B inherits all the property of class, which is public, but class B cannot use the private properties of class A. There are following types of inheritance: Inheritance can be classified to 5 types. 1. Single Inheritance 2. Hierarchical Inheritance 3. Multi Level Inheritance 4. Hybrid Inheritance 5. Multiple Inheritance 1. Single Inheritance when a single derived class is created from a single base class then the inheritance is called as single inheritance. When class a gas inherited in class has
- 118. VISUAL C++ PROGRAMMING VC++ known as base class and B class is know as derived class. Here only two classes have linked to each other. 2. Hierarchical Inheritance When more than one derived class are created from a single base class, then that inheritance is called as hierarchical inheritance. 3. Multi Level Inheritance In this type of inheritance, there are number of level and it has used in that cases where we want to use all properties in number of levels according to the requirement. For example, class A inherited in class b and class b has inherited in class c for class b so on. Where class A is base class c. In another way we can say b is derived class a base class for c and a indirect base class for c is indirect base class for c and c indirect derived class for class A. [email protected] 118
- 119. VISUAL C++ PROGRAMMING VC++ When a derived class is created from another derived class, then that inheritance is called as multi level inheritance. [email protected] 119 4. Hybrid Inheritance Any combination of single, hierarchical and multi level inheritances is called as hybrid inheritance.
- 120. VISUAL C++ PROGRAMMING VC++ [email protected] 120 5. Multiple Inheritance In this type of inheritance, number of classes has inherited in a single class. Where two or more classes are, know as base class and one is derive class. when a derived class is created from more than one base class then that inheritance is called as multiple inheritance. But multiple inheritance is not supported by .net using classes and can be done using interfaces. Handling the complexity that causes due to multiple inheritance is very complex. Hence it was not supported in dotnet with class and it can be done with interfaces. Inheritance, Samples of using inheritance
- 121. VISUAL C++ PROGRAMMING VC++ Inheritance is the property by which one object can inherit the properties of the other object. A general class can be inherited by the other classes. A class that is inherited is called a base class. A class which is inheriting another class is called a derived class. When a class inherits another class, members of the base class become the members of the derived class. The general form of inheritance is:- class derived_name : access_specifier base_name { }; The derived_name is the name of the derived class. The base_name is the name of the base class. The access_specifier can be private, public or protected. If the access_specifier is public then all public members of the base class become public members of the derived class and protected members of the base class become the protected members of the derived class. If the access_specifier is private then all public and protected members of the base class will become private members of the derived class. If the access_specifier is protected then the public and protected members of the base class become the protected members of the derived class. Whether access_specifier is public, private or protected, private members of the base class will not be accessed by the members of the derived class. The access_specifier protected provides more flexibility in terms of inheritance. The private members of the base class cannot be accessed by the members of the derived class. The protected members of the base class remain private to their class but can be accessed and inherited by the derived class. The protected members of the base class will remain private to the other elements of the program. A derived class can inherit one or more base classes. A constructor of the base is executed first and then the constructor of derived class is executed. A destructor of derived class is called before the destructor of base class. The arguments to the base class constructor can be passed as follows:- derived_constructor (argument list): base1 (arg_list) base2(arg_list1) baseN(arg_list) The derived_constructor is the name of the derived class. The argument list is list of the data members of the derived class. The base1 is name of the base class. The arg_list is the list of the members of the base class. Here is a program which illustrates the features of inheritance. [email protected] 121 #includeiostream using namespace std; class shape { private : double length;
- 122. VISUAL C++ PROGRAMMING VC++ [email protected] 122 protected: double breadth; public : double len() { return(length); } shape(double length1,double breadth1) { length=length1; breadth=breadth1; } //shape() { } }; class shape1 { public: double height; shape1(double height1) { height=height1; } //shape1() { } }; class cuboid : public shape, private shape1 { public: cuboid(double length1,double breadth1,double height1):shape(length1,breadth1),shape1(height1) { cout A constructor is called endl; } double volume() { return(height*breadth*len()); } double bre() { return(breadth); } double ht() {
- 123. VISUAL C++ PROGRAMMING VC++ return(height); [email protected] 123 } }; int main() { cuboid c1(2.4,3.5,6.7); cout The length of the cuboid is : c1.len() endl; cout The breadth of the cuboid is : c1.bre() endl; cout The height of the cuboid is : c1.ht() endl; cout The volume of the cuboid is : c1.volume() endl; return(0); } The result of the program is:- The program has two base classes shape and shape1 and one derived class called cuboid which inherits shape as public and shape1 as private. The public and protected members of shape become pubic and protected members of derived class cuboid. The private members of shape remain private to the class shape. The members of shape1 class become the private members of the derived class cuboid. The statement class cuboid : public shape, private shape1 states that class cuboid inherits class shape as public and class shape1 as private. The statement cuboid(double length1,double breadth1,double height1):shape(length1,breadth1),shape1(height1) { cout A constructor is called endl; } declares the constructor of the class cuboid. When constructor of class cuboid is called first constructor of shape is executed and then constructor of shape1 is executed and after that the constructor of cuboid is executed. The statements double volume() { return(height*breadth*len());
- 124. VISUAL C++ PROGRAMMING VC++ [email protected] 124 } calculate the volume of the cuboid. The class cuboid cannot access the private data member length of the shape class. It access the length by calling the function len() which returns the private data member length. The data member breadth becomes the protected member of the class cuboid. The height which is public member of shape1 class becomes the private member of the class cuboid as it inherits the shape 1 class as private. The statements double bre() { return(breadth); } returns the breadth of the cuboid as data member breadth cannot be accessed outside the class as it is protected member of cuboid. The statement double ht() { return(height); } returns the height of the cuboid as data member height cannot be accessed outside the class as height is the private data member of the class cuboid. The statement cuboid c1(2.4,3.5,6.7); creates an object c1 of type cuboid. The constructor is called to initialize the values of the cuboid. The constructor of shape is executed and then constructor of shape1 is executed and then finally constructor of cuboid is executed. The statement cout The length of the cuboid is : c1.len() endl; displays the length of the cuboid as c1.len() calls the len() function of class shape which is also the public member function of cuboid. The statement cout The breadth of the cuboid is : c1.bre() endl; displays the breadth of the cuboid. As the data member breadth cannot be accessed directly as it is protected member of the class cuboid so the function bre() returns the breadth of the cuboid. The statement cout The height of the cuboid is : c1.ht() endl; displays the height of the cuboid. The data member height cannot be accessed directly as it is private member of class cuboid so it is accessed through the function ht() which returns height.
