c++ - how to declare and define global variables in order to access them from all headers/source files properly

Question

Well, I'm learning C++ and never really learned how to do stuff that is not OO. I'm trying to get a bit more experience coding in C style.

GobalInformation.h

#pragma once

#ifndef GLOBALINFORMATION_H
#define GLOBALINFORMATION_H

#include "MapInformation.h"

namespace gi {
    MapInformation mapInf;
};

#endif

I would like to be able to access gi::mapInf from every header and cpp in my project. Right now I'm including globalinformation.h in every header, so I'm getting linker errors with multiple definitions.

How can I work around the problem?

Answer 1

In header file only do

namespace gi {
    extern MapInformation mapInf;
};

In CPP file provide the actual definition.

namespace gi {
    MapInformation mapInf;
};

It will work as you intend.

If you are using the MapInformation across dynamic link library boundaries you might have to link against the library that includes the definition cpp file. Also on Window you might have to use dllimport/dllexport