// the double check pattern
template<class TYPE, class LOCK>
TYPE* Singleton<TYPE, LOCK>::Instance() {
	// this one is for performance
	if (_instance == 0) {
		Guard<LOCK> monitor(_lock);
		// and this one is for thread safeness
		if (_instance == 0)
			_instance = new TYPE;
	}
	return _instance;
}
D. Schmidt in: J. Vlissides, "To Kill a Singleton," C++ Report, vol. 8, pp. 10-19, June 1996.

A note: the pointer assignment needs to be atomic, which is of course not guaranteed by the C++ language.
One has to ask: how expensive is a lock?

An additional note: don't use the double check pattern, it's broken.