单例模式的两种实现方式

1.   饿汉模式：

#include <iostream>using namespace std;class Singleton {public:static Singleton& getInst (void) {return s_inst;}private:Singleton (void) {}Singleton (const Singleton&);static Singleton s_inst;};Singleton Singleton::s_inst;int main (void) {Singleton& s1 = Singleton::getInst ();Singleton& s2 = Singleton::getInst ();Singleton& s3 = Singleton::getInst ();cout << &s1 << ' ' << &s2 << ' ' << &s3 << endl;return 0;}

2 . 懒汉模式

#include <iostream>using namespace std;class Singleton {public:static Singleton& getInst (void) {if (! m_inst)m_inst = new Singleton;++m_cn;return *m_inst;}void releaseInst (void) {if (m_cn && --m_cn == 0)delete this;}private:Singleton (void) {cout << "构造：" << this << endl;}Singleton (const Singleton&);~Singleton (void) {cout << "析构：" << this << endl;m_inst = NULL;}static Singleton* m_inst;static unsigned int m_cn;};Singleton* Singleton::m_inst = NULL;unsigned int Singleton::m_cn = 0;int main (void) {Singleton& s1 = Singleton::getInst ();Singleton& s2 = Singleton::getInst ();Singleton& s3 = Singleton::getInst ();cout << &s1 << ' ' << &s2 << ' ' << &s3 << endl;s3.releaseInst ();s2.releaseInst ();s1.releaseInst ();return 0;}

两种模式的区别：懒汉模式中只有一个静态变量，调一次通过getInst函数返回一次，大家都用的这一个变量。懒汉模式里边维护了一个静态指针变量和一个静态变量做计数器，每次调的时候会先判断有没有构造，如果构造过m_inst就是非空的，计数器直接++就好，调用析构的话就是判断计数器，没人用的时候释放这块内存。