C ++，多重定义

Question

I need to define a constant containing an environment variable (Linux, g++). I would prefer to use string , but std::getenv needs *char (this is not yet my question). To avoid the multiple definition error I used the define workaround, but it is not enough. The program is simple: DataLocation.hpp

#ifndef HEADER_DATALOCATION_H
#define HEADER_DATALOCATION_H

#include <string>

using namespace std;

const char* ENV_APPL_ROOT = "APPL_ROOT";

[...]

#endif

And DataLocation.cpp :

#include <string>
#include <cstdlib>
#include "DataLocation.hpp"

using namespace std;

// Private members
DataLocation::DataLocation()
{
  rootLocation = std::getenv(ENV_APPL_ROOT);
}

[...]

And a test programm, Test.cpp

#include "DataLocation.hpp"
#include <iostream>

using namespace std;

int main() {
  DataLocation *dl;
  dl = DataLocation::getInstance();
  auto s = dl->getRootLocation();

  cout << "Root: " << s << "\n";
}

But compiling, I get the following error:

/tmp/ccxX0RFN.o:(.data+0x0): multiple definition of `ENV_APPL_ROOT'
DataLocation.o:(.data+0x0): first defined here
collect2: error: ld returned 1 exit status
make: *** [Test] Error 1

In my code there is no second definition, and I protect the header from being called twice. What is wrong?

The typical answers to the multiple definition question are either

• to separate declaration / implementation
• the multiple include

Is there a way to separate declaration and implementation in my case?

Edit 1

This question is not linked to this question because mine refers to a constant. In the solution of the cited question I do not see how to solve my problem.

Answer 1

You are including the header twice. Once from DataLocation.cpp (where it finds HEADER_DATALOCATION_H not yet defined and thus defines ENV_APPL_ROOT ), and once from Test.cpp (where it agains finds HEADER_DATALOCATION_H not yet defined and thus defines ENV_APPL_ROOT again.) The "header protection" only protects a header file being included multiple times in the same compilation unit .

You need:

extern const char* ENV_APPL_ROOT;

in the header file, and

const char* ENV_APPL_ROOT = "APPL_ROOT";

in one .cpp file.

Answer 2

Use

extern const char* ENV_APPL_ROOT;

in the header file, and place

const char* ENV_APPL_ROOT = "APPL_ROOT";

in one particular translation unit (eg DataLocation.cpp ).

This should fix it.