ABOUT HEADER FILE CREATION ERROR IN C++

Question

i'm creating header file for .cpp file,where it will contain only declaration or prototype of function

here is the threes programs i have written 1.header.h file//where i have declared functions add( , ) and sub( , )

 #ifndef HEADER_H
 #define HEADER_H
 #pragma once
 int s;
 int add(int a,int b);
 int sub(int a,int b);
 #endif

2.header.cpp,where i have defined functions add( , ) and sub( , )

 #include<iostream>
 #include "header.h"
 using namespace std;


int add(int a,int b){
s=10;   
int c=a+b+s;
return c;
}

 int sub(int a,int b){
 int c=a-b;
 return c;
 }    

3.example.cpp

     #include<iostream>
     #include"header.h"
     using namespace std;
      void main(){

    int a=10,b=20;

    int c=add(a,b);
    int d=sub(c,a);

    cout<<"c"<<c;
    cout<<"d"<<d;

    //cout<<s;
    getchar();
      }

here, i declared variable 's' in header.h and defined in header.cpp file ,which is given as addition to variable c in example.cpp file output.

it showing error header.obj : error LNK2005: "int s" (?s@@3HA) already defined in example.obj Projects\\header\\Debug\\header.exe : fatal error LNK1169: one or more multiply defined symbols found please help me to resolve this error,i'm working from long time for this...thanks in advance

Answer 1

You get the error, because this line in .h is already a definition :

int s;

You need to have this in header:

extern int s;

And then in exactly one .cpp file, usually one with same base file name as the .h file, you need to have the definition:

int s;

---

Related: You don't need the extern keyword with function declarations, because they're just declarations, telling compiler that such a function exists somewhere, and you can do that as many times as you like. But if you put a global (non- static , non- inline ) function definition (with {} function body instead of ; ) to a .h file, you do get similar linker error about multiple definitions.