item2，实现singleton模式

#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 0;
}