Abstract class

 If we can not create any objects of a class, then it is called an abstract class. A class is made abstract by adding at least one pure virtual function to it.

Pure virtual function

A function is said to be a pure virtual function, if it has no definition but has only declaration.

Pure virtual function is defined with the keyword virtual and followed by return type, function name and "=0".

class Shape
{
public:
 virtual void printarea()=0;
};
int main()
{
 Shape obj1;//error
}

Here printarea() function of Shape class is a pure virtual function as it has no body.

To make a function as pure virtual function, you should use =0 at the end of virtual function declaration

Abstract class

When a class has at least one pure virtual function, it is incomplete and no objects can be created from that class. Such a class is called an abstract class.

In the earlier example class Shape is an abstract class, and objects can not be created from that class. And the code  produces compiler error.

Abstract classes are used to enforce some interfaces on their derived classes

Any derived class of abstract class MUST define pure virtual function/s. If not, these derived class also becomes abstract classes.

Let us derive two classes from our previous abstract class A.

class Shape
{
public:
 virtual void printarea()=0;
};
class Circle:public Shape

  {
public:
 void printarea()
 {
 cout<<"area of a circle is pi*r*r";
 }
};
class Rectangle:public Shape
{};
int main()
{
 Shape obj1;//error
 Circle obj2;//valid
 Shape *ptr;/*valid. Object of A is not created*/
 ptr = new Circle;//OK
 Rectangle obj3;//error
}

In the example, Shape is an abstract class. It has two sub-classes - Circle and Rectangle. Circle has defined the pure virtual function print(). But Rectangle class has not. Which means that Circle is a normal class, but Rectangle is an abstract class. And we can see that creating an object of Shape and Rectangle will produce syntax error.

Also note that, in the code above, Shape obj1 produces error, but not Shape *ptr. Because by defining ptr as a pointer of class Shape, we have not yet created any object. And ptr may point to any of the  derived classes of Shape, which might be non-abstract.  

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