Pure Virtual

• Pure virtual functions are a way to force an interface in derived classes without providing an implementation.
  • This is useful when no reasonable default implementation exits.
• A delima
  • You are implementing a shape class for a drawing system.
  • It will hold squares, triangles, ....
  • It seems reasonable that all shapes have a Draw method.
  • But what should the default impementation be? There is none.
  • The solution is to build a pure virtual Draw method in the base class.
• Syntax

  • class Shape {
            ...
         virtual  Draw()const = 0;
    } 

• What does this mean?
  • Shape is an abstract class
  • No instance of Shape can be created.
• See virtual