C++ include file with variables in multiple files

Question

i have code:

main.cpp

#include <iostream>
#include "add.h"
#include "var.h"

using namespace std;

int main() {
    std::cout << width << height << std::endl;
    show();
    return 0;
}

add.cpp

#include <iostream>
#include "add.h"
#include "var.h"

int add(int x, int y) {
    return x + y;
}

int show() {
    std::cout << width << height << std::endl;
    return 0;
}

add.h

#pragma once

int add(int x, int y);
int show();

var.cpp

#include "var.h"

int width = 1024, height = 768;

var.h

#pragma once

int width, height;

i need to include var.h in main.cpp and add.cpp to use variables in those files, but when i start compiling my program, i get an error: C2086 int width redefinition file var.cpp line 3, C2086 int height redefinition file var.cpp line 3

Side note: nothing in this code needs the extra stuff that std::endl does. Use '\n' to end a line unless you have a good reason not to. — Pete Becker
– Pete Becker, Commented Jun 1 at 18:17

user10489 · Accepted Answer · 2025-06-01 16:16:50Z

2

Things in include files should not generate code or object definitions.

int width = 1024, height = 768;

generates memory objects.

This should be in your .cpp file, not in an include file. (And must be in exactly one cpp file.)

The include file should look like

extern int width, height;

which generates declarations, not definitions.

answered Jun 1 at 16:16

user10489

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3 Comments

NathanOliver Jun 1 at 16:17

This should be in your .cpp file, not in an include file. They are in the OP's cpp file

Christian Stieber Jun 1 at 16:30

In most cases, you'll just want to put something like constexpr int width = xxx; into your header. That way, the compiler will just use that as a constant instead of storing it separately.

Aleks Jun 4 at 13:47

and how to declare const variables and arrays in header file?

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