Abstract Class
Abstract Class is a class which contains atleast one Pure Virtual function in it. Abstract classes are used to provide an Interface for its sub classes. Classes inheriting an Abstract Class must provide definition to the pure virtual function, otherwise they will also become abstract class.
Characteristics of Abstract Class
1. Abstract class cannot be instantiated, but pointers and refrences of Abstract class type can be created.
2. Abstract class can have normal functions and variables along with a pure virtual function.
3. Abstract classes are mainly used for Upcasting, so that its derived classes can use its interface.
4. Classes inheriting an Abstract Class must implement all pure virtual functions, or else they will become Abstract too.
Pure Virtual Functions
Pure virtual Functions are virtual functions with no definition. They start with virtual keyword and ends with = 0. Here is the syntax for a pure virtual function,
virtual void f() = 0;
Example of Abstract Class
class Base //Abstract base class
{
public:
virtual void show() = 0; //Pure Virtual Function
};
class Derived:public Base
{
public:
void show()
{
cout << "Implementation of Virtual Function in Derived class";
}
};
int main()
{
Base obj; //Compile Time Error
Base *b;
Derived d;
b = &d;
b->show();
}
Output : Implementation of Virtual Function in Derived class
In the above example Base class is abstract, with pure virtual show() function, hence we cannot create object of base class.
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