在C++标准中，是这样描述的（在标准草案的6.7节中）：

such a variable is initialized the first time control passes through its declaration; such a variable is considered initialized upon the completion of its initialization. If the initialization exits by throwing an exception, the initialization is not complete, so it will be tried again the next time control enters the declaration. If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization. If control re-enters the declaration recursively while the variable is being initialized, the behavior is undefined.

分析

标准关于局部静态变量初始化，有这么几点要求：

1. 变量在代码第一次执行到变量声明的地方时初始化。
2. 初始化过程中发生异常的话视为未完成初始化，未完成初始化的话，需要下次有代码执行到相同位置时再次初始化。
3. 在当前线程执行到需要初始化变量的地方时，如果有其他线程正在初始化该变量，则阻塞当前线程，直到初始化完成为止。
4. 如果初始化过程中发生了对初始化的递归调用，则视为未定义行为

所以一下这种方式，直接就是线程安全的

class Foo 
{ 
public: 
    static Foo *getInstance() 
    { 
        static Foo s_instance; 
        return &s_instance; 
    } 
private: 
    Foo() {} 
};