Friend declaration in C++ - difference between public and private

163
votes

Is there a difference between declaring a friend function/class as private or public? I can't seem to find anything about this online.

I mean the difference between:

class A
{
 public: 
      friend class B;
 };

and

class A
{
 private: //or nothing as the default is private
      friend class B;
 };

Is there a difference?

4
c++privatefriendpublic
Such misinformation... someone didn't deserve to be a friend. It's entirely up to you whether you like your friends tucked in with your privates. – Tony Delroy
may i ask what a friend class is :'(? – I Phantasm I
@I Phantasm - it's a declaration that allows an instance of the friend class to access the members declared private in the class that made the declaration. In the case of this example, an instance of class B can access the private members of class A – BIU
possible duplicate of Does it make a difference whether I put 'friend class xxxxx' in the public or private section? – Suma
This question has earned me way too many points on this site. All right then. – BIU

4 Answers

162
votes

No, there's no difference - you just tell that class B is a friend of class A and now can access its private and protected members, that's all.

39
votes

Since the syntax friend class B doesn't declare a member of the class A, so it doesn't matter where you write it, class B is a friend of class A.

Also, if you write friend class B in protected section of A, then it does NOT mean that B can access only protected and public members of A.

Always remember that once B becomes a friend of A, it can access any member of A, no matter in which section you write friend class B.

1
votes

c++ has the notion of 'hidden friends': http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1601r0.pdf

Which only applies to friend functions that are defined inline. This make it so the functions can only be found via argument-dependent lookups, removing them from enclosing namespace.

-3
votes

The friend declaration appears in a class body and grants a function or another class access to private and protected members of the class where the friend declaration appears.

As such access specifiers have no effect on the meaning of friend declarations (they can appear in private: or in public: sections, with no difference).