/**/#ifdef Implementation1/**/class Singleton {public:    static Singleton* Instance();protected:    Singleton();private:    static Singleton* _instance;};/**/Singleton* Singleton::_instance = 0;Singleton* Singleton::Instance () {    if (_instance == 0) {        _instance = new Singleton;    }    return _instance;}/**/#endif/**/#ifdef Implementation2#include "List.H"#include "stdlib.h"class NameSingletonPair;/**/class Singleton {public:    static void Register(char* name, Singleton*);    static Singleton* Instance();protected:    static Singleton* Lookup(const char* name);private:    static Singleton* _instance;    static List<NameSingletonPair>* _registry;};/**/Singleton* Singleton::Instance () {    if (_instance == 0) {        const char* singletonName = getenv("SINGLETON");        // user or environment supplies this at startup        _instance = Lookup(singletonName);        // Lookup returns 0 if there's no such singleton    }    return _instance;}/**/class MySingleton : public Singleton {public:    MySingleton();};/**/MySingleton::MySingleton() {    // ...    Singleton::Register("MySingleton", this);}/**/static MySingleton theSingleton;/**/#endif/**/#ifdef Singleton/**/class MazeFactory {public:    static MazeFactory* Instance();    // existing interface goes hereprotected:    MazeFactory();private:    static MazeFactory* _instance;};/**/MazeFactory* MazeFactory::_instance = 0;MazeFactory* MazeFactory::Instance () {    if (_instance == 0) {        _instance = new MazeFactory;    }    return _instance;}/**/#else//MazeFactory* MazeFactory::_instance = 0;#include "C++/MazeFactories.H"#include "stdlib.h"#include "strings.h"/**/MazeFactory* MazeFactory::Instance () {    if (_instance == 0) {        const char* mazeStyle = getenv("MAZESTYLE");/**/        if (strcmp(mazeStyle, "bombed") == 0) {            _instance = new BombedMazeFactory;/**/        } else if (strcmp(mazeStyle, "enchanted") == 0) {            _instance = new EnchantedMazeFactory;/**/        // ... other possible subclasses/**/        } else {        // default            _instance = new MazeFactory;        }    }    return _instance;}/**/#endif/**/