Question

Given the following code:

class Screen;

class WindowMgr
{
    WindowMgr& relocateScreen( int r, int c, Screen& s);
};

class Screen
{
    friend WindowMgr& WindowMgr::relocateScreen( int r, int c, Screen& s);
    // ^ cannot access private member declared in class 'WindowMgr'

    int m_nR,
        m_nC;
};

WindowMgr& WindowMgr::relocateScreen( int r, int c, Screen& s)
{
    s.m_nR = r;
    s.m_nC = c;
    return *this;
}

Why can the Screen class not declare the WindowMgr::relocateScreen member function as a friend? Screen is not wanting to use this private member-function of another class, but simply wants that function to be able access its own private members.

Making the relocateScreen function public might be bad design if it’s intented only for use within the WindowMgr class. Equally, making Screen a friend of WindowMgr might be bad design if it is not intented to access private members of WindowMgr in any other case.

Where am I going wrong here? What is the correct approach? Am I making a fool of myself?

Answer 1

The friend declaration doesn’t work because WindowMgr::relocateScreen() is private to WindowMgr.

C++ standard 11.4-7:

“A name nominated by a friend
declaration shall be accessible in the
scope of the class containing the
friend declaration…”

Personally, I would make relocateScreen() a private member function of Screen and make WindowMgr a friend of Screen. That way, WindowMgr can just call relocateScreen() on Screen and won’t have to touch any of the data members of Screen.