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.

Answer 1

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