设计模式之单例模式

个人理解1、不具备线程安全的懒汉单例模式2、线程安全的懒汉单例模式3、返回一个reference指向local static对象4、饿汉单例模式

单例模式顾名思义，保证一个类仅可以有一个实例化对象，并且提供一个可以访问它的全局接口。

#include 
using namespace std;

class Singleton
{
private:
    Singleton(){}
    Singleton(const Singleton& obj) = delete;
    Singleton& operator = (const Singleton& obj) = delete;
    static Singleton* m_instance;
public:
    static Singleton* GetInstance();
};

Singleton* Singleton::m_instance = NULL;

Singleton* Singleton::GetInstance()
{
    if (m_instance == NULL)
        m_instance = new Singleton();
    return m_instance;
}

int main()
{
	Singleton* s = Singleton::GetInstance();
    return 0;
}

  在单例模式构造时加锁防止构造失败

#include 
#include 
using namespace std;

std::mutex mt;

class Singleton
{
private:
    Singleton(){}
    Singleton(const Singleton& obj) = delete;
    Singleton& operator = (const Singleton& obj) = delete;
    static Singleton* m_instance;
public:
    static Singleton* GetInstance();

};

Singleton* Singleton::m_instance = NULL;

Singleton* Singleton::GetInstance()
{
    mt.lock();
    if (m_instance == NULL)
        m_instance = new Singleton();
    mt.unlock();
    return m_instance;
}

int main()
{
	Singleton* s = Singleton::GetInstance();
    return 0;
}

  这种实现方式的好处是不用删除它

#include 

using namespace std;

class Singleton
{
private:
    Singleton(){}
    Singleton(const Singleton& obj) = delete;
    Singleton& operator = (const Singleton& obj) = delete;

public:
    static Singleton& GetInstance();

};

Singleton& Singleton::GetInstance()
{
    static Singleton singleton;
    return singleton;
}

int main()
{
    return 0;
}

 在访问量比较大，或者可能访问的线程比较多时，采用饿汉实现，可以实现更好的性能。这是以空间换时间。

#include 
using namespace std;


class Singleton
{
private:
    Singleton(){}
    Singleton(const Singleton& obj) = delete;
    Singleton& operator = (const Singleton& obj) = delete;

public:
    static Singleton* m_instance;
    static Singleton* GetInstance();

};

Singleton* Singleton::m_instance = new Singleton();

Singleton* Singleton::GetInstance()
{
    return m_instance;
}

int main()
{
    Singleton::GetInstance();
    return 0;
}