template classes & static members

[screetch]

Hey,

Here is my problem :

i am building an exectutable which relies on two Dlls, one is the "core" and the other is "engine"

in Core, i define a Singleton class :

template< class T >
class Singleton
{
   static T* instance;
public:
   Singleton() : instance(reinterpret_cast<T*)(this)) {}
   static T* getInstance() { assert(instance); return instance; }
};

in Engine, i use this Singleton :

class Toto : public Singleton<Toto>
{
};

void register_toto(); //this function creates the singleton from within the engine, just calling new(Toto)

at last, in the exe, i use the instance :

Toto::getInstance();

which raises...or not the assert. Under Visual Studio it seems to work OK, but under mingw the singleton is a static member initialized in engine, and since it's in a header file, the exe also got its own copy of the static member, and thinks it's not initialized. So i have an initialized singleton in Engine and an empty one in the program.

what can I do to have only one static member both for the Dll and the exe?

[screetch]

i've changed my mind, it doesn't work either in Visual Studio

so now how do i make this work? ^_^

here is the code in three different files :

//singleton.h
#include <cassert>

template< class T >
class Singleton
{
   static T* instance;
public:
    Singleton() { instance=reinterpret_cast<T*>(this);}
   static T* getInstance() { assert(instance); return instance; }
};

template<class T>
T* Singleton<T>::instance = 0;

class Toto : public Singleton<Toto>
{
};

#ifdef _USRDLL
__declspec(dllexport)
#else
__declspec(dllimport)
#endif
void register_toto(); //this function creates the singleton from within the engine, just calling new(Toto)

//dll.cpp
#include "singleton.h"

void register_toto()
{
    new Toto;
}

//main.cpp
#include "singleton.h"

int main(void)
{
    register_toto();
    Toto::getInstance();
}

[SeventhStar]

Or you can make code which does not need to call an explicit initializing function.

Like that:

template <class T>
class Singleton
{
public:
    static T& getInstance();
    ~Singleton() {}
protected:
    Singleton() {}
};

...

template <class T>
T& Singleton<T>::getInstance()
{
   static T t;
   return t;
}

and then inherit the desired classes in the manner described by you above.

[jfaust]

And if you're not using vc++ 6.0 (which has a bug), you can also make the destructor private.

Jeff

[screetch]

this doesn't work, the static data in the functions get duplicated to if the code gets duplicated

[jfaust]

This works for me:

#include <iostream>

using namespace std;

template <class T>
class Singleton
{
public:
	static Singleton<T>& getInstance();

	void set(T t) { m_t = t; }
	T get() { return m_t; }

private:
	~Singleton() {}
	Singleton() : m_t() {}

	T m_t;
};

template <class T>
Singleton<T>& Singleton<T>::getInstance()
{
	static Singleton<T> t;
	return t;
}

int main()
{
	// first instances
	{
		Singleton<int>& is = Singleton<int>::getInstance();
		is.set(42);
		Singleton<double>& ds = Singleton<double>::getInstance();
		ds.set(3.14);
	}

	// new instances
	{
		Singleton<int>& is = Singleton<int>::getInstance();
		cout << is.get() << endl;
		Singleton<double>& ds = Singleton<double>::getInstance();
		cout << ds.get() << endl;
	}
}

Jeff

[screetch]

this works in a single file, eventually in a single dll/exe thanks to vague linkage, but won't work through a Dll.

[jfaust]

this works in a single file, eventually in a single dll/exe thanks to vague linkage, but won't work through a Dll.

I missed the dll requirement. This will only work as statically linked.

One option, if you know all the types that will be used, is to move the getInstace to an implementation file and instantiate each instance in the same file. I believe this will solve the dll problem.

If you both an unbounded set of Singletons as well as a dll, I don't know how to solve this problem.

Jeff