Pure Virtual Functions and Abstract Classes in C++

A pure virtual function is a virtual function in C++ for which we need not to write any function definition and only we have to declare it. It is declared by assigning 0 in the declaration.

An abstract class is a class in C++ which have at least one pure virtual function.

• Abstract class can have normal functions and variables along with a pure virtual function.

• Abstract class cannot be instantiated, but pointers and references of Abstract class type can be created.

• Abstract classes are mainly used for Upcasting, so that its derived classes can use its interface.

• If an Abstract Class has derived class, they must implement all pure virtual functions, or else they will become Abstract too.

• We can’t create object of abstract class as we reserve a slot for a pure virtual function in Vtable, but we don’t put any address, so Vtable will remain incomplete.

Example Code

 Live Demo

#include<iostream>
using namespace std;
class B {
   public:
      virtual void s() = 0; // Pure Virtual Function
};

class D:public B {
   public:
      void s() {
         cout << "Virtual Function in Derived class\n";
      }
};

int main() {
   B *b;
   D dobj;
   b = &dobj;
   b->s();
}

Output

Virtual Function in Derived class