单例模式的学习笔记
参见链接1，链接2
#include <iostream>
#include <mutex>using namespace std;/*懒汉模式：只有在用到了才实例化对象并返回（调用了对外的接口才会实例对象）*/
/*构造函数私有化，只对外提供一个接口*//*线程不安全*/
class singleClass {
public:static singleClass* getInstance() // 对外接口，静态成员函数调用静态成员变量{if (m_instance == nullptr) // 在多线程的环境下，在判空的时候可能存在多个线程同时进入if中，此时可能会实例化多个对象{m_instance = new singleClass();}return m_instance;}private:static singleClass* m_instance; // 静态成员变量，类内声明，类外初始化singleClass() {} // 构造函数属性设置为私有singleClass (const singleClass & sc) {} // 拷贝构造函数也设置为私有
};singleClass* singleClass::m_instance = nullptr; // 初始化静态变量/*线程安全*/
class threadSingleClass
{
public:static threadSingleClass* getInstance(){if (m_instance == nullptr) // 先判断是否为空，为空就进入；不为空，说明已经存在实例，返回{//m_mutex.lock(); // 这里如果使用unique_lock就不需要在下面解锁unique_lock<mutex> lock(m_mutex);if (m_instance == nullptr) // 再次判断，确保不会因为加锁期间多个线程同时进入{m_instance = new threadSingleClass();}//m_mutex.unlock();}return m_instance;}private:static threadSingleClass* m_instance;static mutex m_mutex;threadSingleClass() {};threadSingleClass(const threadSingleClass& sc) {}
};threadSingleClass* threadSingleClass::m_instance = nullptr;
mutex threadSingleClass::m_mutex;/*饿汉模式：不管调不调用对外接口，都已经实例化对象了。本身就是线程安全的*/
/*类在加载的时候就实力化对象，所以要提前占用系统资源*/
class hungrySingleClass
{
public:static hungrySingleClass* getInstance(){return m_instance;}private:static hungrySingleClass* m_instance;hungrySingleClass() {}hungrySingleClass(const hungrySingleClass* sc) {} // 拷贝构造私有
};hungrySingleClass* hungrySingleClass::m_instance = new hungrySingleClass();
// 类外定义，main开始执行前，该对象就存在了int main()
{singleClass* singlep1 = singleClass::getInstance();singleClass* singlep2 = singleClass::getInstance();cout << singlep1 << endl;cout << singlep2 << endl;threadSingleClass* thsinglep1 = threadSingleClass::getInstance();threadSingleClass* thsinglep2 = threadSingleClass::getInstance();cout << thsinglep1 << endl;cout << thsinglep2 << endl;hungrySingleClass* hgsinglep1 = hungrySingleClass::getInstance();hungrySingleClass* hgsinglep2 = hungrySingleClass::getInstance();cout << hgsinglep1 << endl;cout << hgsinglep2 << endl;system("pause");return 0;
}
