What's the difference between virtual function instantiations in C++?

Question

What\'s the difference between the following two declarations?

virtual void calculateBase() = 0;  
virtual void calculateBase();

I read the fir

Answer 1

I think you are mixing terms...

virtual void x() = 0;

is a pure virtual function, or abstract function. It is a virtual function without implementation. You talk about a pure abstract class, which is the c++ equivalent of an interface, about a class having only abstract functions.

virtual void x();

is a virtual function, meaning it can be redefined in derived classes, but it's not abstract, so you must provide an implementation for it.