Forward declaration and friend functions

Question

I have the following code

class foo
{
public:
    foo()   {}
private:
    int foo_int;
    friend class bar;  //----->Statement A               
};

class bar 
{
public:
    void someMethod()
    {
        foo f;
        f.foo_int = 13;
    }
};

Now I also read this answer on SO. However I cant put the pieces of puzzle together as to why the compiler recognizes bar as a type. I was under the impression that it would complain of Bar being an incomplete type however that did not happen. My question is why ?

You're not allocating a `bar` at that statement, so why do you expect the compiler to complain? — Jim Balter, Oct 16 '14 at 01:44
Yes that does make sense. Could you put that in as an answer — MistyD, Oct 16 '14 at 01:45
@JimBalter isn't this basically a duplicate of the linked question? I mean the answer is essentially there. — Shafik Yaghmour, Oct 16 '14 at 01:50
@ShafikYaghmour This question is specifically about `friend`, which isn't mentioned at the linked question. The question here is a bit baffling given the link, though. — Jim Balter, Oct 16 '14 at 01:55

score 2 · Accepted Answer · answered Oct 16 '14 at 01:49

friend class bar;

is simply a declaration ... there's nothing for the compiler to complain about. The limitation on incomplete types is when the compiler needs information about the type that it doesn't have, such as its size or, for base classes, its members, but for friend it doesn't need anything other than its name.

Note that it doesn't matter where the friend declaration occurs in the class definition ... that it follows private: doesn't make it private. It's better to put it at the top of the class definition.

score 1 · Answer 2 · answered Oct 16 '14 at 01:51

1

A friend specification of a class that has not been declared yet acts as a declaration of the class. It is perfectly fine to declare an incomplete type as a friend of a class because the compiler only needs to know about the type being declared.

answered Oct 16 '14 at 01:51

David G

94,763
41
167
253

Forward declaration and friend functions

2 Answers2