需要帮助来了解单例模式

Question

can someone explain me this code?

class S
{
    public:
      static S& getInstance()
        {
            static S    instance;
            return instance;
        }
    private:
        S() {}
        S(S const&);              // Don't Implement.
        void operator=(S const&); // Don't implement
};

What I understood is : getInstance is a static method that will return a reference to the instance, but where is this instance created ? I don't see any new S(); so..

Answer 1

A block-scope entity with static storage duration (in your case the static S instance; ) is initialized the first time control passes through its declaration. Before C++11, this is not thread-safe (however, certain compilers do offer options to enforce thread-safe). As for C++11, the standard states that "If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization."