设计模式——经典单例

0、核心要素

// 构造、析构函数私有化（一个进程只允许一个对象存在）
// 对象私有化、静态化（因为接口静态函数）
// 对象调用接口静态化（因为静态函数脱离了类对象，可以直接调用）

一、懒汉

唯一的对象在使用时才进行初始化。存在多线程问题。

#include <iostream>using namespace std;class singleMode {public:static singleMode* getInstance() {if (obj==nullptr) {obj = new singleMode();}return obj;}void printMsg() {cout << "print success." << endl;}private:static singleMode* obj;singleMode() {cout << "instance create." << endl;}~singleMode() {cout << "instance release." << endl;}
};singleMode* singleMode::obj = nullptr;
// singleMode* singleMode::obj = new singleMode();int main()
{{singleMode::getInstance()->printMsg();}return 0;
}

二、饿汉

唯一的对象在定义时就完成初始化。

#include <iostream>using namespace std;class singleMode {public:static singleMode* getInstance() {/*   if (obj==nullptr) {obj = new singleMode();}*/return obj;}void printMsg() {cout << "print success." << endl;}private:static singleMode* obj;singleMode() {cout << "instance create." << endl;}~singleMode() {cout << "instance release." << endl;}
};// singleMode* singleMode::obj = nullptr;
singleMode* singleMode::obj = new singleMode();int main()
{{singleMode::getInstance()->printMsg();}return 0;
}