Question

What does the = 0 part of this declaration imply?

class KeyboardListener
{
  public:
    virtual bool keyPressed(void) = 0; 
} 

Answer 1

This means the member function is pure virtual, which means it has no implementation at all. Consequently, the class cannot be instantiated (it becomes “abstract”), and it can only be used as a base class, whose derived classes must (eventually) implement the virtual member function.

An example would be an abstract Animal class with a pure-virtual member function feed(): Since every animal is always an instance of a concrete (i.e. derived) animal, no purely abstract animal can exist. And while very animal has some way of feeding, there is no universal base implementation that is common to every animal — we only know that feed() exists, but it must always be implemented concretely by a concrete derived animal.

_{(Note that you can actually provide an implementation for a pure-virtual function. You still cannot instantiate such a class, but derived classes can call the base function. This is very rarely useful and probably poor style.)}