What's the difference between friendship and inheritance?

Question

Suppose there are two classes A and B:

class A {};
class B {};

In what aspects differ the two examples below?

Example 1:

Answer 1

There are several big differences. Inheritance and friendship are very different.

With friendship, class C is NOT an instance of class A or class B. Therefore, if you have a function like:

void processMyClass(A* a);

you cannot pass it an instance of C whereas, if C subclasses A (publicly), it IS an instance of A.

With friendship, class A and B can touch all the private member data and functions of C. With inheritance, class C can touch the public and protected members of A and B.

Friendship is not inherited. This means, for example:

class D : public C
{
private:
   void foo() {
      // A and B cannot call this function
   }
}