How to check class friendship?

Question

The standard library provides many useful "is" typetraits that tell about the relationship of two classes, such as is_base_of, is_convertible, is_same, etc. Is there any way to implement a typetrait is_friend to tell whether class A is a friend of class B?

Sample code:

class A {};
class B { friend class A; };
class C { friend class B; };

is_friend<A, B>; // true
is_friend<B, A>; // false
is_friend<B, C>; // true
is_friend<A, C>; // false

I don't know but what would be the point since friend is a hint for compiler? AFAIK, it doesn't generate any code. — , May 20 '19 at 21:04
I have implemented something like this -> https://stackoverflow.com/a/11748131/2565020 To check whether a class is reflectable, instead of checking for the presence of a member/function or creating a flag, I thought it would be more beautiful to check whether the reflectable class is a friend of `reflector` — Nighteen, May 20 '19 at 21:09
@Nighteen May be [this](https://stackoverflow.com/questions/27492132/how-can-i-remove-refactor-a-friend-dependency-declaration-properly) is helpful to some extend. — πάντα ῥεῖ, May 20 '19 at 21:32

score 0 · Answer 1 · answered May 20 '19 at 23:52

No, it’s not possible. In your example, nothing in B has any access restrictions anyway, so A’s friendship has no effect. Since friendship is neither inherited nor transitive, there’s no way to use a trick like inheriting from the class in question and adding a private member.

How to check class friendship?

1 Answers1