Question

I have created custom random number generators and placed their global functions in a file I called SamRandom.h. The file looks as follows:

#ifndef _SAM_RANDOM_H
#define _SAM_RANDOM_H

#include <cstdlib>
#include <ctime>

void InitialiseRandomSeed()
{
    //...
}

//...

#endif

I’m working in a very complex object-oriented program that has many different components from here and there. Everytime I add a file that is anyhow related to this file, I get the following conflict message:

LaserBase.obj:-1: error: LNK2005: "void __cdecl InitialiseRandomSeed(void)" (?InitialiseRandomSeed@@YAXXZ) already defined in main.obj
error: LNK1169: one or more multiply defined symbols found

On MSVC, and on MinGW I get:

In function `Z20InitialiseRandomSeedv':
SamRandom.h:8: multiple definition of `InitialiseRandomSeed()'
error: first defined here
:-1: error: collect2: ld returned 1 exit status

Why is this happening? I thought that preprocessor directive is supposed to prevent this problem from happening… this is really driving me crazy!!!

P.S.: The problem is independent of the function name.

Thanks for any efforts

Answer 1

When you include your file in a .cpp file the preprossesor directly copys it into that file. So when you compile that .cpp file the object file (a step the compiler always does even if you don’t recognize) will contain that function. If you include it in more than one .cpp file each object file will contain the function. When the linker trys to link all object files to an executable it finds the function x-times and gives that error.
“one or more multiply defined symbols found” says it found one of your functions (or variables) more than once and doesnt know how to deal with it.

To avaoid this you should only put the signature of you function e.g.

//whatever.h
#ifndef _SAM_RANDOM_H
#define _SAM_RANDOM_H

void InitialiseRandomSeed();

#endif

in ther header file and put the implementation into a seperate .cpp file:

//whatever.cpp
#include "whatever.h"
#include <cstdlib>
#include <ctime>

void InitialiseRandomSeed()
{
    //...
}