item2，实现singleton模式

单例模式？

只能实现一个实例的类成为单例。

==============

muduo库中单例模式实现

#include<boost/noncopyable.hpp>
//#include "../sort.h"
//#include "getLeastNumber.h"
#include<set>
using namespace std;

template<typename T>
class Singleton:boost::noncopyable{
public:
    static T& instance(){
        pthread_once(&ponce_,&Singleton::init);
        return *value_;
    }
private:
    Singleton();
    ~Singleton();
    static void init(){
        value_ = new T();
    }
private:
    static pthread_once_t ponce_;
    static T* value_;
};

template<typename T>
pthread_once_t Singleton<T>::ponce_ = PTHREAD_ONCE_INIT;

template<typename T>
T* Singleton<T>::value_ = nullptr;

class Foo{
public:
    Foo(){};
    ~Foo(){};
    void show(){std::cout<<"begin"<<"\nend"<<std::endl;}
};

int main() {
    //A bfs;
    //bfs.test();
    Foo& foo = Singleton<Foo>::instance();
    foo.show();
    return ;
}