- 125. VISUAL C++ PROGRAMMING VC++ Unit 8: Pointers, virtual functions and polymorphism (25 HOURS) [email protected] 125 THEORY At the end of the unit the student should be able to: 1. ..employ pointers to objects 2. ..employ the “this” pointer 3. ..employ pointers to derived classes 4. ..describe virtual functions 5. ..describe pure virtual functions Polymorphism and Virtual functions Polymorphism is defined as one interface to control access to a general class of actions. There are two types of polymorphism one is compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is functions and operators overloading. Runtime time polymorphism is done using inheritance and virtual functions. Function Overloading Polymorphism means that functions assume different forms at different times. In case of compile time it is called function overloading. For example, a program can consist of two functions where one can perform integer addition and other can perform addition of floating point numbers but the name of the functions can be same such as add. The function add() is said to be overloaded. Two or more functions can have same name but their parameter list should be different either in terms of parameters or their data types. The functions which differ only in their return types cannot be overloaded. The compiler will select the right function depending on the type of parameters passed. In cases of classes constructors could be overloaded as there can be both initialized and unintialized objects. Here is a program which illustrates the working of compile time function overloading and constructor overloading. #includeiostream using namespace std; class employee { public: int week; int year; double calculate(double salary)
- 126. VISUAL C++ PROGRAMMING VC++ [email protected] 126 { return(salary*week); } int calculate(int salary) { return(salary*week*year); } employee(int week1) { week=week1; } employee(int week1,int year1) { week=week1; year=year1; } }; int main() { int sal; double sal2; employee emp1(10); employee emp2(10,3); cout Enter the no years for first employee endl; cin emp1.year; cout endl Enter the salary per week for first employee endl; cin sal; cout The total salary of first employee is : emp1.calculate(sal) endl; cout endl Enter the salary per week for second employee is : endl; cin sal2; cout The total salary of second employee is for one year: emp2.calculate(sal2) endl; return(0); } The result of the program is:- In the program function calculate() and constructor of class employee are overloaded. The function calculate() is declared two times but with different parameter type and same with
- 127. VISUAL C++ PROGRAMMING VC++ constructor employee which is also declared two times but with different parameter types. The following statements double calculate(double salary) { return(salary*week); } int calculate(int salary) { return(salary*week*year); [email protected] 127 } declare functions with same name calculate but one has parameter type integer and other has parameter type double. The function calculate() is overloaded. The statements employee(int week1) { week=week1; } employee(int week1,int year1) { week=week1; year=year1; } declare two constructors of class employee where one constructor’s parameter list is different from other constructor’s parameter list. The constructor is overloaded. The statement employee emp1(10); declares an object emp1 of type employee. When object is created first constructor is called since the argument list matches with parameter list of the constructor. The constructor initializes the data member week of the object emp1. The statement employee emp2(10,3); declares an object emp2 of type employee. The second constructor is called since the argument list matches with the parameter list of the constructor. The constructor initializes the data members week and year of the object emp2. In the statement cout The total salary of first employee is : emp1.calculate(sal) endl; emp1.calculate(sal) calls the first function calculate as the data type of sal matches with the parameter of the function calculate. It displays the total salary which is salary*week*year. In the following statement cout The total salary of second employee is for one year: emp2.calculate(sal2) endl;
- 128. VISUAL C++ PROGRAMMING VC++ emp2.calculate(sal2) calls the second function calculate as the data type of sal2 which is double matches with the data type of parameter of function calculate and hence second function is called. It displays the total salary which is salary*week. [email protected] 128 Operator Overloading In polymorphism operators can also be overloaded. Operators can be overloaded in order to perform special functions with respect to the class. With the help of operator overloading standard operations such as + , - , * , etc can be applied on the objects of the class. Operators are overloaded by creating operator functions. The keyword operator is used to define operator function. Operator overloading doesn’t allow creating new operators. The general form of operator function is:- return_type operator #(arg_list) { } return_type is the data type of the value returned from the function. The arg_list is the list of the arguments to the function. The operator comes after the keyword operator. Instead of # there will be an operator. Here is a program to show operator overloading. #includeiostream using namespace std; class rectangle { public: int length; int breadth; rectangle(int length1,int breadth1) { length=length1; breadth=breadth1; } int operator+(rectangle r1) { return(r1.length+length); } }; int main () { rectangle r1(10,20); rectangle r2(40,60); int len; len=r1+r2; cout The total length of the two rectangles is : len endl;
