Question

I have created a sample c++ project under Visual Studio 2010 with following files.

A.h

#ifndef A_H
#define A_H

#include <iostream>

void foo();

#endif

A.cpp

#include "A.h"

void foo()
{
    int a = 1;
}

main.cpp

#include "A.h"

int main(int argc, char* argv[])
{
    foo();    

    return 0;
}

I am getting the following output after build:

1>—— Build started: Project: opengl_test, Configuration: Debug Win32 ——

1> main.cpp

1> A.h

1> A.cpp

1> Generating Code…

1>Debug\A.obj : warning LNK4042: object specified more than once; extras ignored

1>main.obj : error LNK2019: unresolved external symbol “void __cdecl foo(void)” (?foo@@YAXXZ) referenced in function _main

1>C:\Users\alp\Projects\Test Samples\opengl_test\Debug\opengl_test.exe : fatal error LNK1120: 1 unresolved externals

========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

What is the reason for this error?

Answer 1

I think what happened is that A.h was in the Source group of the project rather than the Header group, so it was compiled as if it were a .cpp. Since both A.cpp and A.h will generate an object file A.obj, the last one to compile is the only one that got linked. I believe the last one compiled was A.h, which didn’t have an implementation of foo(), thus the linker couldn’t find it.