Undefined Reference/ Multiple Definition for namespace extern variable , c++ structuring issue

Question

I am making a namespace to help me debug a program but I'm having a problem figuring out how structure everything and get it to build without issue.

Here's my current header:

#ifndef HELPER_H
#define HELPER_H
#include <string>
#include <fstream>
#include <sstream>

namespace Helper
{
    enum LOG { ONSCREEN, OFFSCREEN };
    extern std::ofstream logfile;
    //std::ofstream logfile("log.txt", std::ios_base::out | std::ios_base::app );


    void EnableLogging();
    void Log(std::string s, LOG type);

    template <class T>
    std::string ToString(const T& t)
    {
        std::ostringstream sstr;
        sstr << t;
        return sstr.str();
    }
}

#endif // HELPER_H

Here's the Helper cpp file:

#include "Helper.h"
#include <cstdio>

void Helper::EnableLogging()
{
    #ifdef WIN32
        // To use console on pc
    std::ofstream ctt("CON");
    freopen("CON", "w", stdout);
    freopen("CON", "w", stderr);
    #endif
    #ifdef GP2X
        //To log to text file on the caanoo
    logfile.open("log.txt", std::ios_base::out | std::ios_base::app );
    #endif

}

void Helper::Log(std::string s, LOG type)
{
    if(type == OFFSCREEN)
    {
        #ifdef GP2X
        //log << "L" << __LINE__ << "|T" << SDL_GetTicks() << "| " << s << std::endl;
        logfile << s << std::endl;
        #endif
        #ifdef WIN32
        printf("%s",s.c_str());
        #endif
    }
}

At the moment I am getting an undefined reference to Helper::logfile error, which I completely understand is because I've used the extern keyword.

Without the extern keyword I get a different error: multiple definition of Helper::logfile . The error is reported as 'first defined..' in another source file that I am trying to include "Helper.h" in. The line number that error is reported on is a constructor within said source file but I suspect that has little to do with anything.

I'm sure I am structuring the helper code wrong for compile but I can't figure out how I should be doing it?

Answer 1

You need to declare the variable in the header, as you do, to make the name available wherever its needed.

// Helper.h
extern std::ofstream logfile;

You need to define it in the source file; the One Definition Rule requires that you have exactly one definition.

// Helper.cpp
std::ofstream Helper::logfile("log.txt", std::ios_base::out | std::ios_base::app );

With no definition, the variable doesn't exist, hence the "undefined reference" error.

With a definition in the header, it's defined in every translation unit that includes the header, hence the "multiple definition" error.

With a definition in one source file, it's defined once, and the linker is happy.