Reference Parameter, Passing object by reference, constant parameter & Default parameter

BIRLA INSTITUTE OF TECHNOLOGY MESRA,
JAIPUR CAMPUS
:- TARUN TIWARI(MCA/25007/18)
:- NIKHIL AGRAWAL(MCA/25004/18)
TOPIC:- REFERENCE PARAMETERS ,Passing object by reference ,
CONSTANT PARAMETERS & DEFAULT PARAMETER

REFERENCE PARAMETERS &
CONSTANT PARAMETERS
TARUN TIWARI

REFERENCE PARAMETERS
• In call by reference method the called function does not create its own copy rather it
refers to original value only by different name i.e. reference.
• When function is called by reference then, the formal parameter become
reference or alias to the actual parameter in calling function.
• To have a function with multiple outputs, we have to use pass by
reference.

• We use &to denote a parameter that is passed by reference:
Syntax:
<type>& <variable>
Examples:
• void Increment(int& Number);
• void SumAve (double, double, double&, double&);
• The corresponding argument must be a variable.
Increment(Inc);
SumAve(2.5, y+3, sum, mean);

Example of Call By Reference
#include <iostream.h>
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=" << y << ", z="
<< z;
return 0;
}
x=2, y=6, z=14
Void duplicate (int& a, int& b, int& c)
x y z
duplicate( x , y , z )

Advantages of reference parameters
• We can return multiple values from a function
• Because a copy of the argument is not made, it is fast, even when
used with large structs or classes.
• Reference must be initialized
Disadvantages of reference parameters
• Many potential scenarios can occur
• Programs are difficult to understand sometimes
• Un-wanted Aliases

• The const modifier can be applied to formal parameter declarations
• The constant parameter received by the function can not be changed
in the body of the function.
CONSTANT PARAMETERS
• const indicates that the function may not modify the parameter

const type parameter
DECLARATION OF CONSTANT PARAMETERS
return_type FunName(const type1 parameter1, const
type2 parameter2, ..., const typeN parameterN)
{
...
}

EXAMPLE OF CONSTANT PARAMETERS
double Area( const double radius)
{
return double (3.1415 * radius * radius);
}
double area;
area = Area(5.5); area = 95.0304
double r = 7.88;
area = Area(r); area = 195.07

• If you try to change the value of the radius constant in the Area()
function, for example
radius = 2.88;
• the compiler will generate an error:
'radius': you cannot assign to a variable that is const.

PASSING OBJECTS BY
REFERENCE &
DEFAULT PARAMETERS.
NIKHIL AGRAWAL

PASSING OBJECT BY
REFERENCE
 One situation where it is absolutely mandatory to use a reference parameter is when we
pass a Stream object to a function.
 The reason is, When we extract data from a stream or insert data into a stream ,we are
changing the stream.
 As we know, if we passed the stream object by value, any changes that we made to the
stream object would be made to a copy of the stream object, not to the actual object.
 Thus whenever we need to pass a stream to a function, we must pass the stream by
reference.

Example :The following function writes an integer value to a stream
that is passed as the parameter to the function :-
Void OutputValue(ostream &out, int value){
out <<“ Value is “<< Value<<endl;
return;
}
The next line of code uses the OutputValue() to write the value k to
the stream cout :-
OutputValue(cout,k);

• Functions that output values to a stream are
useful when we need to output several values along
with labelling information from several places in a
program.
• We might use such a function to write data to the
display as well as to a log file for later examination.
• Another use would be to help debug the program.

 Example : Suppose , for example, that we wish to “watch” Sort3() function as it sorts the values :-
Void ShowValues(ostream &out , int a, int b, int c) {
Out <<“a: “<<a<<endl;
Out<<“b: “<<b<<endl;
Out<<“c: “<<c<<endl;
return;
}
//sort three numbers into ascending order
Void Sort3(int &a, int &b, int &c) {
If(a>b)
swap(a,b);
ShowValues(clog, a, b, c);
If(a>c)
swap(a,c);
If(b>c)
swap(b,c);

 We can also put the ability to pass an input stream as a parameter to a function to good
use.
 For example :
• Suppose we are writing a program to process data from a file.
• Program development and testing will go faster if the program takes the input from the keyboard.
• We can quickly test the program by typing in data, rather than creating a test file each time we
want to test the program on different data.
• However ,eventually the program will be reading data from a file.
• We do not want to write the program so that it accepts input from the keyboard and then , when the
program is complete , have to make extensive changes to the program so that it operates on a
different stream.
• We can handle this situation by writing any functions that extract input to accept as a parameter
the stream to read data from .

Program :
bool ReadValues (istream &in, int &v1, int &v2, int &v3)
{
if ( in == cin)
cout<<“ Please enter three numbers”;
if ( in >> v1 >> v2 >> v3)
return true;
else
return false;
}

DEFAULT ARGUMENTS
 A default argument is a value provided in function declaration that is automatically
assigned by the compiler if caller of the function doesn’t provide a value for the argument
with default value.
 Syntax: f _type f_name (arg1,arg2,arg3=value).
 When a default argument is placed then all the other arguments after that must be
default arguments only.

DEFAULT ARGUMENTS (EXAMPLE)
int sum(int x,int y,int z=0,int w=0)
{
return(x+y+z+w);
}
int main()
{
cout<<sum(10,15)<<endl;
cout<<sum(10,15,25)<<endl;
cout<<sum(10,15,25,30)<<endl;
return 0;
}
• OUTPUT:
25
50
80

DEFAULT ARGUMENTS
 Default arguments are overwritten when calling function provides values for them. For
example, calling of function sum(10, 15, 25, 30) overwrites the value of z and w to 25 and
30 respectively.
 During calling of function, arguments from calling function to called function are copied
from left to right. Therefore, sum(10, 15, 25) will assign 10, 15 and 25 to x, y and z.
Therefore, default value is used for w only.

DEFAULT ARGUMENTS
The default parameter method is especially useful
when one wants to set default criteria so that the
function can be called with or without parameters.
Default arguments are different from constant
arguments as constant arguments can’t be changed
whereas default arguments can be overwritten if
required.

Reference Parameter, Passing object by reference, constant parameter & Default parameter

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