一個類不管創建多少次對象，永遠只能得到該類型一個對象的實力

常用到的，比如日志模塊，數據庫模塊

餓漢式單例模式：還沒有獲取實例對象，實例對象就已經產生了

懶漢式單例模式：唯一的實例對象，直到第一次獲取它的時候，才產生

餓漢式單例模式? 餓漢式單例模式一定是線程安全的

#include <iostream>
using namespace std;
class Singleton
{
public:static Singleton* getInstance() //3.獲取類的唯一實例對象的接口方法{return &instance;}
private:static Singleton instance;//2.定義一個唯一的類的實例對象Singleton() //1.構造函數私有化{}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;
}

懶漢式單例模式 線程安全的懶漢式單例模式案例

#include <iostream>
#include <mutex>
using namespace std;
std::mutex mtx;
class Singleton
{
public://是不是可重入函數呢？ 鎖＋雙重判斷static Singleton* getInstance() //3.獲取類的唯一實例對象的接口方法{//lock_guard<std::mutex> lck(mtx);//鎖的力度太大了if (instance == nullptr){lock_guard<std::mutex> lck(mtx);if (instance == nullptr){//開辟內存，構造對象，給instance賦值instance = new Singleton();}}return instance;}
private:static Singleton *volatile instance;//2.定義一個唯一的類的實例對象Singleton() //1.構造函數私有化{}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;
}

更加簡便的懶漢式單例模式

#include <iostream>
#include <mutex>
using namespace std;
std::mutex mtx;
class Singleton
{
public://是不是可重入函數呢？ 鎖＋雙重判斷static Singleton* getInstance() //3.獲取類的唯一實例對象的接口方法{static Singleton instance;//2.定義一個唯一的類的實例對象return &instance;}
private:Singleton() //1.構造函數私有化{}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;
}