What are pure virtual functions?

A pure virtual function is a function which has no definition in the base class. Its definition lies only in the derived class ie it is compulsory for the derived class to provide definition of a pure virtual function. Since there is no definition in the base class, these functions can be equated to zero.

The general form of pure virtual function is:

virtual type func-name(parameter-list) = 0;

Consider following example of base class Shape and classes derived from it viz Circle, Rectangle, Triangle etc.

class Shape
{
      int x, y;
      public:
          virtual void draw() = 0;
};

class Circle: public Shape
{
          public:
          draw()
          {
                //Code for drawing a circle 
           }
};

class Rectangle: public Shape
{
        Public:
        void draw()
        {
               //Code for drawing a rectangle
        }
};

class Triangle: public Shape
{
         Public:
         void draw()
         {
                //Code for drawing a triangle
         }
};

Thus, base class Shape has pure virtual function draw(); which is overridden by all the derived classes.