命名空间的类的朋友，未在命名空间中声明和定义

Question

Is it right to do so:

void f();

namespace ns
{
    class C
    {
        int m;
        friend void ::f();
    };
}

void f()
{
    ns::C c;
    c.m = 2;
}

?

The code is compiled successfully in VS. But I can't find anything about qualifying a namespace for a friend declaration in the standard. Is it even legal? Can someone provide the link or the quote from the standard?

Answer 1

Yes it's legal.

A namespace is little more than a way of adding another layer of descrimination for functions, classes, &c..

Any function can be marked as a friend inside a class , even if that class is in a different namespace to the function. The function could be in a namespace too.

There's no specific section in the standard on this, but it's a corollary of the standard. This reference is a starting point: https://en.cppreference.com/w/cpp/language/namespace