- 129. VISUAL C++ PROGRAMMING VC++ [email protected] 129 return(0); } The result of the program is:- The program consists of operator + function which is overloaded. The + operator is used to perform addition of the length of the objects. The statements int operator+(rectangle r1) { return(r1.length+length); } define the operator function whose return type is integer. The operator + is overloaded. The function is used to add the lengths of the two objects. The parameter list consists of one object of type rectangle. The operator()+ takes only one parameter as the operand on the left side of the operator + is passed implicitly to the function through the this operator. The statement len=r1+r2; calls the operator()+ function to calculate the length of the two objects. The return type is of integer. The variable len will contain the total of length of the objects. Virtual Functions A virtual function is a member function of the base class and which is redefined by the derived class. When a derived class inherits the class containing the virtual function, it has ability to redefine the virtual functions. A virtual function has a different functionality in the derived class. The virtual function within the base class provides the form of the interface to the function. Virtual function implements the philosophy of one interface and multiple methods. The virtual functions are resolved at the run time. This is called dynamic binding. The functions which are not virtual are resolved at compile time which is called static binding. A virtual function is created using the keyword virtual which precedes the name of the function. Virtual functions are accessed using a base class pointer. A pointer to the base class can be created. A base class pointer can contain the address of the derived object as the derived object contains the subset of base class object. Every derived class is also a base class. When a base class pointer contains the address of the derived class object, at runtime it is decided which version of virtual function is called depending on the type of object contained by the pointer. Here is a program which illustrates the working of virtual functions. includeiostream using namespace std;
- 130. VISUAL C++ PROGRAMMING VC++ [email protected] 130 class shape { public: int side; virtual int volume() { cout endl Virtual function of base class endl; return(0); } }; class cube: public shape { public: int volume() { cout endl Volume function of cube endl; return(side*side*side); } }; class cuboid:public shape { public: int breadth; int height; int volume() { cout endl Volume function of cuboid endl; return(side*breadth*height); } }; int main() { shape *s,s1; cube c1; cuboid c2; cout Enter the side of the cube endl; cin c1.side; cout endl Enter the side of the cuboid endl; cin c2.side; cout endl Enter the breadth of the cuboid endl; cin c2.breadth; cout endl Enter the height of the cuboid endl; cin c2.height;
- 131. VISUAL C++ PROGRAMMING VC++ s=s1; s-volume(); s=c1; cout endl The volume of the cube s-volume() endl; s=c2; cout endl The volume of the cuboid s-volume() endl; return(0); [email protected] 131 } The result of the program is:- The program has base class shape and class cube and cuboid which inherits base class. The base class has virtual function volume. The statements virtual int volume() { cout endl Virtual function of base class endl; return(0); } define the member function volume() of base class. The function is virtual. The keyword virtual precedes the function return type. In the following statements int volume() { cout endl Volume function of cube endl; return(side*side*side); } the derived class cube which inherits base class shape, redefines the function volume() which is virtual. In the following statements
- 132. VISUAL C++ PROGRAMMING VC++ [email protected] 132 int volume() { cout endl Volume function of cuboid endl; return(side*breadth*height); } the derived class cuboid which inherits the base class shape, redefines the function volume(). The statement shape *s,s1; declares a pointer to the base class shape. The statement s=s1; pointer s contains the address of the base class object s1. The statement s-volume(); calls the function volume of the base class shape as the pointer contains the address of the base class object. The statement s=c1; pointer s now contains the address of the derived class object c1. The statement cout endl The volume of the cube s-volume() endl; displays the volume of the cube as s-volume() calls the function volume of the derived class cube. The pointer s contains the address of the object of derived class cube. In the statement s=c2; pointer s now contains the address of the derived class object c2. The statement cout endl The volume of the cuboid s-volume() endl; displays the volume of the cuboid as s-volume() calls the function volume of the derived class cuboid. The pointer s now contains the address of the object of derived class cuboid. When a virtual function is not defined by the derived class, the version of the virtual function defined by the base class is called. When a derived class which contains the virtual function is inherited by another class, virtual function can be overloaded for the new derived class. This means that virtual function can be inherited. A pure virtual function is a function which contains no definition in the base class. The general form of virtual function is:-
- 133. VISUAL C++ PROGRAMMING VC++ virtual return_type function_name(para_list)=0; The return_type is the type of the value returned. The function_name is the name of the function and para_list is the parameter list. Each derived class should contain the definition of virtual functions. If the derived class does not define virtual function then compiler will generate an error. A class which contains one or more pure virtual function is called abstract class. Objects of abstract class cannot be created as it does not contain definition of one or more functions. The pointers of abstract class can be created and the references to the abstract class can be made. Here is a same program which shows how pure virtual function is defined. [email protected] 133 #includeiostream using namespace std; class shape { public: int side; virtual int volume()=0; }; class cube: public shape { public: int volume() { cout endl Volume function of cube endl; return(side*side*side); } }; class cuboid:public shape { public: int breadth; int height; int volume() { cout endl Volume function of cuboid endl; return(side*breadth*height); } }; int main() { shape *s;
- 134. VISUAL C++ PROGRAMMING VC++ cube c1; cuboid c2; cout Enter the side of the cube endl; cin c1.side; cout endl Enter the side of the cuboid endl; cin c2.side; cout endl Enter the breadth of the cuboid endl; cin c2.breadth; cout endl Enter the height of the cuboid endl; cin c2.height; s=c1; cout endl The volume of the cube s-volume() endl; s=c2; cout endl The volume of the cuboid s-volume() endl; return(0); [email protected] 134 } The result of the program is:- The statement virtual int volume()=0; declares pure virtual function volume() which has no definition. The class shape is now abstract class and objects of type shape cannot be created.
