WHY "already defined"?

Question

Please suggest me a hint over here:

class UIClass
{
public:
    UIClass::UIClass();
};

#ifndef __PLATFORM__
#define __PLATFORM__
    UIClass Platform;
#else
    extern UIClass Platform;
#endif

I'm including this two times and getting:

LNK2005 - Platform already defined in .obj (MSVS13).

As you can guess, the idea was to define Platform only once. Why does #ifndef or #define fail? How should I fix this?

Answer 1

#define's are translation unit-local, but definitions are not. You need to put extern UIClass Platform; in your header and UIClass Platform; in an implementation file.

If you really want to have the definition in your header you can use some template class magic:

namespace detail {
    // a template prevents multiple definitions
    template<bool = true>
    class def_once_platform {
        static UIClass Platform;
    };

    // define instance
    template<bool _> def_once_platform<_> def_once_platform<_>::Platform;

    // force instantiation
    template def_once_platform<> def_once_platform<>::Platform;
}

// get reference
UIClass& Platform = detail::def_once_platform<>::Platform;