关于Muduo网络库中Singleton代码的疑问

[chudong...@gmail.com]

// 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-functions

template<typename T>

struct has_no_destroy

{

#ifdef __GXX_EXPERIMENTAL_CXX0X__

  template <typename C> static char test(decltype(&C::no_destroy));

#else

  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;

};

}

template<typename T>

class Singleton : boost::noncopyable

{

 public:

  static T& instance()

  {

    //保证init_routine()函数在本进程执行序列中仅执行一次。

    pthread_once(&ponce_, &Singleton::init);

    assert(value_ != NULL);

    return *value_;

  }

 private:

  //默认构造函数

  Singleton();

  //默认析构函数

  ~Singleton();

  static void init()

  {

    //初始化value_，并把其清零

    value_ = new T();

    if (!detail::has_no_destroy<T>::value)

    {

      //在进程退出时回收资源

      ::atexit(destroy);

    }

  }

  //回收资源

  static void destroy()

  {

    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:

  //pthread_once参数

  static pthread_once_t ponce_;

  //指向一个实例

  static T*             value_;

};

//static需要在类外进行初始化

template<typename T>

pthread_once_t Singleton<T>::ponce_ = PTHREAD_ONCE_INIT;

template<typename T>

T* Singleton<T>::value_ = NULL;

}

#endif

想问一下学长学姐下面这段代码的用法

typedef char T_must_be_complete_type[sizeof(T) == 0 ? -1 : 1];

T_must_be_complete_type dummy; (void) dummy;