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c++c++11friend

friend functions and declarations

I had class which had a friend function for example if the header was:

 class A{
       friend void foo();
       public:
             A(){}
 };

 void foo();

So what I cannot get is what does the compiler do with the second declaration?

I've all so noticed I can write as many of them as I want with no effect, e.g

void foo();
void foo();
...

(Assume the implementation is in the cpp file)

Plus I couldn't find where does the standard say the declaring some function as a friend also declares the function itself.

Solution

  • If the friend declaration were the only declaration, then the function would be in the surrounding namespace, but couldn't be found by the normal lookup rules; it could only be found by argument dependent lookup.

    In this case, it means that it couldn't be found at all, since there are no arguments; so you also need a declaration in the namespace to make it accessible.

    Sometimes you want that behaviour; for example, a streaming operator can be found by ADL without declaring it in the namespace:

    struct A {
    friend std::ostream & operator<<(std::ostream & s, A const & a) {
        return s << "Hello";
    }
};

A a;
std::cout << a;   // finds the friend function by ADL, based on the argument 'a'

    I've all so noticed I can write as many of them as I want with no effect

    Yes, you can repeat declarations as many times as you like.

    Plus I couldn't find where does the standard say the declaring some function as a friend also declares the function itself.

    Most of the specification for declarations (C++11 clause 7) is the same whether or not the declaration contains a friend specifier. 7.3.1.2/3 specifies that the name is in the namespace, not the class, and also specifies the lookup rules.