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 

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 

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;
}