//这里作者模拟实现了一下：仅供参考
namespace hungry
{class Singleton{public:static Singleton& Create(){return slt;}bool Add(const pair<string, string>& kv){hash.insert(kv);return true;}void print(){for (const auto& e : hash){cout << e.first << " " << e.second << endl;}}private://先将构造函数私有化，防止轻易构造新对象Singleton(){}//以hash表为例子unordered_map<string, string> hash;//如何确保只有一个对象？使用staticstatic Singleton slt;//!!!记住要防止拷贝Singleton& operator=(const Singleton& st) = delete;Singleton(const Singleton& st) = delete;};Singleton Singleton::slt;//这里有点小奇怪，就理解为用singleton实例化singletion的slt吧
}//懒汉模式：直接用指针代替，需要的时候再进行实例化
namespace lazy
{class Singleton{public:static Singleton& Create(){if (slt == nullptr){slt = new Singleton;return *slt;}else{return *slt;}}// 一般单例不用释放。// 特殊场景：1、中途需要显示释放  2、程序结束时，需要做一些特殊动作（如持久化）,比如写入文件static void Delete(){if (slt){delete slt;slt = nullptr;}else return;}bool Add(const pair<string, string>& kv){hash.insert(kv);return true;}void print(){for (const auto& e : hash){cout << e.first << " " << e.second << endl;}}class GC{public:~GC(){//垃圾回收站，这是为了方便多个单例对象需要持久化或者手动释放使用的Singleton::Delete();}};private://先将构造函数私有化，防止轻易构造新对象Singleton(){}~Singleton(){//在这里可以做一些持久化操作，将数据写入文件cout << "destructor" << endl;}unordered_map<string, string> hash;static Singleton* slt;static Singleton::GC gc;Singleton& operator=(const Singleton& st) = delete;Singleton(const Singleton& st) = delete;};Singleton* Singleton::slt=nullptr;Singleton::GC Singleton::gc;
}

//只能在堆上实例化
class Heaponly1
{
public:Heaponly1(){ }void Delete(){delete this;//巧妙}private:~Heaponly1(){//...}//要防止在栈上创建空间，还要将拷贝构造和复制构造deleteHeaponly1& operator=(const Heaponly1& hp) = delete;Heaponly1(const Heaponly1& hp) = delete;
};class Heaponly2
{
public:static Heaponly2* Create(){return new Heaponly2;}
private:Heaponly2(){//...}Heaponly2& operator=(const Heaponly2& hp) = delete;Heaponly2(const Heaponly2& hp) = delete;
};

//只能在栈上创建对象的类
class Stackonly1
{
public:static Stackonly1 Create(){Stackonly1 st;return st;}
private:Stackonly1(){}
};class Stackonly2
{
public:Stackonly2(){ }
private:void* operator new(size_t size) = delete;
};