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

The first one doesn't have to be implemented in the base class, but enforces it to be implemented in inherited classes.

You have to implement the second one in the base class and it can be implemented in inherited classes.