// 不好的解法一: 只适用于单线程环境
class Singleton1 {public:static Singleton1* getInstance(void) {if(instance == NULL) {instance = new Singleton1();}return instance;}private:static Singleton1 *instance;private:Singleton1(void) {}Singleton1(const Singleton1&);Singleton1& operator=(const Singleton1&);~Singleton1(void) {}class NestedClass {public:~NestedClass(void) {delete Singleton1::instance;Singleton1::instance =  NULL;}};static NestedClass justForSingleton; // 实现程序结束时, 释放 instance 占用的动态内存
};

// 不好的解法一(扩展): 只适用于单线程环境
class Singleton2 {public:static Singleton2& getInstance(void) {static Singleton2 instance; // 使用局部静态变量, 实现 instance 占用内存的释放return instance;}private:Singleton2(void) {}Singleton2(const Singleton2&);Singleton2& operator=(const Singleton2&);~Singleton2(void) {}};

// 可行的解法: 加同步锁前后两次判断实例是否存在
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
class Singleton1 {public:static Singleton1* getInstance(void) {if(instance == NULL) {pthread_mutex_lock(&mutex);if(instance == NULL) {instance = new Singleton1();}pthread_mutex_unlock(&mutex);}return instance;}private:static Singleton1 *instance;private:Singleton1(void) {}Singleton1(const Singleton1&);Singleton1& operator=(const Singleton1&);~Singleton1(void) {}class NestedClass {public:~NestedClass(void) {delete Singleton1::instance;Singleton1::instance =  NULL;}};static NestedClass justForSingleton;
};Singleton1* Singleton1::instance = NULL;

// 强烈推荐的解法: 利用静态成员变量
class Singleton3 {public:static Singleton3* getInstance(void) {return instance;}private:static Singleton3 *instance;private:Singleton3(void) {}Singleton3(const Singleton3&);Singleton3& operator=(const Singleton3&);~Singleton3(void) {}class NestedClass {public:~NestedClass(void) {delete Singleton3::instance;Singleton3::instance =  NULL;}};static NestedClass justForSingleton;
};Singleton3* Singleton3::instance = new Singleton3(); // 在程序开始时就会产生一个该类的实例，并在以后仅返回此实例

// 强烈推荐的的解法: 实现按需创建实例, 但这里并没有实现(C++中实现按需创建, 似乎只能借助于锁机制)
class Singleton4 {public:static Singleton4* getInstance(void) {return &NestedClass::instance; // 实例在进入主程序前已完成初始化}private:Singleton4(void) {}Singleton4(const Singleton4&);Singleton4& operator=(const Singleton4&);~Singleton4(void) {}class NestedClass {public:static Singleton4 instance;};
};
Singleton4 Singleton4::NestedClass::instance;

在单例类内部定义专有的嵌套类；
在单例类内定义私有的专门用于释放的静态成员；
利用程序在结束时析构全局变量的特性，选择最终的释放时机；
使用单例的代码不需要任何操作，不必关心对象的释放。