- 135. VISUAL C++ PROGRAMMING VC++ Unit 9: Working with files (25 HOURS) [email protected] 135 THEORY At the end of the unit the student should be able to: 1. ..employ classes for file stream operators 2. ..open and close a file 3. ..detect an end-of-line 4. ..distinguish between file modes 5. ..know about file pointers and their manipulators 6. ..employ sequential input and output operations 7. ..to update a file: random access 8. ..take care of error handling during file operations 9. ..use command-line arguments Input/Output with files C++ provides the following classes to perform output and input of characters to/from files: • ofstream: Stream class to write on files • ifstream: Stream class to read from files • fstream: Stream class to both read and write from/to files. These classes are derived directly or indirectly from the classes istream, and ostream. We have already used objects whose types were these classes: cin is an object of class istream and cout is an object of class ostream. Therfore, we have already been using classes that are related to our file streams. And in fact, we can use our file streams the same way we are already used to use cin and cout, with the only difference that we have to associate these streams with physical files. Let's see an example: // basic file operations #include iostream #include fstream using namespace std; int main () { ofstream myfile; myfile.open (example.txt); myfile Writing this to a file.n; myfile.close(); return 0; } [file example.txt]
- 136. VISUAL C++ PROGRAMMING VC++ [email protected] 136 Writing this to a file This code creates a file called example.txt and inserts a sentence into it in the same way we are used to do with cout, but using the file stream myfile instead. But let's go step by step: Open a file The first operation generally performed on an object of one of these classes is to associate it to a real file. This procedure is known as to open a file. An open file is represented within a program by a stream object (an instantiation of one of these classes, in the previous example this was myfile) and any input or output operation performed on this stream object will be applied to the physical file associated to it. In order to open a file with a stream object we use its member function open(): open (filename, mode); Where filename is a null-terminated character sequence of type const char * (the same type that string literals have) representing the name of the file to be opened, and mode is an optional parameter with a combination of the following flags: ios::in Open for input operations. ios::out Open for output operations. ios::binary Open in binary mode. ios::ate Set the initial position at the end of the file. If this flag is not set to any value, the initial position is the beginning of the file. ios::app All output operations are performed at the end of the file, appending the content to the current content of the file. This flag can only be used in streams open for output-only operations. ios::trunc If the file opened for output operations already existed before, its previous content is deleted and replaced by the new one. All these flags can be combined using the bitwise operator OR (|). For example, if we want to open the file example.bin in binary mode to add data we could do it by the following call to member function open(): ofstream myfile; myfile.open (example.bin, ios::out | ios::app | ios::binary); Each one of the open() member functions of the classes ofstream, ifstream and fstream has a default mode that is used if the file is opened without a second argument: class default mode parameter ofstream ios::out
- 137. VISUAL C++ PROGRAMMING VC++ [email protected] 137 ifstream ios::in fstream ios::in | ios::out For ifstream and ofstream classes, ios::in and ios::out are automatically and respectivelly assumed, even if a mode that does not include them is passed as second argument to the open() member function. The default value is only applied if the function is called without specifying any value for the mode parameter. If the function is called with any value in that parameter the default mode is overridden, not combined. File streams opened in binary mode perform input and output operations independently of any format considerations. Non-binary files are known as text files, and some translations may occur due to formatting of some special characters (like newline and carriage return characters). Since the first task that is performed on a file stream object is generally to open a file, these three classes include a constructor that automatically calls the open() member function and has the exact same parameters as this member. Therefor, we could also have declared the previous myfile object and conducted the same opening operation in our previous example by writing: ofstream myfile (example.bin, ios::out | ios::app | ios::binary); Combining object construction and stream opening in a single statement. Both forms to open a file are valid and equivalent. To check if a file stream was successful opening a file, you can do it by calling to member is_open() with no arguments. This member function returns a bool value of true in the case that indeed the stream object is associated with an open file, or false otherwise: if (myfile.is_open()) { /* ok, proceed with output */ } Closing a file When we are finished with our input and output operations on a file we shall close it so that its resources become available again. In order to do that we have to call the stream's member function close(). This member function takes no parameters, and what it does is to flush the associated buffers and close the file:
- 138. VISUAL C++ PROGRAMMING VC++ [email protected] 138 myfile.close(); Once this member function is called, the stream object can be used to open another file, and the file is available again to be opened by other processes. In case that an object is destructed while still associated with an open file, the destructor automatically calls the member function close(). Text file streams are those where we do not include the ios::binary flag in their opening mode. These files are designed to store text and thus all values that we input or output from/to them can suffer some formatting transformations, which do not necessarily correspond to their literal binary value. Data output operations on text files are performed in the same way we operated with cout: // writing on a text file #include iostream #include fstream using namespace std; int main () { ofstream myfile (example.txt); if (myfile.is_open()) { myfile This is a line.n; myfile This is another line.n; myfile.close(); } else cout Unable to open file; return 0; } [file example.txt] This is a line. This is another line. Data input from a file can also be performed in the same way that we did with cin: // reading a text file #include iostream #include fstream #include string using namespace std; int main () { string line;
