Mulitiple definitions

[dellthinker]

Hi all, I'm trying to write to a file using a global variable. For example....

/* prog1.cpp */

#include "header.h"
ofstream out;
out.open(test,ios::app);
out << "Prog1 " << endl;
out.close();

/* prog2.cpp */

#include "header.h"
ofstream out;
out.open(test,ios::app);
out << "Prog2 " << endl;
out.close();

/* header.h */

char test[MAX_PATH]="file.txt";

However when I compile I get the multiple definition of `test' error for each source file. Does anyone know how I can sort this out? Thanx.

[Chris_F]

http://www.gamedev.net/reference/art...rticle1798.asp

[dellthinker]

Is it possible to get an answer with out being redirected to a article? Does anyone have a C++ answer for me?

[Chris_F]

You should probably read the article regardless, as I believe you'd not only find the answer to your current question, but probably your second, and third, etc that you are likely to run into later.

If you want the short answer, it's because of multiple inclusion.

#ifndef HEADER_H_
#define HEADER_H_

/* header.h */
char test[MAX_PATH]="file.txt";

#endif /*FUNCS_H_*/

[Russco]

In one cpp file define the variable, in all other translation units that need the variable declare as extern.

[itsmeandnobodyelse]

In one cpp file define the variable, in all other translation units that need the variable declare as extern.

Russco is right. The error is not because of multiple inclusion but cause the global variable was DEFINED in the header so that each cpp that includes the header would create a new instance of that variable (symbol). The linker then will complain because of that.

So, in the header you would need only a DECLARATION what is made by the extern keyword. And one and only cpp MUST have a definition of the variable simply like

// header
...
extern char test[MAX_PATH];
...

// first.cpp 
#include "header.h"

char test[MAX_PATH] = "file.txt";

You see the use of shared global variables among source files is quite tricky. You better use a class and define a static public member.

// global.h
#ifndef GLOBAL_H
#define GLOBAL_H

class Global
{
public:
    static char * getTest()
    {
           static char test[MAX_PATH] = "file.txt";
           return test;
    }
};
#endif

// any cpp 
#include "global.h"
...
    // set another file
    strcpy(Global::getTest(), "anotherfile.txt");

    // use filename
    ifstream ifs(Global::getTest());

Beside of using plain char pointers the above is much easier and a C++ solution. And the char pointer easily could be replaced by a std::string.

[dellthinker]

Thanx for the solution itsmeandnobodyelse, I've been trying to understand the concept of classes public and private over and over. Reading many many tutorials about it and it just seems to slip my mental grasp but I'll keep trying to learn and understand it. However, I'm having the same error. I declared it in the first .cpp like you said and extern in the header and the same error exists. What gives?

[hypheni]

Just one thing. Rather than using

#ifndef GLOBAL_H
#define GLOBAL_H

....
......

#endif

you can simply use #pragma once

[laserlight]

Just one thing. Rather than using

#ifndef GLOBAL_H
#define GLOBAL_H

....
......

#endif

you can simply use #pragma once

But be warned that #pragma once is non-standard. I suggest that you use normal header inclusion guards.