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c++inheritancefriend

What's the difference between friendship and inheritance?

Suppose there are two classes A and B:

class A {};
class B {};

In what aspects differ the two examples below?

Example 1:

class C : public A, public B {};

Example 2: 

class C
{
    //private
    friend class A;
    friend class B;
}
Solution

  • A friend can touch the private parts (pun only slightly intentional! ;) ) of whatever it is friend of, but nothing of A and B are part of C - it just means that "A and B can touch C's private bits"). Anything "less" than private is of course also available to A and B, so if C has protected or public members, that will also be available.

    When you inherit, the A and B becomes part of C. Any private sections of A and B are not available to C. In the "is-a" vs. "has-a" nomenclature, C now is-a A and is-a B - in other words, it's inherited from A, so it "behaves like A from an interface perspective.