Generic C++ singleton implementation

singleton.hpp

#ifndef SINGLETON_H
#define SINGLETON_H

#include <string>

using namespace std;

template<typename T>
class Singleton
{
public:
    static bool isBuilt();
    static T* instance();

protected:
    static T* _instance;
    static void buildInstance();

    Singleton(){}
    ~Singleton(){}
};

template<typename T>
T* Singleton<T>::_instance = 0;

template<typename T>
void Singleton<T>::buildInstance()
{
    if ( _instance )
        throw "Singleton already constructed";
    _instance = new T();
}

template<typename T>
T* Singleton<T>::instance() {
    if ( ! isBuilt() )
      buildInstance();
    return _instance;
}

template<typename T>
bool Singleton<T>::isBuilt() {
    return (_instance != 0);
}

#endif