Already defined in login.obj

[DEhqDjzh]

I am trying to create a porgram that have all the global variables in OS.h but it gives this error:
SLNK2005 "int userid" (?userid@@3HA) already defined in login.obj

OS.h

#pragma once
#include <iostream>
#include <stdio.h>
#include <Windows.h>
#include <experimental/filesystem>
#include <fstream>

#define log(x) std::cout << x
#define loge(x) std::cout << x << std::endl;
#define get(x) std::cin >> x
#define gete(x) std::cin >> x >> std::endl;



int userid;
namespace fs = std::experimental::filesystem;
 

Login.h

#include "OS.h"
bool login()
void intro()
 

OS.cpp

 
#include "OS.h"
#include "login.h"
int main()
{
	if (login() == true)
	{
		loge("succes");
		system("pause");
	}



}

Can you post the fixed code ?

[TwilightSpectre]

If you define a global variable in a header file, it will be defined multiple times – once for each source file that header file is (eventually) included in. Remember that header files are basically just copy/pasted into the source files.

The fix is to use extern in the header file, and define it in just one cpp file:

// A.h
#ifndef A_H_INCLUDED
#define A_H_INCLUDED

extern int A;

#endif // A_H_INCLUDED 

// A.cpp
#include "A.h"
int A = 0;

// B.cpp
#include "A.h"
int main() {
    std::cout << A << std::endl;
    return 0;
}

[DEhqDjzh]

@TwilightSpectre I cant use extern for namespace

[TwilightSpectre]

You don't need it for namespaces aliases (or typedefs or using-declarations or function prototypes or...), only for variables – they're just aliases rather than actual "objects", and hence can be redefined in different object files with no consequence.

[MikeyBoy]

If you define a global variable in a header file, it will be defined multiple times

Note that, in C++, this is only a problem for non-const variables. If you define a file-scope const variable, it has internal linkage only, so it's no problem if you have them defined multiple times in different translation units.