模板类和typedef

Question

There is a template class

template <ACE_SYNCH_DECL, class TIME_POLICY = ACE_System_Time_Policy>
class ACE_Task : public ACE_Task_Base

and the ACE_SYNCH_DECL can be

class ACE_Export ACE_NULL_SYNCH
{
public:
  typedef ACE_Null_Mutex MUTEX;
  typedef ACE_Null_Mutex NULL_MUTEX;
  typedef ACE_Null_Mutex PROCESS_MUTEX;
  typedef ACE_Null_Mutex RECURSIVE_MUTEX;
  typedef ACE_Null_Mutex RW_MUTEX;
  typedef ACE_Null_Condition CONDITION;
  typedef ACE_Null_Condition RECURSIVE_CONDITION;
  typedef ACE_Null_Semaphore SEMAPHORE;
  typedef ACE_Null_Mutex NULL_SEMAPHORE;
};

or

class ACE_Export ACE_MT_SYNCH
{
public:
  typedef ACE_Thread_Mutex MUTEX;
  typedef ACE_Null_Mutex NULL_MUTEX;
  typedef ACE_Process_Mutex PROCESS_MUTEX;
  typedef ACE_Recursive_Thread_Mutex RECURSIVE_MUTEX;
  typedef ACE_RW_Thread_Mutex RW_MUTEX;
  typedef ACE_Condition_Thread_Mutex CONDITION;
  typedef ACE_Condition_Recursive_Thread_Mutex RECURSIVE_CONDITION;
  typedef ACE_Thread_Semaphore SEMAPHORE;
  typedef ACE_Null_Semaphore NULL_SEMAPHORE;
};

But the ACE_SYNCH_DECL is not used in ACE_Task, so how does these typedef work?

Answer 1

When you instantiate an ACE_Task the ACE_SYNCH_DECL parameter is replaced with one of ACE_MT_SYNCH or ACE_NULL_SYNCH depending on whether you want a synchronized queue in the ACE_Task class.

If the ACE_Task is only used in one thread then the queue synchronization overhead isn't needed and one can use ACE_NULL_SYNCH. However, if it will be used from multiple threads, use ACE_MT_SYNCH.

There is an example of ACE_NULL_SYNCH in tests/Message_Queue_Test.cpp

BTW, this is explained in much more detailed in C++NPv2 sections 6.2 and 6.3.