C++单例模式

 C++ 单例模式_c++单例模式-CSDN博客

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#include 
#include "widget.h"
using namespace std;

/*
*	版本1 SingletonPattern_V1 存在以下两个问题
*
*	1. 线程不安全, 非线程安全版本
*	2. 内存泄露
*/

class SingletonPattern_V1
{
private:
    SingletonPattern_V1() {
        cout << "constructor called!" << endl;
    }
    SingletonPattern_V1(SingletonPattern_V1&) = delete;
    SingletonPattern_V1& operator=(const SingletonPattern_V1&) = delete;
    static SingletonPattern_V1* m_pInstance;

public:
    ~SingletonPattern_V1() {
        cout << "destructor called!" << endl;
    }
    //在这里实例化
    static SingletonPattern_V1* Instance() {
        if (!m_pInstance) {
            m_pInstance = new SingletonPattern_V1();
        }
        return m_pInstance;
    }
    void use() const { cout << "in use" <use();
    p2->use();
    return 0;
}

constructor called!

in use

in use

容易知道此时是内存泄漏的 

线程安全:

多线程中:

static SingletonPattern_V1* Instance() {
		if (!m_pInstance) {
			m_pInstance = new SingletonPattern_V1();
		}
		return m_pInstance;
	}

执行该段代码时,if(!m_pInstance),如果线程A和线程B都判断为空,都开始初始化实例,那么会初始化两次.此时需要加锁.

解决问题后的代码:

#include 
using namespace std;
#include  // C++11 shared_ptr头文件
#include   // C++11 mutex头文件
/*
*	版本2 SingletonPattern_V2 解决了V1中的问题
*
*	1. 通过加锁让线程安全了
*	2. 通过智能指针(shareptr 基于引用计数)内存没有泄露了
*/
class SingletonPattern_V2 
{
public:
	~SingletonPattern_V2() {
		std::cout << "destructor called!" << std::endl;
	}
	SingletonPattern_V2(SingletonPattern_V2&) = delete;
	SingletonPattern_V2& operator=(const SingletonPattern_V2&) = delete;
 
	//在这里实例化
	static std::shared_ptr Instance() 
	{
		//双重检查锁
		if (m_pInstance == nullptr) {
			std::lock_guard lk(m_mutex);
			if (m_pInstance == nullptr) {
				m_pInstance = std::shared_ptr(new SingletonPattern_V2());
			}
		}
		return m_pInstance;
	}
 
private:
	SingletonPattern_V2() {
		std::cout << "constructor called!" << std::endl;
	}
	static std::shared_ptr m_pInstance;
	static std::mutex m_mutex;
};
 
//在类外初始化静态变量
std::shared_ptr SingletonPattern_V2::m_pInstance = nullptr;
std::mutex SingletonPattern_V2::m_mutex;
 
int main()
{
	std::shared_ptr p1 = SingletonPattern_V2::Instance();
	std::shared_ptr p2 = SingletonPattern_V2::Instance();
 
	system("pause");
	return 0;
}

用智能共享指针和锁把上面的问题给解决了. 

SingletonPattern_V2(SingletonPattern_V2&) = delete;

C++11中=delete的巧妙用法_= delete-CSDN博客