Question

Regarding when call copy constructor of base class, we have the form:

DerivedClass ( const DerivedClass& obj ) : BaseClass ( obj ) 
{ 
// ... 
} 

I just do not understand why we pass object of derived class to the base class copy constructor.

Answer 1

What else would you want to pass? Because the only other option is a magical pony conjured up from the air.

It’s a typical “real type hidden behind a reference to parent” thing. Base class’ copy ctor has signature base(const base&). It doesn’t care if you pass a most derived type, or semi derived type, or maybe base type. It only sees the portion it cares about, which is base, and nothing else, so it can do its job properly.

Of course that only works for types derived using public inheritance, which forms is-a relationship between the two.