两种单例模式

双重检查锁(DCLP)

• 通过 std::mutex 保证线程安全
• 双重检查减少了锁的开销，只有第一次创建时才会加锁

#include <iostream>
#include <mutex>

class Singleton {
private:
    static Singleton* instance;
    static std::mutex mtx;

    // 私有构造函数，禁止外部创建
    Singleton() {
        std::cout << "Constructor called\n";
    }

public:
    // 删除拷贝和赋值，防止拷贝产生多个实例
    Singleton(const Singleton&) = delete;
    Singleton& operator=(const Singleton&) = delete;

    static Singleton* getInstance() {
        if (instance == nullptr) { // 第一次检查
            std::lock_guard<std::mutex> lock(mtx);
            if (instance == nullptr) { // 第二次检查
                instance = new Singleton();
            }
        }
        return instance;
    }

    void show() {
        std::cout << "Singleton instance address: " << this << "\n";
    }
};

// 静态成员初始化
Singleton* Singleton::instance = nullptr;
std::mutex Singleton::mtx;

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

    s1->show();
    s2->show();

    return 0;
}

静态局部变量

• C++11 标准保证静态局部变量的初始化是线程安全的
• 不需要手动加锁

#include <iostream>

class Singleton {
private:
    Singleton() {
        std::cout << "Constructor called\n";
    }

public:
    Singleton(const Singleton&) = delete;
    Singleton& operator=(const Singleton&) = delete;

    static Singleton& getInstance() {
        static Singleton instance; // C++11 保证线程安全
        return instance;
    }

    void show() {
        std::cout << "Singleton instance address: " << this << "\n";
    }
};

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

    s1.show();
    s2.show();

    return 0;
}