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

Question

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?

Sash

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. – Tabby

may i ask what a friend class is :'(? – Poorly

@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 – Sash

possible duplicate of Does it make a difference whether I put 'friend class xxxxx' in the public or private section? – Jara

This question has earned me way too many points on this site. All right then. – Sash

Answer 1

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.

Gaze

so I guess whoever told me that just didn't know what they were talking about. Thanks :) – Sash

but for documentation purposes, would you consider a friend an implementation detail or part of the interface? – Epexegesis

@TemplateRex: IMO that's part of interface - it's quite a strong claim that there's some (random) class Friend which can access all private members of the current class. – Gaze

for random class, yes. But say you implement operator==(T, T) using private data members of T, and use friend as an implementation detail so that operator== can appear as a non-member. IMO, this friendship should not appear in the public interface (as will be generated by Doxygen e.g.) – Epexegesis

I read some time ago here in stackoverflow that some compilers, I guess some old buggy ones, will get confused if a friend declaration happens within a non-public region. – Cayser

Answer 2

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.

Thistledown

so I guess whoever told me that just didn't know what they were talking about. Thanks :) – Sash

Answer 3

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.

Soldier

Answer 4

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).

Inborn

Answering a seven-year-old question without being new. Please take away from making things more duplicated. – Wheedle