- 139. VISUAL C++ PROGRAMMING VC++ ifstream myfile (example.txt); if (myfile.is_open()) { while (! myfile.eof() ) { [email protected] 139 getline (myfile,line); cout line endl; } myfile.close(); } else cout Unable to open file; return 0; } This is a line. This is another line. This last example reads a text file and prints out its content on the screen. Notice how we have used a new member function, called eof() that returns true in the case that the end of the file has been reached. We have created a while loop that finishes when indeed myfile.eof() becomes true (i.e., the end of the file has been reached). Checking state flags In addition to eof(), which checks if the end of file has been reached, other member functions exist to check the state of a stream (all of them return a bool value): bad() Returns true if a reading or writing operation fails. For example in the case that we try to write to a file that is not open for writing or if the device where we try to write has no space left. fail() Returns true in the same cases as bad(), but also in the case that a format error happens, like when an alphabetical character is extracted when we are trying to read an integer number. eof() Returns true if a file open for reading has reached the end. good() It is the most generic state flag: it returns false in the same cases in which calling any of the previous functions would return true. In order to reset the state flags checked by any of these member functions we have just seen we can use the member function clear(), which takes no parameters. get and put stream pointers
- 140. VISUAL C++ PROGRAMMING VC++ All i/o streams objects have, at least, one internal stream pointer: ifstream, like istream, has a pointer known as the get pointer that points to the element to be read in the next input operation. ofstream, like ostream, has a pointer known as the put pointer that points to the location where the next element has to be written. Finally, fstream, inherits both, the get and the put pointers, from iostream (which is itself derived from both istream and ostream). These internal stream pointers that point to the reading or writing locations within a stream can be manipulated using the following member functions: tellg() and tellp() These two member functions have no parameters and return a value of the member type pos_type, which is an integer data type representing the current position of the get stream pointer (in the case of tellg) or the put stream pointer (in the case of tellp). seekg() and seekp() These functions allow us to change the position of the get and put stream pointers. Both functions are overloaded with two different prototypes. The first prototype is: [email protected] 140 seekg ( position ); seekp ( position ); Using this prototype the stream pointer is changed to the absolute position position (counting from the beginning of the file). The type for this parameter is the same as the one returned by functions tellg and tellp: the member type pos_type, which is an integer value. The other prototype for these functions is: seekg ( offset, direction ); seekp ( offset, direction ); Using this prototype, the position of the get or put pointer is set to an offset value relative to some specific point determined by the parameter direction. offset is of the member type off_type, which is also an integer type. And direction is of type seekdir, which is an enumerated type (enum) that determines the point from where offset is counted from, and that can take any of the following values: ios::beg offset counted from the beginning of the stream ios::cur offset counted from the current position of the stream pointer
- 141. VISUAL C++ PROGRAMMING VC++ ios::end offset counted from the end of the stream The following example uses the member functions we have just seen to obtain the size of a file: [email protected] 141 // obtaining file size #include iostream #include fstream using namespace std; int main () { long begin,end; ifstream myfile (example.txt); begin = myfile.tellg(); myfile.seekg (0, ios::end); end = myfile.tellg(); myfile.close(); cout size is: (end-begin) bytes.n; return 0; } size is: 40 bytes. Random access files We use two new functions which are write(), a member of ofstream and read(), a member of the ifstream. The pararmeters of read() and write() are the data or variable and its length. Their prototypes are: write ( memory_block, size ); read ( memory_block, size ); // Files.cpp : Defines the entry point for the console application. // #include stdafx.h #include fstream.h #include iostream.h ofstream dout;
- 142. VISUAL C++ PROGRAMMING VC++ [email protected] 142 int main() { dout.open(stud.txt, ios::out); char text[5]; coutEnter textendl; cintext; dout.write(text,sizeof(text)); dout.close(); return 0; } Binary files In binary files, to input and output data with the extraction and insertion operators ( and ) and functions like getline is not efficient, since we do not need to format any data, and data may not use the separation codes used by text files to separate elements (like space, newline, etc...). File streams include two member functions specifically designed to input and output binary data sequentially: write and read. The first one (write) is a member function of ostream inherited by ofstream. And read is a member function of istream that is inherited by ifstream. Objects of class fstream have both members.; Where memory_block is of type pointer to char (char*), and represents the address of an array of bytes where the read data elements are stored or from where the data elements to be written are taken. The size parameter is an integer value that specifies the number of characters to be read or written from/to the memory block. // reading a complete binary file #include iostream #include fstream using namespace std; ifstream::pos_type size; char * memblock;
- 143. VISUAL C++ PROGRAMMING VC++ int main () { ifstream file (example.txt, ios::in|ios::binary|ios::ate); if (file.is_open()) { size = file.tellg(); memblock = new char [size]; file.seekg (0, ios::beg); file.read (memblock, size); file.close(); cout the complete file content is in memory; delete[] memblock; } else cout Unable to open file; return 0; } the complete file content is in memory In this example the entire file is read and stored in a memory block. Let's examine how this is done: First, the file is open with the ios::ate flag, which means that the get pointer will be positioned at the end of the file. This way, when we call to member tellg(), we will directly obtain the size of the file. Notice the type we have used to declare variable size: [email protected] 143 ifstream::pos_type size; ifstream::pos_type is a specific type used for buffer and file positioning and is the type returned by file.tellg(). This type is defined as an integer type, therefore we can conduct on it the same operations we conduct on any other integer value, and can safely be converted to another integer type large enough to contain the size of the file. For a file with a size under 2GB we could use int: int size; size = (int) file.tellg(); Once we have obtained the size of the file, we request the allocation of a memory block large enough to hold the entire file: memblock = new char[size]; Right after that, we proceed to set the get pointer at the beginning of the file (remember that we opened the file with this pointer at the end), then read the entire file, and finally close it: file.seekg (0, ios::beg);
