muduo网络库源码学习————线程本地单例类封装

muduo库中线程本地单例类封装代码是ThreadLocalSingleton.h
如下所示：

//线程本地单例类封装
// 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_THREADLOCALSINGLETON_H
#define MUDUO_BASE_THREADLOCALSINGLETON_H

#include <boost/noncopyable.hpp>
#include <assert.h>
#include <pthread.h>

namespace muduo
{

template<typename T>
class ThreadLocalSingleton : boost::noncopyable//不可被拷贝
{
 public:
//返回单例对象的引用
  static T& instance()
  {
    if (!t_value_)//如果指针为空
    {
      t_value_ = new T();//创建对象
      deleter_.set(t_value_);//将t_value_传入deleter_，以便deleter_可以调用destructor销毁
    }
    return *t_value_;//返回
  }
//返回单例对象的指针
  static T* pointer()
  {
    return t_value_;
  }

 private:
//销毁对象
  static void destructor(void* obj)
  {
    assert(obj == t_value_);
    typedef char T_must_be_complete_type[sizeof(T) == 0 ? -1 : 1];
    delete t_value_;
    t_value_ = 0;
  }
//一个嵌套类只是为了能够调用destructor自动销毁t_value_
  class Deleter
  {//借助线程特定数据来实现
   public:
    Deleter()//构造函数的回调函数为ThreadLocalSingleton::destructor
    {
      pthread_key_create(&pkey_, &ThreadLocalSingleton::destructor);
    }

    ~Deleter()
    {
      pthread_key_delete(pkey_);
    }

    void set(T* newObj)
    {
      assert(pthread_getspecific(pkey_) == NULL);
      pthread_setspecific(pkey_, newObj);
    }

    pthread_key_t pkey_;
  };

  static __thread T* t_value_;//T类型的指针，__thread关键字表示每个线程都有一份
  static Deleter deleter_;//主要用于销毁上面那个指针所指向的对象
};

template<typename T>
__thread T* ThreadLocalSingleton<T>::t_value_ = 0;

template<typename T>
typename ThreadLocalSingleton<T>::Deleter ThreadLocalSingleton<T>::deleter_;

}
#endif

测试程序如下：

//线程本地单例类封装测试程序
#include <muduo/base/ThreadLocalSingleton.h>
#include <muduo/base/CurrentThread.h>
#include <muduo/base/Thread.h>

#include <boost/bind.hpp>
#include <boost/noncopyable.hpp>
#include <stdio.h>

class Test : boost::noncopyable
{
 public:
  Test()
  {
    printf("tid=%d, constructing %p
", muduo::CurrentThread::tid(), this);
  }

  ~Test()
  {
    printf("tid=%d, destructing %p %s
", muduo::CurrentThread::tid(), this, name_.c_str());
  }

  const std::string& name() const { return name_; }
  void setName(const std::string& n) { name_ = n; }

 private:
  std::string name_;
};

void threadFunc(const char* changeTo)
{//打印线程tid，线程单例对象地址，线程单例对象名称
  printf("tid=%d, %p name=%s
",
         muduo::CurrentThread::tid(),
         &muduo::ThreadLocalSingleton<Test>::instance(),
         muduo::ThreadLocalSingleton<Test>::instance().name().c_str());
  //重新设置一下名称
  muduo::ThreadLocalSingleton<Test>::instance().setName(changeTo);
  //再次打印
  printf("tid=%d, %p name=%s
",
         muduo::CurrentThread::tid(),
         &muduo::ThreadLocalSingleton<Test>::instance(),
         muduo::ThreadLocalSingleton<Test>::instance().name().c_str());
//线程退出时会自动销毁，对象周期结束
  // no need to manually delete it
  // muduo::ThreadLocalSingleton<Test>::destroy();
}

int main()
{//muduo::ThreadLocalSingleton<Test>::instance()返回一个线程单例对象，每个线程都有一个Test对象
  muduo::ThreadLocalSingleton<Test>::instance().setName("main one");
  //创建两个线程，threadFunc为回调函数
  muduo::Thread t1(boost::bind(threadFunc, "thread1"));
  muduo::Thread t2(boost::bind(threadFunc, "thread2"));
  //启动两个线程
  t1.start();
  t2.start();
  t1.join();
  //打印主线程的
  printf("tid=%d, %p name=%s
",
         muduo::CurrentThread::tid(),
         &muduo::ThreadLocalSingleton<Test>::instance(),
         muduo::ThreadLocalSingleton<Test>::instance().name().c_str());
  t2.join();
//退出主线程
  pthread_exit(0);
}

运行结果如下：