Why does #ifndef not work in this situation?

Question

Note: The question has already been answered here undirectly

The problem is not include guards : they won't help across different translation units

Note: I know the solution is to use extern keyword.

I'm new to C++. I have a problem understanding #ifndef in header files. When I do something like this, I get an error saying that the variables game_over and turn are already defined.

/*chess.h*/
#ifndef CHESS
#define CHESS
#include <iostream>
#include "chessboard.h"
using namespace std;

bool game_over;
char turn;
chessboard board;

int main();
#endif

/*bishop.cpp*/
#include "bishop.h"
#include "chess.h"
bishop::bishop(string pos, char color)
{
    int x = pos[0] - 97;
    int y = pos[1] - 1;
    name = "bishop";
    this->color = color;
    board.add_new(*this);
}

/*chess.cpp*/
#include "chess.h"

int main()
{
    ...
}

Why are the variables defined twice in here? I thought that first time when chess.h is included, CHESS is defined. So in bishop.cpp, #include "chess.h" will not do anything since the header will skip to #endif from #ifndef CHESS. But it does not work like that obviously. Why am I wrong?

Do you have any *other* source files that also `#include "chess.h"` that you're linking with this one? — Greg Hewgill, Feb 17 '15 at 20:01
Are you including this file in multiple cpp files? If so, each one will have the variable declarations in it. — Retired Ninja, Feb 17 '15 at 20:01
Yes, I do. So I don't get why we use #ifndef if it does not prevent this problem from happening? — omerfarukdogan, Feb 17 '15 at 20:02
possible duplicate of [When to use extern in C++](http://stackoverflow.com/questions/10422034/when-to-use-extern-in-c) — Retired Ninja, Feb 17 '15 at 20:03
possible duplicate of [How to prevent multiple definitions of functions from .h file](http://stackoverflow.com/questions/25407905/how-to-prevent-multiple-definitions-of-functions-from-h-file) — Quentin, Feb 17 '15 at 20:03

score 2 · Accepted Answer · answered Feb 18 '15 at 02:55

The #ifndef only blocks the code if the symbol is defined within the same translation unit, at a point before the #ifndef. A translation unit is a source file (.cpp) and all the files that are included into it. Since you're compiling two source files, they'll both have a complete include of the .h file.

You already appear to know how to handle the problem of defining global variables in a header file: declare them extern in the header, and put a definition into one of the sources. I would be remiss though if I didn't warn you to avoid globals in the first place, as your programs grow they'll make your life difficult.

Why does #ifndef not work in this situation?

1 Answers1