• 单例在内存中只有一份，在内存中不会占用太多，程序退出时会随着程序统一释放内存，所以可以不必特意释放

一、懒汉式

不是线程安全的，所以多线程使用要加锁

class Singleton{
public:
    static Singleton* getInstance(){
        if(s_instance == nullptr){
            s_instance = new Singleton;
        }
        return s_instance;
    }

    // 可有可无，程序结束时，系统会自动回收程序的所有内存
    class Garbo{
        ~Garbo(){
            if(s_instance){
                cout << "gc..." << endl;
                delete s_instance;
            }
        }
    };

private:
    Singleton(){
        cout << "Singleton()..." << endl;
    }
    Singleton(const Singleton&) = delete;
    Singleton& operator=(const Singleton&) = delete;
    
    static Singleton* s_instance;
    static Garbo s_gc;
};
Singleton* Singleton::s_instance = nullptr;
int main()
{
    cout << "main()..." << endl;
    Singleton* s = Singleton::getInstance();
    return 0;
}

输出：

main()...
Singleton()...

二、饿汉式

在 main 函数之前，单例就创建好了，所以是线程安全的

class Singleton{
public:
    static Singleton* getInstance(){
        return s_instance;
    }
private:
    Singleton(){
        cout << "Singleton()..." << endl;
    }
    Singleton(const Singleton&) = delete;
    Singleton& operator=(const Singleton&) = delete;
    
    static Singleton* s_instance;
};
Singleton* Singleton::s_instance = new Singleton;
int main()
{
    cout << "main()..." << endl;
    Singleton* s = Singleton::getInstance();
    return 0;
}

输出：

Singleton()...
main()...

三、更加简单的方式

class Singleton{
public:
    static Singleton* getInstance(){
        static Singleton* s_instance = new Singleton;
        return s_instance;
    }
    void sayHello(){
        cout << "Hi there" << endl;
    }
private:
    Singleton(){
        cout << "Singleton()..." << endl;
    }
    Singleton(const Singleton&) = delete;
    Singleton& operator=(const Singleton&) = delete;
};
int main()
{
    cout << "main()..." << endl;
    Singleton* s1 = Singleton::getInstance();
    cout << s1 << endl;
    s1->sayHello();
    Singleton* s2 = Singleton::getInstance();
    cout << s2 << endl;
    s2->sayHello();
    return 0;
}

输出：

main()...
Singleton()...
0x55cc7de532c0
Hi there
0x55cc7de532c0
Hi there