Why can't I declare a friend in one class that is a private member of another class?

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 开发者_Python百科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?

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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.

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The WindowMgr will have to declare Screen as a friend. You can use a forward declaration.

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Why not factor the WindowMgr::relocateScreen out into a different class, say WindowMgrFoo witht the 1 relocateScreen function. Delcare WindowMgrFoo a friend of Screen in Screen and have WindowMgr inherit privately from WindowMgrFoo? Or just have WindowMgr have a reference to a WindowMgrFoo, but that you need to change how it's called by users if you make it public.

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In Silico - Nice one for citing the standard. Having slept on it I now see the rationale:

By declaring WindowMgr::relocateScreen with its paramater list to be a friend in Screen, the Screen class becomes dependent on the private implementation of the WindowMgr class. This violates encapsulation/information hiding.

In order to not violate the tenets of OOD, only public member-functions of class can be declared as friends in another, because otherwise the latter becomes dependent on the private implementation of the former.