单例模式

来源：互联网发布：淘宝大学在哪里找编辑：程序博客网时间：2024/06/04 01:36

1.饿汉模式

#include<iostream>using namespace std;class singleton1{private:    static singleton1 s;    int i;    singleton1(int x) :i(x){}    singleton1(const singleton1 &);    singleton1& operator=(singleton1 &);public:    static singleton1& instance(){//static singleton1&        return s;    }    int getValue(){        return i;    }    void setValue(int x){        i = x;    }};singleton1 singleton1::s(47);//此处不能写staticint main(){    singleton1& s = singleton1::instance();//&，否则错误    cout << s.getValue() << endl;    singleton1& s2 = singleton1::instance();    s2.setValue(9);    cout << s.getValue() << endl;    return 0;}

2.懒汉模式

#include<iostream>using namespace std;class singleton2{private:    int i;    singleton2(int x) :i(x){}    singleton2(const singleton2&);    singleton2& operator=(singleton2&);public:    static singleton2& instance(){        static singleton2 s(47);        return s;    }    int getVal(){        return i;    }    void setVal(int x){        i = x;    }};int main(){    singleton2& s = singleton2::instance();    cout << s.getVal() << endl;    singleton2& s2 = singleton2::instance();    s2.setVal(9);    cout << s.getVal() << endl;    return 0;}

结果都是
47
9

3.线程安全

#include<iostream>#include<mutex>using namespace std;class Lock{private:    mutex* mutexPtr;public:    Lock(mutex* pm) : mutexPtr(pm){        mutexPtr->lock();    }    ~Lock(){        mutexPtr->unlock();    }};class singleton3{private:    int i;    static mutex* m;//static    static singleton3* pInstance;    singleton3(int x) :i(x){}    singleton3(const singleton3&);    singleton3& operator=(singleton3&);public:    static singleton3* instance(){        if (pInstance == NULL){            Lock l(m);            if (pInstance == NULL){                pInstance = new singleton3(47);            }        }        return pInstance;    }    void setVal(int x){        i = x;    }    int getVal(){        return i;    }};singleton3* singleton3::pInstance = 0;mutex* singleton3::m = new mutex();int main(){    singleton3* s = singleton3::instance();    cout << s->getVal() << endl;     singleton3* s2 = singleton3::instance();    s->setVal(9);    cout << s->getVal() << endl;    return 0;}

之所以在Instantialize函数里面对pInstance 是否为空做了两次判断，因为该方法调用一次就产生了对象。

pInstance == NULL 大部分情况下都为false，如果按照原来的方法，每次获取实例都需要加锁，效率太低。
而改进的方法只需要在第一次调用的时候加锁，可大大提高效率。

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