c++ Global Variables Across Multiple Files

Question

I have some code spread over three files and I would like to use a fourth "gloabls" file to store some physical constants such as the value of pi. This would avoid repetitive definitions of pi = 4*atan(1.0). After poking around, what I've tried is creating a global header file:

/*globals.h*/
extern double g_pi;

and a global cpp file:

/*globals.cpp*/
#include "math.h"
#include "globals.h"
double g_pi = 4*atan(1.0);

Then I include these file in my main files:

/*mainFile.cpp*/
//Include math and other libraries 
#include globals.h"
int main() {
/*
....
*/
double x = 2*g_pi
/*
....
*/
}

This gives me an undefined reference error to g_pi. I'm using a g++ compiler on Ubuntu. Hopefully its a simple fix! Your suggestions are much appreciated. If more details are necessary I'll be happy to supply them.

The problem may lie with the way you build the executable, so you should add whatever you use to do that to your question (Makefile, g++ command line...). — alexisdm
– alexisdm, Commented Mar 14, 2012 at 19:51
Can you include the exact command you execute to compile? And the exact error message? — JaredC
– JaredC, Commented Mar 14, 2012 at 19:52

drahnr · Accepted Answer · 2012-03-14 20:15:06Z

18

You could simply use M_PI from the include (there are other constants too).

Edit: your setup is correct. I got a working minmal example:

globals.h

extern double g_tst;

globals.cpp

#include "globals.h"
double g_tst = 4.0;

main.cpp

#include "globals.h"
#include <stdio.h>
#include <stdlib.h>

int main()
{
fprintf (stderr, "g_tst  = %lf \n", g_tst);
return 0;
}

The problem is within your buildsystem

See wikipedia

edited Mar 14, 2012 at 20:15

answered Mar 14, 2012 at 19:52

drahnr

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Comments

Crymson · Accepted Answer · 2012-10-23 07:15:28Z

7

I think the problem is that you've got #include gobals.h instead of #include globals.h. This would give you the undefined references because it isn't inserting globals.h. The C++ precompiler doesn't fail when it can't find a header file. Instead you get an undefined reference message at compilation.

answered Oct 23, 2012 at 7:15

Crymson

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Matthias · Accepted Answer · 2012-03-14 19:59:44Z

2

The order of linking might be the problem. Try to link the global object file as the last one.

answered Mar 14, 2012 at 19:59

Matthias

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1 Comment

MSalters Over a year ago

Can't be the problem. The linker must handle any order, as C++ allows bidirectional use of globals. E.g. A.cpp defines int a and uses b, whereas B.cpp uses a and defines int b. This must link, and obviously one file has to precede the other in the link order.

Smittii · Accepted Answer · 2012-03-14 19:56:26Z

1

g_pi musst not be declared extern in one translation unit. You could use a small #define for this

in globals.cpp

#define MY_EXTERN_CPP

in /*globals.h

#ifdef MY_EXTERN_CPP
    #define MY_CONFIGURATION_EXTERN
#else
    #define MY_CONFIGURATION_EXTERN extern
#endif

MY_CONFIGURATION_EXTERN double g_pi;

so g_pi will be extern in all translation units you include it except globals.cpp

answered Mar 14, 2012 at 19:56

Smittii

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1 Comment

Smittii Over a year ago

oh, i just saw this isn't your problem :-)

sirian_ye · Accepted Answer · 2019-05-30 15:23:42Z

0

I had the same problem. I was using CMAKE, the problem was I didn't put my globals.cpp file into CMAKE executable. This problem should be something similar, nothing wrong with the implementation itself.

answered May 30, 2019 at 15:23

sirian_ye

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