1. Make friend function declaration in class in *.h file
e.g.

class fooclass
{
public:
friend int foo();
};

2. Put cursor on declaration of friend int foo(), enter alt + enter and hit enter again to create a prototype in associated *.cpp file
3. It creates:

int fooclass::foo()
{
}

But must create:

int foo()
{
}

Where you can see the confirmation:
https://en.wikipedia.org/wiki/Friend_function
http://www.cplusplus.com/doc/tutorial/inheritance/