Question

I have a simple project broken up into the following source files:

my.h:

#ifndef MY_H
#define MY_H

extern int val;     
void print_val();   

#endif

my.cpp:

#include "my.h"
#include <iostream>

void print_val()
{
    std::cout << val;
}

main.cpp:

#include "my.h"

int main()
{
    val = 4;      
    print_val();  

    return 0;
}

When I compile I receive the following errors:

1>main.obj : error LNK2001: unresolved external symbol "int val" (?val@@3HA)             
1>my.obj : error LNK2001: unresolved external symbol "int val" (?val@@3HA)     
1>c:\...\test.exe : fatal error LNK1120: 1 unresolved externals

Why is this so? I simply want to declare a variable and function in a header and define the function in a separate source file. I use Visual C++ 2010 Express, and the project is a Win32 Console app.

Answer 1

In your my.cpp you probably wanted to define that extern variable of yours, like this:

int val; // optionally you can also initialize it here, like this: int val=0;

This will make sure the compiler actually allocates an object for this variable that can be later linked into your executable.

On a related note – make sure there’s only one definition of such extern variable in all of your source files otherwise you’ll have duplicate symbols and linker won’t be happy with that