Abstract Classes and Pure Virtual Functions
Abstract class is a class that serves only as a base class from which classes are derived. No objects of an abstract base class are created. A base class that contains pure virtual functions is an abstract base class.
Pure virtual function can be defined as
virtual void show ( ) = 0 ; // pure virtual function
The equal sign here has nothing to do with assignment; the value 0 is not assigned to anything. The =0 syntax is simply how we tell the compiler that a function will be pure virtual function.
A pure virtual function is a function declared in a base class that has no definition relative to the base class. In such cases, the compiler requires each derived class to either define the function or redeclare it as a pure virtual function. Remember that a class containing pure virtual functions cannot be used to declare any objects of its own. Such classes are called abstract base classes. The main objective of an abstract base class is to provide some traits to the derived classes and to create a base pointer required for achieving runtime polymorphism. .
class Base
{
public :
virtual void display ( ) = 0 ;
} ;
class Derived1 : public Base
{
public :
void display ( )
{
cout << " derived1 " ;
}
} ;
class Derived2 : public Base
{
public :
void display ( )
{
cout << " derived2 " ;
}
} ;
int main ( )
{
Base b; // error can't make object from abstract class
Base * ar [ 2 ] ; // array of pointers to base class
Derived1 d1;
Derived2 d2;
ar [ 0 ] = & d1 ;
ar [ 1 ] = & d2 ;
ar [ 0 ] ->display ( ) ; // execute derived1's display ( )
ar [ 1 ] ->display ( ) ; // execute derived2's display ( )
return 0 ;
}
However, programmer can declare pointers to an abstract class to achieve runtime polymorphism.