Best way to implement globally scoped data

Question

I'd like to make program-wide data in a C++ program, without running into pesky LNK2005 errors when all the source files #includes this "global variable repository" file.

I have 2 ways to do it in C++, and I'm asking which way is better.

The easiest way to do it in C# is just public static members.

C#:

public static class DataContainer
{
    public static Object data1 ;
    public static Object data2 ;
}

In C++ you can do the same thing

C++ global data way#1:

class DataContainer
{
public:
    static Object data1 ;
    static Object data2 ;
} ;
Object DataContainer::data1 ;
Object DataContainer::data2 ;

However there's also extern

C++ global data way #2:

class DataContainer
{
public:
    Object data1 ;
    Object data2 ;
} ;
extern DataContainer * dataContainer ; // instantiate in .cpp file

In C++ which is better, or possibly another way which I haven't thought about?

The solution has to not cause LNK2005 "object already defined" errors.

Answer 1

If you absolutely have to have some global objects then the simplest way is to just to declare them extern in a header file included anywhere that needs access to them and define them in a single source file.

Your way #1 uses a class with only static members which means that it is essentially doing the job of a namespace so why not just use a namespace?

Way #2 aggregates both objects in a single class but if there is no true interdependency between the two objects there is no particular benefit to this.

I'd recommend putting the objects in a namespace prevent pollution of the global namespace with potentially common identifiers like data1,

// File: globaldata.h
#ifndef GLOBALDATA_H
#define GLOBALDATA_H

#include "Object.h" // Definition of Object might be here

namespace GlobalData
{
    extern Object data1;
    extern Obejct data2;
}

#endif GLOBALDATA_H

.

// File: globaldata.cc
#include globaldata.h

namespace GlobalData
{

    Object data1;
    Object data2;
}

Then you can access them in other places like this.

#include "globaldata.h"

// Does something with global data
void f()
{
    GlobalData::data1.doSomething();
}