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

c++ private friend public

39,038

Solution 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.

Solution 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.

Solution 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.

39,038

BIU

Web Developer, Jerusalem !לא על אנש רחיצנא, ולא על בר אלהין סמיכנא. אלא באלהא דשמיא

Updated on May 11, 2021

Comments

BIU about 3 years
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?
- Tony Delroy about 13 years
  
  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.
- BIU about 13 years
  
  @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 over 10 years
  
  This question has earned me way too many points on this site. All right then.
- Suma
  
  possible duplicate of Does it make a difference whether I put 'friend class xxxxx' in the public or private section?
BIU about 13 years

so I guess whoever told me that just didn't know what they were talking about. Thanks :)
BIU about 13 years

so I guess whoever told me that just didn't know what they were talking about. Thanks :)
TemplateRex almost 10 years

but for documentation purposes, would you consider a friend an implementation detail or part of the interface?
sharptooth almost 10 years

@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.
TemplateRex almost 10 years

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.)
Peregring-lk almost 5 years

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.
MAChitgarha about 4 years

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