C++中单例的线程安全延迟构造

#include<iostream>
#include <mutex>

class Singleton {
public:
    static Singleton* getInstance() {
        if (instance == nullptr) {
            std::lock_guard<std::mutex> lock(mutex);
            if (instance == nullptr) {
                instance = new Singleton;
            }
        }
        return instance;
    }

private:
    Singleton() {}

    static Singleton* instance;
    static std::mutex mutex;
};

Singleton* Singleton::instance = nullptr;
std::mutex Singleton::mutex;

int main() {
    Singleton* s1 = Singleton::getInstance();
    Singleton* s2 = Singleton::getInstance();

    if (s1 == s2) {
        std::cout << "s1 and s2 are the same instance"<< std::endl;
    } else {
        std::cout << "s1 and s2 are different instances"<< std::endl;
    }

    return 0;
}