1. #include <iostream>
    3. using namespace std;
    5. class Singleton {
    6. private:
    7.     Singleton() {
    8.         cout << "Constructor Called" << endl;
    9.     }
    11. public:
    12.     Singleton(Singleton &) = delete;
    14.     Singleton &operator=(Singleton &) = delete;
    16.     ~Singleton() {
    17.         cout << "Destructor Called" << endl;
    18.     }
    20.     static Singleton &Instance() {
    21.         // 注意返回引用
    22.         // 使用local static对象替换non-local static对象
    23.         static Singleton singleton;
    24.         return singleton;
    25.     }
    26. };
    28. int main() {
    29.     cout << "main begin" << endl;
    30.     Singleton &s1 = Singleton::Instance();
    31.     cout << "1" << endl;
    32.     Singleton &s2 = Singleton::Instance();
    33.     cout << "2" << endl;
    34.     Singleton &s3 = s1;
    35.     cout << "main end" << endl;
    36.     return 0;
    37. }

    输出
    image.png
    可见构造函数只调用了一次，并且local static的初始化是在第一次执行到其声明式时，生命周期维持到程序结束。

    并且该方法在c++11之后是线程安全的！！！

    If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization. 如果当变量在初始化的时候，并发同时进入声明语句，并发线程将会阻塞等待初始化结束。