Singleton Template does not compile in Visual C++

Question

Possible Duplicate:
Why should the implementation and the declaration of a template class be in the same header file?

I'm currently trying to implement a singleton template. I've taken the code straight from the german Wikipedia (you should be able to read the code). But I always get a weird compile error in Visual C++:

game.obj : error LNK2019: unresolved external symbol ""protected: __thiscall singleton<class game>::singleton<class game>(void)" (??0?$singleton@Vgame@@@@IAE@XZ)" in function ""protected: __thiscall game::game(void)" (??0game@@IAE@XZ)".
fatal error LNK1120: 1 unresolved externals.

(Running Visual Studio 2010)

I don't know what I did wrong in my code besides splitting it on multiple pages.

I define a template class singleton that will be inherited by the class game that should become the singleton.

singleton.hpp:

template <class T_DERIVED>
class singleton {
        public:
                static T_DERIVED& get_instance();

        protected:
                singleton();

        private:
                singleton(const singleton&);
                singleton& operator=(const singleton&);
};

singleton.cpp:

#include "singleton.hpp"

template <class T_DERIVED>
singleton<T_DERIVED>::singleton()
{
}

template <class T_DERIVED>
T_DERIVED& singleton<T_DERIVED>::get_instance()
{
        static T_DERIVED instance;
        return instance;
}

template <class T_DERIVED>
singleton<T_DERIVED>& singleton<T_DERIVED>::operator=(
                const singleton<T_DERIVED>&)
{
        return *this;
}

game.hpp:

#include "singleton.hpp"

class game: public singleton<game> {
        friend class singleton<game>;
        protected:
                game();
};

game.cpp:

#include "game.hpp"

game::game()
{
}

main.cpp:

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

int main()
{
        game& a = game::get_instance();

        return 0;
}

score 0 · Accepted Answer · answered Jul 25 '11 at 21:09

Template methods definitions must be available to the end-user of that template. Thus, they (usually) should be in the header file where template is declared. So splitting your template into parts and putting definitions into source (cpp) files results in linker errors.

Singleton Template does not compile in Visual C++

1 Answers1