锦州网站建设怎样市场营销的策划方案
1. 模式说明
单例模式保证类只有一个实例;创建一个对象,当你创建第二个对象的时候,此时你获取到的是已经创建过的对象,而不是一个新的对象;
1.1 使用场景
共享资源的访问权限;任务的管理类;数据库的操作等;
1.2 要素
- 私有化类的构造函数;
- 使用类的私有静态指针变量指向类的唯一实例
- 公有静态方法获取该实例;
1.3 类型
- 懒汉模式
- 饿汉模式
2. 懒汉模式
单例实例在第一次被使用时才进行初始化,延迟初始化
2.1 基本
示例
/** @brief : design patterns singleton* @author : your name* @compile : g++ -g singleton_main.cc -o d -std=c++11* @date : 2023/04/18* @lastEditorDate: */#include <iostream>
#include <string>//懒汉模式
class Singleton
{
public:static Singleton* getSingletonInstance() //访问函数{if(instance == nullptr){instance = new Singleton;}return instance;}public:void addCount() {m_count++;}int32_t getCount() {return m_count;}private:static Singleton* instance; //静态指针变量Singleton(){} //私有化类的构造函数int32_t m_count{0};
};Singleton* Singleton::instance = nullptr; //静态成员类外初始化int main(int argc, char* argv[])
{Singleton* single_0 = Singleton::getSingletonInstance();single_0->addCount();std::cout<<"single_0 value is: "<<single_0->getCount()<<std::endl<<std::endl;Singleton* single_1 = Singleton::getSingletonInstance();single_1->addCount();std::cout<<"single_0 value is: "<<single_0->getCount()<<std::endl;std::cout<<"single_1 value is: "<<single_1->getCount()<<std::endl<<std::endl; return 0;
}
输出
single_0 value is: 1single_0 value is: 2
single_1 value is: 2
2.2 线程安全
2.1中的示例是线程不安全的,假如有多个线程同时调用getSingletonInstance(),此时检测到instance是nullptr,就会导致单例的崩溃,造成内存泄漏;
通过加锁来实现
示例
/** @brief : design patterns singleton* @author : your name* @compile : g++ -g singleton_main.cc -o d -std=c++11* @date : 2023/04/18* @lastEditorDate: */#include <iostream>
#include <string>
#include <mutex>//懒汉模式
std::mutex mtx; //全局的锁
class Singleton
{
public:static Singleton* getSingletonInstance() //访问函数{if(instance == nullptr){std::lock_guard<std::mutex> guard(mtx); if(instance == nullptr){instance = new Singleton;} }return instance;}public:void addCount() {m_count++;}int32_t getCount() {return m_count;}private: static Singleton* instance; //静态指针变量Singleton(){} //私有化类的构造函数int32_t m_count{0};
};
Singleton* Singleton::instance = nullptr; //静态成员类外初始化int main(int argc, char* argv[])
{Singleton* single_0 = Singleton::getSingletonInstance();single_0->addCount();std::cout<<"single_0 value is: "<<single_0->getCount()<<std::endl<<std::endl;Singleton* single_1 = Singleton::getSingletonInstance();single_1->addCount();std::cout<<"single_0 value is: "<<single_0->getCount()<<std::endl;std::cout<<"single_1 value is: "<<single_1->getCount()<<std::endl<<std::endl; return 0;
}
输出
single_0 value is: 1single_0 value is: 2
single_1 value is: 2
使用 c++11 中局部静态变量是线程安全的性质
示例
/** @brief : design patterns singleton* @author : your name* @compile : g++ -g singleton_main.cc -o d -std=c++11* @date : 2023/04/18* @lastEditorDate: */#include <iostream>
#include <string>
#include <mutex>//懒汉模式
class Singleton
{
public:static Singleton* getSingletonInstance() //访问函数{static Singleton instance; //静态指针变量return &instance;}public:void addCount() {m_count++;}int32_t getCount() {return m_count;}private: Singleton(){} //私有化类的构造函数int32_t m_count{0};
};int main(int argc, char* argv[])
{Singleton* single_0 = Singleton::getSingletonInstance();single_0->addCount();std::cout<<"single_0 value is: "<<single_0->getCount()<<std::endl<<std::endl;Singleton* single_1 = Singleton::getSingletonInstance();single_1->addCount();std::cout<<"single_0 value is: "<<single_0->getCount()<<std::endl;std::cout<<"single_1 value is: "<<single_1->getCount()<<std::endl<<std::endl; return 0;
}
输出
single_0 value is: 1single_0 value is: 2
single_1 value is: 2
3. 饿汉模式
指单例实例在程序运行时被立即执行初始化
示例
/** @brief : design patterns singleton* @author : your name* @compile : g++ -g singleton_main.cc -o d -std=c++11* @date : 2023/04/18* @lastEditorDate: */#include <iostream>
#include <string>
#include <mutex>//饿汉模式
class Singleton
{
public:static Singleton* getSingletonInstance() //访问函数{ return instance;}public:void addCount() {m_count++;}int32_t getCount() {return m_count;}private:static Singleton* instance; //静态指针变量Singleton(){} //私有化类的构造函数int32_t m_count{0};
};Singleton* Singleton::instance = new Singleton; //静态成员类外初始化int main(int argc, char* argv[])
{Singleton* single_0 = Singleton::getSingletonInstance();single_0->addCount();std::cout<<"single_0 value is: "<<single_0->getCount()<<std::endl<<std::endl;Singleton* single_1 = Singleton::getSingletonInstance();single_1->addCount();std::cout<<"single_0 value is: "<<single_0->getCount()<<std::endl;std::cout<<"single_1 value is: "<<single_1->getCount()<<std::endl<<std::endl; return 0;
}
输出
single_0 value is: 1single_0 value is: 2
single_1 value is: 2
4. 在开发过程中的使用
在项目的使用过程中,可以将单例设计成为类模板;这样其它类在设计的时候不用考虑,是否需要将其设计成为单例;
其它类如果需要单例属性,只需要通过单例模板对其进行赋能即可;
示例
/** @brief : design patterns singleton* @author : your name* @compile : g++ -g singleton_main.cc -o d -std=c++11* @date : 2023/04/18* @lastEditorDate: */#include <iostream>
#include <string>
#include <mutex>template<class T>
class Singleton
{
public:static T* getSingletonInstance() //访问函数{static T instance; //静态指针变量return &instance;}
private: Singleton(){} //私有化类的构造函数
};class C1
{
public:void addCount() {m_count++;}int32_t getCount() {return m_count;} private: int32_t m_count{0};
};int main(int argc, char* argv[])
{auto re1 = Singleton<C1>::getSingletonInstance();re1->addCount();std::cout<<"re1->getCount() value is: "<<re1->getCount()<<std::endl;auto re2 = Singleton<C1>::getSingletonInstance();re2->addCount();std::cout<<"re1->getCount() value is: "<<re1->getCount()<<std::endl;std::cout<<"re2->getCount() value is: "<<re2->getCount()<<std::endl;return 0;
}
输出
re1->getCount() value is: 1
re1->getCount() value is: 2
re2->getCount() value is: 2