Object Oriented Programming (OOP) using C++ - Lecture 4

Object Oriented
Programming using C++
By Mohamed Gamal
© Mohamed Gamal 2024

The topics of today’s lecture:
Agenda

#include <iostream>
#include <cstring> //for strcpy(), etc
using namespace std;
class String
{
private:
char *str; //pointer to string
public:
String(char *s) //constructor, one arg
{
int length = strlen(s); //length of string argument
str = new char[length + 1]; //get memory
strcpy(str, s); //copy argument to it
}
~String() //destructor
{
cout << "Deleting str.n";
delete[] str; //release memory
}
void display() //display the String
{
cout << str << endl;
}
};
int main()
{
String s1 = "Who knows nothing doubts nothing.";
cout << "s1 = "; //display string
s1.display();
return 0;
}
A String class using
new and delete

#include <iostream>
class Distance
{
private:
int feet;
float inches;
public:
void getdist() {
cout << "nEnter feet: "; cin >> feet;
cout << "Enter inches: "; cin >> inches;
}
void showdist() {
cout << feet << "' - " << inches << '"';
}
};
int main()
{
Distance dist; //define a named Distance object
dist.getdist(); //access object members
dist.showdist(); // with dot operator
Distance* distptr; //pointer to Distance
distptr = new Distance; //points to new Distance object
distptr->getdist(); //access object members
distptr->showdist(); // with -> operator
return 0;
}
Pointer to objects
// ok but inelegant
(*distptr).getdist();
// create Distance object (access with dot)
Distance& dist = *(new Distance);

#include <iostream>
struct link //one element of list
{
int data; //data item
link *next; //pointer to next link
};
class linklist //a list of links
{
private:
link *first; //pointer to first link
public:
linklist() //no-argument constructor
{
first = NULL; //no first link
}
void additem(int d); //add data item (one link)
void display(); //display all links
};
void linklist::additem(int d) //add data item
{
link *newlink = new link; //make a new link
newlink -> data = d; //give it data
newlink -> next = first; //it points to next link
first = newlink; //now first points to this
}
void linklist::display() //display all links
{
link *current = first; //set ptr to first link
while (current != NULL) //quit on last link
{
cout << current -> data << endl; //print data
current = current -> next; //move to next link
}
}
int main()
{
linklist li; //make linked list
//add four items to list
li.additem(25);
li.additem(36);
li.additem(49);
li.additem(64);
li.display(); //display entire list
return 0;
}
Linked List

Virtual Functions
– Virtual means existing in appearance but not in reality.
– When virtual functions are used, a program that appears to be calling a
function of one class may in reality be calling a function of a different
class.
– Polymorphism means different forms.

#include <iostream>
class Base //base class
{
public:
void show() {
cout << "Base Class.n";
}
};
class Derv1 : public Base //derived class 1
{
public:
void show() {
cout << "Derv1 Classn";
}
};
{
public:
void show() {
}
};
int main()
{
Derv1 dv1; //object of derived class 1
Base *ptr; //pointer to base class
ptr = &dv1; //put address of dv1 in pointer
ptr -> show(); //execute show()
return 0;
}
Example 1
Normal Member Functions
Accessed with Pointers

#include <iostream>
{
public:
//virtual function
virtual void show() {
cout << "Base Classn";
}
};
{
public:
void show() {
}
};
{
public:
void show() {
}
};
int main()
{
return 0;
}
Example 2
Virtual Member Functions
Accessed with Pointers
The same function call ptr -> show();
executes different functions, depending
on the contents of ptr.

Late/Dynamic Binding
– Which version of show() does the compiler call?
– In fact the compiler doesn’t know what to do, so it arranges for the decision to
be deferred until the program is running.
– At runtime, when it is known what class is pointed to by ptr, the appropriate
version of draw will be called. This is called late binding or dynamic binding.
– Choosing functions in the normal way, during compilation, is called early
binding or static binding.
– <
– Late binding requires some overhead but provides increased power and
flexibility.

Abstract Classes and Pure Virtual Functions
– When we will never want to instantiate objects of a base class, we call
it an abstract class.
– Such a class exists only to act as a parent of derived classes that will be
used to instantiate objects.
– It may also provide an interface for the class hierarchy.
– How can we can mark a class as abstract? By placing at least one pure
virtual function in the base class.
– A pure virtual function is one with the expression =0 added to the
declaration.

