Friend function can no longer access private data members of class after class is encapsulated in namespace

Question

I have a class called Circle that I encapsulated inside a namespace.

namespace my_name_space {
    class Circle;
}

class my_name_space::Circle {
private:
    double radius;
public:
    friend std::ostream& operator << (std::ostream &os, const Circle &c);
};

Here is the function in the implementation file:

std::ostream& operator << (std::ostream &os, const Circle &c)
{
    os << "Radius: " << c.radius;

    return os;
}

Before encapsulating the class within the namespace, everything was working fine. Now the friend function can no longer access the private data members. I don't understand what's wrong.

Answer 1

When you declare operator << as the friend of Circle, it'll become the member of the innermost enclosing namespace of Circle. That means when you put Circle into namespace my_name_space, operator << becomes the member of namespace my_name_space too. The definiton of operator<< doesn't match that, it defines operator<< at globle scope.

A name first declared in a friend declaration within class or class template X becomes a member of the innermost enclosing namespace of X

You can move the definition of operator<< into namespace my_name_space:

namespace my_name_space {
    std::ostream& operator << (std::ostream &os, const Circle &c) {
        ...
    }
}

Or if you still want to keep operator<< at global scope:

// delcaration
std::ostream& operator << (std::ostream &os, const my_name_space::Circle &c);
class my_name_space::Circle {
    ...
    friend std::ostream& ::operator << (std::ostream &os, const Circle &c);
    //                   ~~
};
// definition
std::ostream& operator << (std::ostream &os, const my_name_space::Circle &c) {
    ...
}