- 144. VISUAL C++ PROGRAMMING VC++ [email protected] 144 file.read (memblock, size); file.close(); At this point we could operate with the data obtained from the file. Our program simply announces that the content of the file is in memory and then terminates. Buffers and Synchronization When we operate with file streams, these are associated to an internal buffer of type streambuf. This buffer is a memory block that acts as an intermediary between the stream and the physical file. For example, with an ofstream, each time the member function put (which writes a single character) is called, the character is not written directly to the physical file with which the stream is associated. Instead of that, the character is inserted in that stream's intermediate buffer. When the buffer is flushed, all the data contained in it is written to the physical medium (if it is an output stream) or simply freed (if it is an input stream). This process is called synchronization and takes place under any of the following circumstances: • When the file is closed: before closing a file all buffers that have not yet been flushed are synchronized and all pending data is written or read to the physical medium. • When the buffer is full: Buffers have a certain size. When the buffer is full it is automatically synchronized. • Explicitly, with manipulators: When certain manipulators are used on streams, an explicit synchronization takes place. These manipulators are: flush and endl. • Explicitly, with member function sync(): Calling stream's member function sync(), which takes no parameters, causes an immediate synchronization. This function returns an int value equal to -1 if the stream has no associated buffer or in case of failure. Otherwise (if the stream buffer was successfully synchronized) it returns 0. Details of file I/O seem to be buried at the back, missing, or overly complicated in most C++ manuals. This page provides a quick reference for the most frequently used methods. This page only discusses basic options that seem to be common to all my C++ references. Apparently there is a lot of variation from one manual to another, and from one implementation to another. I believe the methods below to be safe and portable, at least for ASCII (human-readable text) files. Header files
- 145. VISUAL C++ PROGRAMMING VC++ To do input and output, you will need to load the iostream header file. You may also need to load the fstream (file I/O) and/or iomanip (format manipulation) header files. Put some/all of the following lines at the top of your code file (or in the header file for your program, if you are using one). #include iostream.h // I/O #include fstream.h // file I/O #include iomanip.h // format manipulation [email protected] 145 Getting a stream Three streams just exist: cout (terminal output), cin (terminal input), and cerr (error output, which also goes to the terminal). When writing error messages, use cerr rather than cout. In simple examples, the two appear to be the same. However, they behave differently when you use Unix redirection operators when you run your program. In particular, if you redirect output to a file, error messages printed to cerr will still appear on the user's terminal whereas error messages printed to cout will be mixed into the output file. File streams are of type ifstream (input) or ofstream (output). ifstream fp_in; // declarations of streams fp_in and fp_out ofstream fp_out; fp_in.open(myfile.txt, ios::in); // open the streams fp_out.open(myfile.txt, ios::out); fp_in.close(); // close the streams fp_out.close(); A file should be closed if you are done with it, but the program will continue running for a while longer. This is particularly important when you intend to open a lot of files, as there may be a limit on how many you can have open at once. It is also a good idea if you intend to open a file for input, and then re-open the file for output. Declaring the pointer and opening the file can be combined: ifstream fp_in(myfile.txt, ios::in); // declare and open The parameters ios::in and ios::out specify the mode in which the file is to be opened.
- 146. VISUAL C++ PROGRAMMING VC++ Depending on the implementation of C++, it may be possible to specify a variety of other mode options, such as appending to the end of an existing file, triggering an error rather than overwriting an existing file, or specifying that a file is binary for operating systems (e.g. MS-DOS) which distinguish binary and ASCII files. [email protected] 146 Passing streams to functions File streams must be passed to functions by reference, not by value. void myfunction(ifstream fp, ...) // use this void myfunction(ifstream fp, ...) // not this If you pass streams by value, the C++ compiler will not complain. However, mysterious bad things will start happening, often in parts of the code which don't appear to be related to the offending function. Item by item input and output If each input item is surrounded by whitespace (blanks, tabs, newlines), the items can be read easily using the extraction operator . int myinteger; // declarations float myfloat; char mychar; char *mystring; // two ways to declare a string char mystring[64]; fp_in myinteger; // input from file pointer or standard input cin myfloat; fp_in mychar; cin mystring; The extraction operator works for numbers (ints, floats), characters (char), and strings (declared as arrays of type char or pointers to type char). The extraction operator returns a zero value if it encounters a problem (typically, the end of the file). Therefore, it can be used as the test in an if statement or a while loop. WARNING: when reading data into a character string, bad things will happen if the input word is longer than your string. To avoid problems, use the operator setw to force excessively long input to be broken up. (You must include the iomanip header file.) The input to setw should be the length of your string (including the null character '0' at the end of the string).
- 147. VISUAL C++ PROGRAMMING VC++ cin setw(length) mystring; Numbers, characters, and strings can be written to a file, standard output, or the standard error using the insertion operator . cout Value of myinteger myinteger endl; cout My string is mystring plus a null charactern flush; To insert a line break, either insert the magic variable endl or write the end-of-line character ('n') to the output. To make a pointer print out as a pointer, not as whatever type of data it points to, cast it to the type (void *). To make a character print as a number, cast it to type int. Similarly, you can use a cast to convince C++ to print an integer as the corresponding character. [email protected] 147 cout (void *)ptr; cout (int)ch; cout (char)ival; Buffering and flush When you send output to a stream, it does not necessarily get printed immediately. Rather, it may wait in a buffer until some unspecified event, e.g. buffer full enough, reading from input, or exit from program. The details may vary. Buffering makes it faster to print a large amount of output, particularly if you are producing the output bit-by-bit (e.g. one character at a time). However, it is a nuisance for output used in debugging (e.g. a statement that informs you that step 3 of your algorithm has been finished). Forcing all buffered output to actually be printed is known as flushing the stream. A flush can be forced by calling the flush function associated with each output stream, inserting the magic variable flush into the stream, or inserting endl. cout flush; cout.flush(); cout endl; Other input operations
