【muduo】单例模式

Singleton.h

// Use of this source code is governed by a BSD-style license// that can be found in the License file.//// Author: Shuo Chen (chenshuo at chenshuo dot com)#ifndef MUDUO_BASE_SINGLETON_H#define MUDUO_BASE_SINGLETON_H#include <boost/noncopyable.hpp>#include <assert.h>#include <stdlib.h> // atexit#include <pthread.h>namespace muduo{namespace detail{// This doesn't detect inherited member functions!// http://stackoverflow.com/questions/1966362/sfinae-to-check-for-inherited-member-functionstemplate<typename T>struct has_no_destroy{#ifdef __GXX_EXPERIMENTAL_CXX0X__  // C++11 利用decltype来进行类型推断  template <typename C> static char test(decltype(&C::no_destroy));#else  // typeof 类型推断  template <typename C> static char test(typeof(&C::no_destroy));#endif  template <typename C> static int32_t test(...);  const static bool value = sizeof(test<T>(0)) == 1;};}// 单例模式的共同基类template<typename T>class Singleton : boost::noncopyable{ public:  // 利用 pthread_once保证只有一个实例被创建  static T& instance()  {    pthread_once(&ponce_, &Singleton::init);    assert(value_ != NULL);    return *value_;  } private:  Singleton();  ~Singleton();  // 只有单例模式第一次被创建的时候被执行  static void init()  {    value_ = new T();    if (!detail::has_no_destroy<T>::value)    {      ::atexit(destroy);    }  }  // 销毁单例模式中的唯一变量  static void destroy()  {    // //类T一定要完整，否则delete时会出错，这里当T不完整时将数组的长度置为-1，这是不合法的，因而会报错      typedef char T_must_be_complete_type[sizeof(T) == 0 ? -1 : 1];    T_must_be_complete_type dummy; (void) dummy;    delete value_;    value_ = NULL;  } 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_ = NULL;}#endif