Why does the implicit copy constructor calls the base class copy constructor and the defined copy constructor doesn't?

Question

Consider a class hierarchy where A is the base class and B derives from A.

If the copy constructor is not defined in B

Answer 1

The simple (possibly trite) answer is because you didn't tell it to. Since you are writing the derived copy constructor, you completely control how it behaves. Failure to specify a call to the base and the compiler generates code to initialize the base class by calling the base classes default constructor.