设计模式 —— 单例模式

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最新推荐文章于 2024-04-26 16:22:39 发布

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分类专栏： C++学习算法文章标签： c++ c语言

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本文介绍了三种C++实现单例模式的方法：懒汉式（线程安全和非线程安全版本）、饿汉式。懒汉式通过双重检查锁定确保线程安全，避免内存泄漏；懒汉式非线程安全版本利用静态局部变量在首次调用时初始化，达到线程安全；饿汉式则在类加载时即创建单例，保证线程安全。

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//******************************* 懒汉式1 *******************************
#include <iostream>
#include <memory> // shared_ptr
#include <mutex>  // mutex

// version 2: thread is safe now and memory doesn't leak

class Singleton 
{
public:
	typedef std::shared_ptr<Singleton> Ptr;

	~Singleton() 
	{
		std::cout << "destructor called!" << std::endl;
	}

	Singleton(Singleton&) = delete;
	
	Singleton& operator=(const Singleton&) = delete;
	
	static Ptr get_instance() 
	{
		//注意这里双重判断
		if (m_instance_ptr == nullptr) 
		{
			std::lock_guard<std::mutex> lk(m_mutex);//C++11加互斥锁，可以自动解锁
			if (m_instance_ptr == nullptr) 
			{
				m_instance_ptr = std::shared_ptr<Singleton>(new Singleton);
			}
		}

		return m_instance_ptr;
	}

private:
	Singleton() 
	{
		std::cout << "constructor called!" << std::endl;
	}

	static Ptr m_instance_ptr;
	static std::mutex m_mutex;
};

// initialization static variables out of class
Singleton::Ptr Singleton::m_instance_ptr = nullptr;
std::mutex Singleton::m_mutex;

int main() 
{
	Singleton::Ptr instance1 = Singleton::get_instance();
	Singleton::Ptr instance2 = Singleton::get_instance();

	return 0;
}


//******************************* 懒汉式2 *******************************
//懒汉式在不使用锁机制的情况下保证线程安全
#include <iostream>

class Singleton1{
public:
	~Singleton1() 
	{
		std::cout << "destructor called!" << std::endl;
	}
	
	Singleton1(const Singleton1&) = delete;
	
	Singleton1& operator=(const Singleton1&) = delete;
	
	static Singleton1& get_instance() 
	{
		static Singleton1 instance;
		return instance;

	}

private:
	Singleton1() 
	{
		std::cout << "constructor called!" << std::endl;
	}
};

int main(int argc, char* argv[])
{
	Singleton1& instance_1 = Singleton1::get_instance();
	Singleton1& instance_2 = Singleton1::get_instance();

	return 0;
}


//******************************* 饿汉式1 *******************************
//全局的单例实例在类装载时构建。
#include <iostream>
using namespace std;

class Singleton2{
private:
	static Singleton2* pInstance;

	Singleton2()
	{
		cout << "constructor called!" << std::endl;
	}

	Singleton2(const Singleton2&) = delete;

	Singleton2& operator = (const Singleton2&) = delete;

public:
	static Singleton2* getInstance();

	~Singleton2() 
	{
		std::cout << "destructor called!" << std::endl;
	}

	void print()
	{
		cout << "this = " << this << endl;
	}
};

Singleton2* Singleton2::pInstance = new Singleton2;   //的单例实例在类装载时构建。

// 静态方法返回该实例
Singleton2* Singleton2::getInstance()
{
	return pInstance;
}

int main()
{
	Singleton2* s1 = Singleton2::getInstance();
	Singleton2* s2 = Singleton2::getInstance();

	s1->print();
	s2->print();

	return 0;
}