C + + Primer, an example on a friend function (by Sybase)

This article attempts to explain the example of C + + primer screen and window_mgr, why is it possible to put two classes in two files that cannot be compiled?

Write two classes in the same file, explaining the problem in three examples:

The first is a question of wording:

When compiling to screen, because the screen class uses the member function to Window_mgr, although the Window_mgr declaration is given earlier, the full definition of window_mgr is also clear, so the compilation error.

Class Window_mgr

Class screen

{

Public

Friend window_mgr& Window_mgr::relocate (Window_mgr::index R, Window_mgr::index C, screen& s);

Private

int height;

int width;

}

Class Window_mgr

{

Public

typedef STD::STRING::SIZE_TYPE Index;

window_mgr& window_mgr::relocate (Index R, index C, screen& s)

{

S.height + = r;

S.width + = C;

return *this;

}

}

The second way of writing the question is:

When compiling to relocate, because the implementation of screen& S is used to the member variable of screen, although the previous screen's declaration, but also clear the full definition of screen, so the compilation error.

Class screen;

Class Window_mgr

{

Public

typedef STD::STRING::SIZE_TYPE Index;

window_mgr& window_mgr::relocate (Index R, index C, screen& s)

{

S.height + = r;

S.width + = C;

return *this;

}

}

Class screen

{

Public

Friend window_mgr& Window_mgr::relocate (Window_mgr::index R, Window_mgr::index C, screen& s);

Private

int height;

int width;

}

The third type of notation:

Moving the implementation of window_mgr::relocate to the end, the problem is solved because the implementation details of the screen&s are not required for compiling the class Window_mgr

Class screen;

Class Window_mgr

{

Public

typedef STD::STRING::SIZE_TYPE Index;

window_mgr& window_mgr::relocate (Index R, index C, screen& s);

}

Class screen

{

Public

Friend window_mgr& Window_mgr::relocate (Window_mgr::index R, Window_mgr::index C, screen& s);

Private

int height;

int width;

}

window_mgr& Window_mgr::relocate (Window_mgr::index R, Window_mgr::index C, screen& s)

{

S.height + = r;

S.width + = C;

return *this;

}

It can be seen that these two classes require a strict alternating order if the compilation succeeds

This also explains why it is not possible to compile in two files.

Appendix:

The first implementation of the two files that cannot be compiled

The implementation is as follows: Window_mgr.h

#ifndef window_mgr//In order to avoid nesting of two files

#define Window_mgr

#include <string>

#include <Screen.h>

Class Window_mgr

{

Public

typedef STD::STRING::SIZE_TYPE Index;

window_mgr& window_mgr::relocate (Index R, index C, screen& s)

{

S.height + = r;

S.width + = C;

return *this;

}

}

#endif

Screen.h

#ifndef screen

#define Screen

#include "Window_mgr.h"

Class screen

{

Public

Friend window_mgr& Window_mgr::relocate (Window_mgr::index R, Window_mgr::index C, screen& s);

Private

int height;

int width;

}

#endif