muduo庫源碼學習(base)ThreadLocal

// 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_THREADLOCAL_H
#define MUDUO_BASE_THREADLOCAL_H

#include <muduo/base/Mutex.h>  // MCHECK
#include <muduo/base/noncopyable.h>

#include <pthread.h>

namespace muduo
{

template<typename T>
class ThreadLocal : noncopyable
{
 public:
  ThreadLocal()
  {
    MCHECK(pthread_key_create(&pkey_, &ThreadLocal::destructor));//系統調用這個回調函數
  }

  ~ThreadLocal()
  {
    MCHECK(pthread_key_delete(pkey_));//只是從線程裏刪除key
  }

  T& value()
  {
    T* perThreadValue = static_cast<T*>(pthread_getspecific(pkey_));
    if (!perThreadValue)
    {
      T* newObj = new T();
      MCHECK(pthread_setspecific(pkey_, newObj));
      perThreadValue = newObj;
    }
    return *perThreadValue;
  }

 private:

  static void destructor(void *x)//靜態函數。線程結束時，系統調用它（不是public函數！）刪除
  {
    T* obj = static_cast<T*>(x);//如果沒有調用value()呢？
    typedef char T_must_be_complete_type[sizeof(T) == 0 ? -1 : 1];//T_must_be_complete_type現在是個類型，char[10]
    T_must_be_complete_type dummy; (void) dummy;
    delete obj;
  }

 private:
  pthread_key_t pkey_;//線程局部對象依靠這個key得到對象
};

}
#endif