Friend function is not visible in the class

Question

I have the following code:

struct M {
    friend void f() {}
    M() {
        f(); // error: \'f\' was not declared in this scope
    }
};

int main() {
            


        

Answer 1

This quote from the C++ Standard

A friend function defined in a class is in the (lexical) scope of the class in which it is defined.

means the following

9 Name lookup for a name used in the definition of a friend function (11.3) defined inline in the class granting friendship shall proceed as described for lookup in member function definitions.

That is any name used in the function is searched starting from the class scope.

However the function itself is not visible in the namespace until it will be declared outside the class.

So in your case it is enough to declare the function before the class definition

void f() {}

struct M {
    friend void f();
    M() {
        f(); 
    }
};

int main() {
    M m;
}

Or

void f();

struct M {
    friend void f() {}
    M() {
        f(); 
    }
};

int main() {
    M m;
}