一個(gè)類只有一個(gè)實(shí)例

• 將所有構(gòu)造函數(shù)私有化，防止外部創(chuàng)建對(duì)象
• 給出共有靜態(tài)成員函數(shù)為獲取對(duì)象實(shí)例的唯一渠道
• 應(yīng)用場(chǎng)景：緩存，日志，工具類，配置，線程池等

分類：

餓漢式：程序啟動(dòng)就創(chuàng)建對(duì)象，不管用不用

懶漢式：需要用的時(shí)候創(chuàng)建，不用了銷毀

餓漢式

程序啟動(dòng)就創(chuàng)建對(duì)象實(shí)例，不管有沒有使用該對(duì)象

#include<iostream>
using namespace std;
//餓漢式  程序啟動(dòng)就創(chuàng)建對(duì)象實(shí)例，不管有沒有使用該對(duì)象
class single
{
    int m_data;
    //1. 把所有構(gòu)造函數(shù)私有
    single(int data = 0) {}
    single(single const& single1) {}
    //2.靜態(tài)成員對(duì)象
    static single s_single;
    //3.提供靜態(tài)公有方法返回該私有靜態(tài)成員對(duì)象
public:
    static single& getinstance()
    {
        return s_single;
    }
};
single single::s_single(10);
int main()
{
    single& s1 = single::getinstance();
    single& s2 = single::getinstance();
    cout << &s1 << " " << &s2 << endl;
    return 0;
}

懶漢式

你需要用對(duì)象才創(chuàng)建該對(duì)象實(shí)例

#include<iostream>
using namespace std;
//懶漢式 你需要用對(duì)象才創(chuàng)建該對(duì)象實(shí)例
class singleton
{
    int m_data;
    //1. 把所有構(gòu)造函數(shù)私有
    singleton(int data = 0) {}
    singleton(singleton const& single1) {}

    //2.靜態(tài)成員對(duì)象指針
    static singleton* s_single;
    //3.提供靜態(tài)公有方法返回該私有靜態(tài)成員對(duì)象
public:
    static singleton& getinstance()
    {
        if (s_single == NULL)
        {
            s_single = new singleton(20);
        }
        return *s_single;
    }
    ~singleton()
    {
        if (s_single)
            delete s_single;
        s_single = NULL;
    }
};
singleton* singleton::s_single = NULL;
int main()
{
    singleton& st1 = singleton::getinstance();
    singleton& st2 = singleton::getinstance();
    cout << &st1 << " " << &st2 << endl;

    return 0;
}