Using a namespace to make global functions, but getting multiply defined symbols error

Question

These functions are Utility type things that most of my programs objects will use. I want to have them in a namespace and have them global. This namespace is defined in a header and then added to my precompiled header. However so far I have used the function from this namespace in 2 different objects and the compiler is throwing the multiply defined symbols error on these 2 objects.

namespace file

#ifndef UTILS_H
#define UTILS_H

#include <random>
#include <cmath>


namespace Utils
{
    extern int GetRandomBetween(int low, int high)
    {
        if (low < 0 || low >= high)
            return 0;
        int seed = high - low;

        return (rand() % seed) + low;
    }
};

#endif

and my precomp header

// stdafx.h : include file for standard system include files,
// or project specific include files that are used frequently, but
// are changed infrequently
//

#pragma once

#include "targetver.h"

//#define WIN32_LEAN_AND_MEAN             // Exclude rarely-used stuff from Windows headers
// Windows Header Files:
#include <windows.h>

// C RunTime Header Files
#include <stdlib.h>
#include <malloc.h>
#include <memory.h>
#include <tchar.h>
#include <random>


#define SAFE_DELETE( p )       { if( p ) { delete ( p );     ( p ) = NULL; } }
#define SAFE_DELETE_ARRAY( p ) { if( p ) { delete[] ( p );   ( p ) = NULL; } }
#define SAFE_RELEASE( p )      { if( p ) { ( p )->Release(); ( p ) = NULL; } }
// TODO: reference additional headers your program requires here

#include "Utils.h"
#include "Manager.h" // this object uses utils
#include "Bot.h"    // this object uses utils
#include "LinkedList.h"
#include "Village.h"  // this object will use utils in the future

The linker error message:

Manager.obj : error LNK2005: "int __cdecl Utils::GetRandomBetween(int,int)" (?GetRandomBetween@Utils@@YAHHH@Z) already defined in Bot.obj stdafx.obj : error LNK2005: "int __cdecl Utils::GetRandomBetween(int,int)" (?GetRandomBetween@Utils@@YAHHH@Z) already defined in Bot.obj c:\users\lee\documents\visual studio 2010\Projects\AI\Debug\AI.exe : fatal error LNK1169: one or more multiply defined symbols found

it also maybe worth noting that in my Manager class header I forward declared Bot. Same with Village class header.

Answer 1

Your function definition (ie: the source code) should not be in a header. The reason you are getting multiple definitions is that extern cannot convert a function definition (the source code) into a function declaration (ie: just the prototype). So you need to do this:

Util.h:

namespace Utils
{
    int GetRandomBetween(int low, int high);
};

SomeSourceFile.cpp (probably Util.cpp):

namespace Utils
{
    int GetRandomBetween(int low, int high);
    {
        if (low < 0 || low >= high)
            return 0;
        int seed = high - low;

        return (rand() % seed) + low;
    }
};

Alternatively, you can declare the function inline in the header:

namespace Utils
{
    inline int GetRandomBetween(int low, int high)
    {
        if (low < 0 || low >= high)
            return 0;
        int seed = high - low;

        return (rand() % seed) + low;
    }
};

Though you should only use this for small functions.