C++ 实现单例模式（懒汉式和饿汉式）

单例模式

懒汉式

#include <thread>
#include <cstdio>
class Singleton
{
private:
    Singleton(){printf("new Singleton.\n");};
    static Singleton* m_Instance;
public:
    static Singleton* GetInstance()
    {
        if(m_Instance==nullptr)
        {
            if(m_Instance == nullptr)
            {
                m_Instance = new Singleton();
            }
        }
        return m_Instance;
    }
};
Singleton* Singleton::m_Instance = nullptr;
void func()
{
    Singleton *s = Singleton::GetInstance();
}
int main()
{

    std::thread t1(func);
    std::thread t2(func);
    t1.join();
    t2.join();
}

可以看到上面的代码是线程不安全的：
如果两个线程同时调用GetInstance方法且同时检测到Instance是NULL，则两个线程会同时构造一个实例给Instance，这样就发生错误。

懒汉式-上锁

#include <thread>
#include <mutex>
#include <cstdio>
std::mutex mylock;
class Singleton
{
private:
    Singleton(){printf("new Singleton.\n");};
    static Singleton* m_Instance;
public:
    static Singleton* GetInstance()
    {
        if(m_Instance==nullptr)
        {
            mylock.lock();
            if(m_Instance == nullptr)
            {
                m_Instance = new Singleton();
            }
            mylock.unlock();
        }
        return m_Instance;
    }
};
Singleton* Singleton::m_Instance = nullptr;
void func()
{
    Singleton *s = Singleton::GetInstance();
}
int main()
{

    std::thread t1(func);
    std::thread t2(func);
    t1.join();
    t2.join();
}

饿汉式

饿汉式一开始就创建了类的实例，所以直接将创建好的实例返回就好。

#include <cstdio>
class Singleton
{
private:
    /* data */
    Singleton(){printf("New Singleton.\n");};
public:
    static Singleton* GetInstance()
    {
        static Singleton m_Instance;
        return &m_Instance;
    }
};
int main()
{
    Singleton* m_Instance = Singleton::GetInstance();
    return 0;
}