class Singleton
{
private:
    static Singleton* m_instance;
    Singleton(){}
public:
    static Singleton* getInstance();
};

Singleton* Singleton::getInstance()
{
    if(NULL == m_instance)
    {
        Lock();//借用其它类来实现，如boost
        if(NULL == m_instance)
        {
            m_instance = new Singleton;
        }
        UnLock();
    }
    return m_instance;
}
//上面的代码getInstance我觉得第一个if是很有必要，如果改为如下方式，不管m_instance 有没有创建好都会去参与锁的竞争，会严重降低效率，因此第一个if是很有必要的
Singleton* Singleton::getInstance()
{
      
        Lock();//借用其它类来实现，如boost
        if(NULL == m_instance)
        {
            m_instance = new Singleton;
        }
        UnLock();
   
    return m_instance;
}

 1 class SingletonInside
 2 {
 3 private:
 4     SingletonInside(){}
 5 public:
 6     static SingletonInside* getInstance()
 7     {
 8         Lock(); // not needed after C++0x
 9         static SingletonInside instance;
10         UnLock(); // not needed after C++0x
11         return instance; 
12     }
13 };

 1 class SingletonStatic
 2 {
 3 private:
 4     static const SingletonStatic* m_instance;
 5     SingletonStatic(){}
 6 public:
 7     static const SingletonStatic* getInstance()
 8     {
 9         return m_instance;
10     }
11 };
12 
13 //外部初始化 before invoke main
14 const SingletonStatic* SingletonStatic::m_instance = new SingletonStatic;