How can I use variables in multiple files?

Question

I want do define a variable in the header and be able to use it in multiple files. e.g. have variable a defined somewhere and be able to use/change it in both

Answer 1

Collin already described the problem and the solution, but I want to emphasize just how bad a style this is.

As you've found out even from this simple exercise, using globals in this manner is often a Bad Thing. Aside from the compiler issues you've already found, sharing state between functions using globals causes the following problems:

• it makes the function itself harder to reuse in other programs, because the function is expecting the presence of those symbols;

• changes to one function may cause problems in other functions that share that state (i.e., the changes are not localized to the function);

• code becomes harder to debug, since anyone can modify a and b at any time.

Ideally, functions should communicate exclusively through parameters, return values, and exceptions (where applicable); there are times where sharing state is the right answer, but those are relatively few and far between.

Since you're obviously using a C++ compiler, I'm going to do this the C++ way, using references instead of pointers. I'm also going to use C++ iostream routines instead of cstdio (write either C or C++; trying to mix the two only leads to heartburn).

vars.hpp:

#ifndef VARS_H  // Include guard to prevent multiple inclusion
#define VARS_H

void double_vars(int &, float &);

#endif

vars.cpp:

#include "vars.hpp"

void double_vars(int &x, float &y)
{
  x *= 2;    
  y *= 2.0;  
}

main.cpp:

#include 
#include "vars.hpp"

int main(void)
{
  int a = 1;
  float b = 2.2;

  std::cout << "a: " << a << "; b: " << b << std::endl;
  double_vars(a, b);
  std::cout << "a: " << a << "; b: " << b << std::endl;

  return 0;
}

Since x and y were declared as reference types (using the &), writing to the expressions x and y in double_vars is equivalent to writing to a and b in main.

The C way of doing this would be with pointers instead of references:

vars.h:

#ifndef VARS_H  // Include guard to prevent multiple inclusion
#define VARS_H

void double_vars(int *, float *); 
#endif

vars.c:

#include "vars.h"

void double_vars(int *x, float *y)
{
  *x *= 2;    
  *y *= 2.0;  
}

main.c:

#include 
#include "vars.h"

int main(void)
{
  int a = 1;
  float b = 2.2;

  printf("a: %d; b: %f\n", a, b);
  double_vars(&a, &b);
  printf("a: %d; b: %f\n", a, b);
  return 0;
}

In this case, we're using pointers instead of references, so we need to pass the result of the expressions &a and &b to double_vars; within the function, writing to the expressions *x and *y is the same as writing to a and b in main.