- 148. VISUAL C++ PROGRAMMING VC++ All of the following are illustrated using the standard input, but they work just the same on file streams. [email protected] 148 cin.get(char ch) Puts the next input character in the variable ch. Returns an integer value, which is zero if it encountered a problem (e.g. end of file). cin.getline(char *buffer, int length) Reads characters into the string buffer, stopping when (a) it has read length-1 characters or (b) when it finds an end-of-line character ('n') or the end of the file. Stores a null character ('0') after the last character read. cin.read(char *buffer, int n) Reads n bytes (or until the end of the file) from the stream into the buffer. cin.gcount() Returns the number of characters read by a preceding get, getline, or read command. cin.ignore(int n) Remove the next n characters (or until end of file) from the input stream, throwing them away into the Great Bit Bucket. cin.putback(char ch) Puts character ch back onto the stream. Bad things will happen if this character is not the one most recently extracted from the stream. These operations all return zero if something goes wrong, e.g. they hit the end of the file. Therefore, they can be used as the condition in an if statement or while loop. In addition, there are two more input operations, get and peek. These functions return EOF (which is secretly -1) if they encounter the end of the file. The output of these functions should be put into an integer (not a char) variable. cin.get() Returns the next character in the stream. cin.peek() Returns the next character in the stream but does not remove it from the stream. The following functions can be used to test the status of a stream. They return an integer, which is either zero or non-zero. cin.good() Returns 0 if the stream has encountered problems such as reading the end of file, non-existent file. cin.bad() Returns non-zero value if the stream is totally unusable, e.g. the file cannot be opened (but not if the stream has merely hit the end of the file). cin.eof()
- 149. VISUAL C++ PROGRAMMING VC++ Returns a non-zero value if the stream has hit the end of the file. Notice that the stream's status will change to bad (not good, eof, etc) after the first read request which encounters a problem. So, to use one of these functions, you attempt to do what you wanted to do (e.g. open the file, read the next number from the file, ...). If the action can't succeed, the program won't crash, though some of your variables might not contain the values you intended. Next, use the status function to check whether the action succeeded. [email protected] 149 Other output operations Other options for writing data to an output stream are: cout.put(char ch) Puts ch onto the stream. cout.write(char *str, int n) Puts n characters onto the stream, reading them from the string str. The function setf can be used to change formatting parameters for an output stream. For example, the following causes numbers to be left justified. cout.setf(ios::left); set option cout.unsetf(ios::left); unset option The most obviously useful parameters are: ios::left Left justify output. ios::right Right justify output. ios::scientific Print floating point numbers using scientific notation. ios::fixed Print floating point numbers using fixed point notation. ios::showpoint Print a decimal point for all floating point numbers, even when it's not needed (e.g. the number is exactly an integer). The precision of numbers can be changed as follows. You can also set the width, i.e. the minimum number of spaces used to print the next. These featuers are used, for example, to make output items line up in columns when printed. Both of these features require that you include the iomanip header file. cout setprecision(2); print two digits after decimal point cout.precision(2); an alternative syntax
- 150. VISUAL C++ PROGRAMMING VC++ cout setw(8); make item occupy 8 characters cout.width(8); an alternative syntax Setting the width to k forces the item to occupy at least k characters. If its printed representation is shorter, blanks will be added so that it occupies k characters. If its printed representation is longer, it will occupy more than k characters. When you reset parameters such as the precision, it is not clear whether the new value lasts only for the next item printed or whether it persists for subsequent values. Apparently this varies with the parameter and the C++ compiler. Therefore, if you care about the value of some formatting parameter, explicitly set it to the right thing before you output each item. [email protected] 150 Repositioning and error states In general, it is possible to move to any position in a file stream. This is a capability most programmers use rarely, if at all. For fairly obvious reasons, don't try to use repositioning operations on the standard input, output, or error streams. The most likely reason for using a repositioning command is to rewind a file to the start, so that you can read its contents again. This is done as follows: fp_in.clear(); // forget we hit the end of file fp_in.seekg(0, ios::beg); // move to the start of the file Data Conversion The following functions convert data types: atof() converts an ASCII character to a double (to float in c++) atoi() converts an ASCII character to an integer atol() converts an ASCII to a long integer itoa() converts an integer to a character array Example: /* convert a string to an integer */ #include stdio.h #include stdlib.h char string[] = “1234”;
- 151. VISUAL C++ PROGRAMMING VC++ [email protected] 151 int main(void) { int sum; sum = atoi( string); printf(“Sum = %dn”, sum); return 0; } Another example: /* convert a string to an integer */ #include stdio.h #include stdlib.h int main(void) // remember that the default return data is int { int sum; char buff[20]; printf(“Enter in an integer “); scanf (“%d”, sum); printf(“As a string it is %sn”, itoa(sum, buff, 10)); return 0; } Note that itoa() takes three parameters: • The integer to be converted • A character buffer into which the resultant string is stored • A radix value (10=decimal, 16=hexadecimal); Built in functions for character handling The following character handling functions are defined in ctype.h isalnum tests for alphanumeric character isalpha tests for alphabetic character isascii tests for ASCII character iscntrl tests for control character isdigit tests for 0 to 9 isgraph tests for printable character islower tests for lowercase isprint tests for printable character ispunct tests for punctuation character isspace tests for space character isupper tests for uppercase character isxdigit tests for hexadecimal toascii converts character to ASCII code tolower converts character to lowercase toupper converts a character to uppercase
- 152. VISUAL C++ PROGRAMMING VC++ To convert a string array to uppercase one character at a time using touper() [email protected] 152 #include stdio.h #include ctype.h int main(void) { char name[80]; int loop; printf(“Enter in a name in lowercasen”); scanf(“%s”, name); for (loop = 0; name[loop] !=0; loop++) name[loop] = toupper(name[loop]); printf(“The name in uppercase is |%sn”, name); return 0; } Sample program output Enter in a name in lowercase samuel The name In uppercase in SAMUEL Don't worry if this lesson seems too complicated. Pay attention only to the keywords that are summarized in the bullet points on top of this page. A new section is under construction which will list all the standard libraries (as defined by ASCII) and will sort all keywords alphabetically and by library. Pointers What is a Pointer? So what is a pointer? A pointer is a way to get at another object. Essentially it is a way to grab an instance of an object and then either pass that instance a message or retreive some data from that object. A pointer is actually just an address of where an instance is held in memory. Some piece of your program can either possess an instance of an object, or know where an instance of an object is. An instance of an object is a chunk of memory that is big enough to store all the member data of that object. A pointer is an address that explains how to get to where the instance is actually held in memory. Here's a quick example: Our Player object has three pieces of data that it owns: strength, agility, and health. These are a part of the player object. That makes sense in real world terms. The player knows about two
