[C++知识库]设计模式--单例模式（c++实现）

定义：顾名思义，确保一个类在任何情况下都只有一个实例。

#include<iostream>
using namespace std;

class SingletonDP
{
public:
	static SingletonDP* GetInstance();
private:
	SingletonDP() 
	{ 
		cout << "Constructor" << endl; 
	};//构造函数私有
	static SingletonDP* pSingleInst;
};

SingletonDP* SingletonDP::pSingleInst = nullptr;
SingletonDP* SingletonDP::GetInstance()
{
	cout << "Return Instance By GetInstance()" << endl;
	if (!pSingleInst)
		pSingleInst = new SingletonDP();
	return pSingleInst;
}

int main()
{
	cout << "SingletonDPTest" << endl;
	SingletonDP *pTemp1 = SingletonDP::GetInstance();
	SingletonDP *pTemp2 = nullptr;
	if (pTemp1) pTemp2 = pTemp1->GetInstance();
	cout << "pTemp1 Address: " << pTemp1 << endl;
	cout << "pTemp2 Address: " << pTemp2 << endl;
	system("pause");
	return 0;
}

从上述例子输出结果可以看出：1.在获取该类实例对象时才会走构造函数；2.两者地址相同满足单例模式。

但是，这种方式是线程不安全的，如果多个线程都第一次访问该类实例的时候，可能会创建多个实例。

?

#include<iostream>
#include<mutex>
using namespace std;

class SingletonDP
{
public:
	static SingletonDP* GetInstance();
private:
	SingletonDP() 
	{ 
		cout << "Constructor" << endl; 
	};//构造函数私有
	static SingletonDP* pSingleInst;
	static mutex sMutex;
};

SingletonDP* SingletonDP::pSingleInst = nullptr;
mutex SingletonDP::sMutex;
SingletonDP* SingletonDP::GetInstance()
{
	cout << "Return Instance By GetInstance()" << endl;
	if (!pSingleInst)
	{
		sMutex.lock();
		if(!pSingleInst)
			pSingleInst = new SingletonDP();
		sMutex.unlock();
	}
	return pSingleInst;
}

int main()
{
	cout << "SingletonDPTest" << endl;
	SingletonDP *pTemp1 = SingletonDP::GetInstance();
	SingletonDP *pTemp2 = nullptr;
	if (pTemp1) pTemp2 = pTemp1->GetInstance();
	cout << "pTemp1 Address: " << pTemp1 << endl;
	cout << "pTemp2 Address: " << pTemp2 << endl;
	system("pause");
	return 0;
}

如上述代码所示：在GetInstance()方法中通过Double Check解决线程不安全问题。（也可以通过在创建线程之前先实例化一个实例对象来解决该问题）

#include<iostream>
using namespace std;

class SingletonDP
{
public:
	static SingletonDP* GetInstance()
	{
		static SingletonDP OnlyInstance;
		return &OnlyInstance;
	}
private:
	SingletonDP()
	{
		cout << "Constructor" << endl;
	};//构造函数私有
};


int main()
{
	cout << "SingletonDPTest" << endl;
	SingletonDP *pTemp1 = SingletonDP::GetInstance();
	SingletonDP *pTemp2 = nullptr;
	if (pTemp1) pTemp2 = pTemp1->GetInstance();
	cout << "pTemp1 Address: " << pTemp1 << endl;
	cout << "pTemp2 Address: " << pTemp2 << endl;
	system("pause");
	return 0;
}

?上述模式在类创建的同时就已经创建好一个静态对象，以后不会改变，是线程安全的。