Can derived class use friend function of the basis class?

If I have some class Basis, and derived from it Derived, inside basis I have friend function

friend int operator!=(const Basis&, const Basis&)

Inside derived class I don't have such function So my question is if I have inside my main

If( derived1 != derived2 ) ...

why does it work? i don't have any constructor for casting for != th开发者_JS百科anks in advance If I write if ( derived != basis ) will it work?

The compiler is comparing them as objects of class Basis. Since you can always implicitly convert from a derived class to a base class, the compiler is able to pass them to the Basis overload of operator !=. Of course, this comparison can only use fields declared in Basis, so if you want the comparison to be more specific by using members of Derived, you'll have to define a separate operator != overload.

The friendship declaration isn't relevant when it comes to calling operator !=; all it does is allow operator != to access private members declared in Basis.

It sounds like your friend function only compares the Basis parts of Derived. So, it works after a fashion, but ignores any data in Derived.

Because your Derived class inherits everything that your Basis class has, which in this case is an operator overload for !=, your Derived objects (derived1 and derived2) also have them defined.