- 153. VISUAL C++ PROGRAMMING VC++ other pieces of data: the weapon and the armor that the player possesses. Here's a diagram for an instance of the player object. So that is how to conceptually think of pointers. Now what's really going on? Memory in a computer is a complicated thing, but let's reduce it to it's simplest form: one large string of slots with addresses that data can be put in. As in the following picture: If we were to access the spot in memory with address 3, we would get the value 45. If we were to access the spot in memory with address 2 we would get the value Dave. The previous diagram over simplifies two important concepts, however. First, each spot that has an address is the same size as every other spot. Second, what's held in memory is simply data. That is, the information there is just a string of binary data, 1's and 0's. The way that the data is viewed gives it meaning. With these new ideas in mind let's Take a look at our previous diagram about the player and see what's really going on. Here is the pseudo code for the Player in a little more detail than we have seen: [email protected] 153 class Player { // attributes int health; int strength; int agility; // pointers to weapons and armor Pointer weapon; Pointer armor;
- 154. VISUAL C++ PROGRAMMING VC++ // actions that a player knows how to perform void move(); void attackMonster(); void getTreasure(); }; What follows could be a view of memory. This will seem fairly complex and is not imperative to understanding the c++ language. If it does not make sense right away, come back to it. I have translated the actual binary values to their corresponding values in the context of our program. In this view an instance of a Player is stored at memory address 4098. The first attribute (instance variable) is there: health is 12. The second attribute is strength, 14. Then agility 16. The next attribute, stored at address 4104 is a pointer to an instance of a weapon object. If we follow that to the bottom of thediagram, there is the name of the weapon, club stored as ASCII values at address 6144 through address 6148 (the 0 ends the string). The next attribute for a weapon after name is rating, in this case 2, which means poor because I have decided that the rating system should be: [email protected] 154 1 = very poor 2 = poor 3 = average 4 = good 5 = very good Notice that all the memory contains is binary data. It is how that data is viewed that actually gives it meaning. Pointers Simplified constant pointer dereferencing operator expanded memory
- 155. VISUAL C++ PROGRAMMING VC++ [email protected] 155 extended memory pointer What You'll Learn • Inside memory • Pointer variables • The marriage of arrays and pointers • Pointers to characters • Arrays of pointers Definition Pointer variables point to other variables. This unit takes you into the world of advanced Visual C++ programming by teaching you about pointer variables. Be warned: At first, pointer variables seem to add more work without offering any advantages. Be patient! Before this unit ends, you'll see how pointers let you write programs that manipulate string data in arrays. Until you learn about pointers, you can't keep track of string data using an array. Pointer variables are often just called pointers. Although pointers are considered advanced, the true power of Visual C++ doesn't appear until you master pointers. Many programming languages, such as COBOL, FORTRAN, and QBasic, don't support pointers. Those languages can't provide for the advanced data management that you'll read about in Lesson 9. Enough convincing! Let's get to the point. Inside Memory To understand pointers, you must understand computer memory. Definition RAM stands for random access memory. The memory inside your PC, often called RAM, holds your program, data, and operating system.
- 156. VISUAL C++ PROGRAMMING VC++ There are really two types of memory if you count the disk drive. The disk, however, is long-term memory. Your PC can't process any program or data on the disk until you or another program reads the disk contents into RAM. To keep things simple, RAM will be called memory throughout the rest of this book. ROM, a special kind of read-only memory, isn't discussed here. Your computer can process only data stored in memory. Therefore, your computer's memory is vital and closely tied to your computer's processor. Until now, you didn't need to know much about memory to run Visual C++ programs. However, a thorough understanding of pointers isn't possible until you really understand the makeup of your computer's memory storage. Figure 16.1 shows an overview of memory as viewed by Visual C++. When you run Windows or OS/2, you'll have to give up even more memory for your operating environment, but usually there's still enough memory left to write virtually any program you need. [email protected] 156 Your PC's memory layout. The Address column in Figure 16.1 is very important for this unit. Each memory location in your PC has a different address, just as your house has an address different from the rest of the world. The PC's addresses are sequential numbers that begin at 0 and increment at each memory location through the highest memory location. When you define a variable, Visual C++ finds the next empty address in the data area and sticks the variable at that location. Visual C++ keeps track, through an internal table, of where each variable resides. As you already know, Visual C++'s data types consume different amounts of memory, so variables are rarely stored in back-to-back addresses. Integers take two bytes, long integers take four, and so on. After the variable definitions main() { int i = 19; char c = 'X'; float f = 3.453; } Visual C++ would store the data in memory to look something like the memory in Figure 16.2. Of course, the variables probably wouldn't be placed exactly at the addresses in the figure, but the important thing to note is that each variable starts at a given address and different data types take different amounts of memory.
- 157. VISUAL C++ PROGRAMMING VC++ Three defined variables in memory. Aren't variable names great? Without variable names, you would have to remember the address of every data value your program worked with. Each memory location in your computer contains a specific address. When you define variables, Visual C++ finds unused memory locations and stores the variables there. When you refer to a variable by name, Visual C++ finds that variable's address and works with the value stored there. There is no Stop and Type section here due to this section's conceptual nature. [email protected] 157