Originally posted by neotransotaku

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try placing this as the first two lines of the shared header file

#ifdef __headerfile
#define __headerfile

and this as the end of the shared header file

#endif


usually this is the techinique to ensure a header file is included only once

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Originally posted by Disch

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If you define a variable to be global, it will have global scope (Global meaning it exists for the whole program). If you #include a header file which has a var declaration in several .cpp files, each .cpp file will create its own variable with the same name. This is where your problems are coming from.

The solution here is to make the variable once, then link it so that all .cpp files have access to it.


int myVar; /* will actually make the variable -- only have 1 of these */

extern int myVar; /* will not make the variable, will link to it after compile time -- have one of these for each .cpp file beyond the first */


Of course extern int myVar will only work if 'int myVar' is declared somewhere in your program.

For a halfway decent analogy of why it works this way... making the actual variable (int myVar) actaully makes the variable with a "body". Similar to how making a function with a body gives the function it's body. If you have a function in your program which has several bodies, you'll get errors much like the ones you were getting -- because you were making several different variables which have the same body -- and the compiler doesn't know which one you want to use.

Making it 'extern' says "okay, this variable exists... but not in this .cpp file, so we'll link to it, but it won't be in this compile" -- which is kind of like a body-less variable. Equivilent to having a function prototype (function without a body). It's making the function/var available for use, but without actually making it. Of course if you actually call the function, it will need to have a function body found at link time. Likewise if you actually use an extern var, it will need to find a var body at link time. Otherwise you get errors.


So yeah, only 1 cpp file has the variable. The others use 'extern' to link to that var. Yeah it's a pain in the ass -- which is one of the reasons why I avoid global vars like the plague.

EDIT:

To avoid a pain in the ass, you can sort of cheat with clever use of #defines

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// your header file (referred to as "include.h" below)

#ifdef DO_NOT_MAKE_EXTERN
#define _EXTERN
#else
#define _EXTERN extern
#endif


_EXTERN int Var1;
_EXTERN int Var2;
//etc

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// in **ONE** of your cpp files
#define DO_NOT_MAKE_EXTERNS
#include "include.h"

// rest of file here

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// in all the other cpp files

#include "include.h" // note, no #define here

//rest of file here

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Originally posted by beneficii

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Okay, so I would have to add _EXTERN in front of every variable in the header file?

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Also, put in DO_NOT_MAKE_EXTERN and DO_NOT_MAKE_EXTERNS or was that a typo? Thanks.

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Originally posted by Disch

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Originally posted by beneficii

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Okay, so I would have to add _EXTERN in front of every variable in the header file?

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Yes. That will make it bodied or body-less depending on whether or not DO_NOT_MAKE_EXTERNS was defined before including the header.

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Also, put in DO_NOT_MAKE_EXTERN and DO_NOT_MAKE_EXTERNS or was that a typo? Thanks.

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sorry, that was a typo. It does matter what it is as long as it's the same thing both times. *me goes to edit post to correct that*

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Originally posted by Disch

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don't init the var when it's declared.

int myvar = 5; <--- don't do that, it won't like it when the var is extern

Instead init all your vars at program startup -- like call an Init() function or something which sets everything to what you want it to be.

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Originally posted by Disch

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If they're constant then declare them as constant and then you don't have to worry about that extern crap:

const int myArray[5] = {0,1,2,3,4};

that should work fine. If you still get problems (which I doubt), try making it static as well:

static const int myArray[5] = {0,1,2,3,4};

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