一个类不管创建多少次对象，永远只能得到该类型一个对象的实力

常用到的，比如日志模块，数据库模块

饿汉式单例模式：还没有获取实例对象，实例对象就已经产生了

懒汉式单例模式：唯一的实例对象，直到第一次获取它的时候，才产生

饿汉式单例模式  饿汉式单例模式一定是线程安全的

#include <iostream>
using namespace std;
class Singleton
{
public:static Singleton* getInstance() //3.获取类的唯一实例对象的接口方法{return &instance;}
private:static Singleton instance;//2.定义一个唯一的类的实例对象Singleton() //1.构造函数私有化{}Singleton(const Singleton&) = delete;Singleton& operator=(const Singleton&) = delete;
};
Singleton Singleton::instance;
int main()
{Singleton* p1 = Singleton::getInstance();Singleton* p2 = Singleton::getInstance();Singleton* p3 = Singleton::getInstance();cout << p1 << " " << p2 << " " << p3 << " " << endl;return 0;
}

懒汉式单例模式 线程安全的懒汉式单例模式案例

#include <iostream>
#include <mutex>
using namespace std;
std::mutex mtx;
class Singleton
{
public://是不是可重入函数呢？ 锁＋双重判断static Singleton* getInstance() //3.获取类的唯一实例对象的接口方法{//lock_guard<std::mutex> lck(mtx);//锁的力度太大了if (instance == nullptr){lock_guard<std::mutex> lck(mtx);if (instance == nullptr){//开辟内存，构造对象，给instance赋值instance = new Singleton();}}return instance;}
private:static Singleton *volatile instance;//2.定义一个唯一的类的实例对象Singleton() //1.构造函数私有化{}Singleton(const Singleton&) = delete;Singleton& operator=(const Singleton&) = delete;
};
Singleton* volatile Singleton::instance = nullptr;
int main()
{Singleton* p1 = Singleton::getInstance();Singleton* p2 = Singleton::getInstance();Singleton* p3 = Singleton::getInstance();cout << p1 << " " << p2 << " " << p3 << " " << endl;return 0;
}

更加简便的懒汉式单例模式

#include <iostream>
#include <mutex>
using namespace std;
std::mutex mtx;
class Singleton
{
public://是不是可重入函数呢？ 锁＋双重判断static Singleton* getInstance() //3.获取类的唯一实例对象的接口方法{static Singleton instance;//2.定义一个唯一的类的实例对象return &instance;}
private:Singleton() //1.构造函数私有化{}Singleton(const Singleton&) = delete;Singleton& operator=(const Singleton&) = delete;
};
int main()
{Singleton* p1 = Singleton::getInstance();Singleton* p2 = Singleton::getInstance();Singleton* p3 = Singleton::getInstance();cout << p1 << " " << p2 << " " << p3 << " " << endl;return 0;
}