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 second function must have an implementation in the class that declare it (lack of '= 0'), and can be overriden by subClasses.

The first function may or not have an implementation in the class that declare it, and has to be implemented by subClasses