friend functions of a class inside a namespace

Question

I\'ve two question about this code bellow:

namespace A { class window; }

void f(A::window);

namespace A
{
    class window
    {
    private:
       int a;         


        

Answer 1

1) because function f is declared and defined outside of the current namespace. If you moved the definition of your class into the same namespace as the function whether global or otherwise you wouldn't need to.

2) you always need to declare a function before it is referenced. Your class references the function with the friend statement.