Friend functions in C++

Question

I have a doubt related to friend functions in C++. Friend function is not a member function of the claas and can be invoked directly from the main. So, what difference does it make if we keep the friend function within the private or the public part of the class. I have generally noticed that the friend functions are always in the public part. In what scenario we should keep the friend function within private.

I don't know why this is getting close votes... This is a perfectly sensible (if poorly worded) question. — ildjarn, Jun 19 '12 at 19:19
+1 - "when is it good design to make a private friend function" is a perfectly fine question. (Or why is it never.) — djechlin, Jun 19 '12 at 19:19
"Friend declaration can be placed anywhere in the class declaration. It is not affected by the access control keywords." - http://en.wikipedia.org/wiki/Friend_function — , Jun 19 '12 at 19:21

score 20 · Answer 1 · answered Jun 19 '12 at 19:18

20

The compiler does not pay any attention to whether a friend function is in the private or public (or protected) section of a class. Most people put it in the public section, but it'll be publicly visible regardless of where you put it.

answered Jun 19 '12 at 19:18

Jerry Coffin

476,176
80
629
1,111

score 4 · Answer 2 · answered Jun 19 '12 at 19:22

4

It does not matter if you declare it in the public: or private: part of the class. It will function the same regardless.

answered Jun 19 '12 at 19:22

Brandon Kreisel

1,596
1
17
23

score 2 · Answer 3 · answered Jun 19 '12 at 19:32

2

One reason for having the friend declarations in the private section is that it can keep them together with the member functions or objects they are supposed to have access to.

Other than that, there is no difference.

answered Jun 19 '12 at 19:32

Bo Persson

90,663
31
146
203

score 2 · Answer 4 · edited Apr 08 '13 at 03:22

2

The friend keyword is just here to grant private access to another function which is not part of your class. Since it's not part of your class, it's not affected by public/private specifiers.

edited Apr 08 '13 at 03:22

Shafik Yaghmour

154,301
39
440
740

answered Jun 19 '12 at 19:36

Frédéric Terrazzoni

2,190
19
25

score 0 · Answer 5 · answered Jan 26 '17 at 17:30

it doesn’t matter where you put the friendship declaration. It may exists inside any of the class parts (public, private or protected) but must be put outside any function or aggregate.

Here is a nice example and explanation from www.cprogramming.com:

It is often useful for one class to see the private variables of another class, even though these variables should probably not be made part of the public interface that the class supports. For instance, if you were writing a binary tree, you might want to use a Node class that has private data, but it would still be convenient for the functions that actually combine nodes together to be able to access the data directly without having to work through the Node interface. At times, it may not even be appropriate for an accessor function to ever give even indirect access to the data.

Friend functions in C++

5 Answers5

Linked