#include <iostream>
{
public:
virtual void show() = 0; //pure virtual function
};
{
public:
void show() {
}
};
{
public:
void show() {
}
};
int main()
{
// Base bad; //can’t make object from abstract class
ptr-> show(); //execute show()
return 0;
}
Example 1
Pure Virtual
Function
Abstract Class
=0 expression

#include <iostream>
class person //person class
{
protected:
char name[40];
public:
void getName() {
cout << " Enter name: "; cin >> name;
}
void putName() {
cout << "Name: " << name << endl;
}
virtual void getData() = 0; //pure virtual func
virtual bool isOutstanding() = 0; //pure virtual func
};
class student : public person //student class
{
private:
float gpa; //grade point average
public:
void getData() //get student data from user
{
person::getName();
cout << " Enter student's GPA: "; cin >> gpa;
}
bool isOutstanding() {
return (gpa > 3.5) ? true : false;
}
};
class professor : public person //professor class
{
private:
int numPubs; //number of papers published
public:
void getData() //get professor data from user
{
person::getName();
cout << " Enter number of professor's publications: ";
cin >> numPubs;
}
bool isOutstanding() {
return (numPubs > 100) ? true : false;
}
};
int main()
{
person *persPtr[100]; //array of pointers to persons
int n = 0; //number of persons on list
char choice;
do {
cout << "Enter student or professor (s/p): ";
cin >> choice;
if (choice == 's') //put new student
persPtr[n] = new student; // in array
else //put new professor
persPtr[n] = new professor; // in array
persPtr[n++] -> getData(); //get data for person
cout << " Enter another (y/n)? "; //do another person?
cin >> choice;
} while (choice == 'y'); //cycle until not ‘y’
//print names of all persons, and say if outstanding
for (int j = 0; j < n; j++)
{
persPtr[j] -> putName();
if (persPtr[j] -> isOutstanding())
cout << " This person is outstanding!n";
}
return 0;
}
Example 2
Pure Virtual
Function
Abstract Class
=0 expression

Friend Function
– A friend function is a function which operates on private data from
objects of two different classes.
– The function will take objects of the two classes as arguments and
operate on their private data.

#include <iostream>
class beta; //needed for frifunc declaration
class alpha
{
private:
int data;
public:
alpha() : data(3)
{ }
friend int frifunc(alpha, beta); //friend function
};
class beta
{
private:
int data;
public:
beta() : data(7)
{ }
friend int frifunc(alpha, beta); //friend function
};
int frifunc(alpha a, beta b) //function definition
{
return (a.data + b.data);
}
int main()
{
alpha aa;
beta bb;
cout << frifunc(aa, bb) << endl; //call the function
return 0;
}
Example 1
Friend Function

#include <iostream>
class Distance //English Distance class
{
private:
int feet;
float inches;
public:
Distance() : feet(0), inches(0.0)
{ }
Distance(float fltfeet) //convert float to Distance
{
feet = static_cast<int>(fltfeet); //feet is integer part
inches = 12 * (fltfeet - feet); //inches is what's left
}
Distance(int ft, float in) //constructor (two args)
{
feet = ft; inches = in;
}
void showdist() //display distance
{
cout << feet << "' - " << inches << '“’;
}
Distance operator + (Distance);
};
//add this distance to d2
Distance Distance::operator + (Distance d2) //return the sum
{
int f = feet + d2.feet; //add the feet
float i = inches + d2.inches; //add the inches
if (i >= 12.0) //if total exceeds 12.0
{
i -= 12.0;
f++;
}
return Distance(f, i); //return new Distance with sum
}
int main()
{
Distance d1 = 2.5; //constructor converts
Distance d2 = 1.25; //float feet to Distance
Distance d3;
cout << "nd1 = "; d1.showdist();
d3 = d1 + 10.0; //distance + float: OK (Distance(float fltfeet) ctor)
// d3 = 10.0 + d1; //float + Distance: ERROR
// cout << "nd3 = "; d3.showdist();
return 0;
}
Example 2
Without Friend
Function

#include <iostream>
class Distance
{
private:
int feet;
float inches;
public:
Distance() { //constructor no args
feet = 0; inches = 0.0;
}
Distance(float fltfeet) //constructor (one arg)
{ //convert float to Distance
feet = int(fltfeet); //feet is integer part
inches = 12 * (fltfeet - feet); //inches is what's left
}
Distance(int ft, float in) //constructor (two args)
{
feet = ft; inches = in;
}
void showdist() {
cout << feet << "' - " << inches << '“’;
}
friend Distance operator + (Distance, Distance); //friend
};
Distance operator + (Distance d1, Distance d2) //add d1 to d2
{
int f = d1.feet + d2.feet; //add the feet
float i = d1.inches + d2.inches; //add the inches
if (i >= 12.0) //if inches exceeds 12.0,
{
i -= 12.0;
f++;
}
return Distance(f, i); //return new Distance with sum
}
int main()
{
Distance d1 = 2.5; //constructor converts
Distance d2 = 1.25; //float-feet to Distance
Distance d3;
d3 = d1 + 10.0; //distance + float: OK
d3 = 10.0 + d1; //float + Distance: OK
return 0;
}
Example 3
Friend Function
A fix for the pervious
example

#include <iostream>
//class beta;
class alpha
{
private:
int data1;
public:
alpha() : data1(99)
{ }
friend class beta; //beta is a friend class
//friend beta; // same (requires top definition)
};
class beta
{ //all member functions can access private alpha data
public:
void func1(alpha a) { cout << "ndata1 = " << a.data1; }
void func2(alpha a) { cout << "ndata1 = " << a.data1; }
};
int main()
{
alpha a;
beta b;
b.func1(a);
b.func2(a);
return 0;
}
Friend Classes
Example
Friend Class

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