Re: [Boost-users] Re: Re: thread::thread() in Windows

From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2004-12-09 11:33:58

Yuval Ronen wrote:
>>>>A default-constructed thread is documented as being non-joinable:
>>>
>>>Ok, I can't join it. So what can I do with it?
>>
>>Not a whole heck of a lot aside from comparing it to other threads.
> 
> 
> This brings me some thoughts.
> The thread class is non-copyable (which is fine by me) and also it has an
> operator==. I think these two things contradict each other. When defining an
> equal operator, we are actually defining semantics for copying: copying an
> object will result in a second object that will be equal, in terms of
> operator==, to the first object. Defining an equal operator means that
> objects can be equal, or in other works, they are a copies of the same
> value!
<snip>

As I see it, the problem is that thread combines the functions of thread 
ID and thread handle.  Joinable and non-joinable thread objects are two 
very different types and the type system should reflect that.  I think 
that thread should be separated into two classes:

class thread_id  // Exposition only
{
public:
   // construct/copy/destruct
   thread_id();
   thread_id(const thread_id &);
   ~thread_id();

   // modifier
   thread_id & operator=(const thread_id &);

   // comparison
   bool operator==(const thread_id &) const;
   bool operator!=(const thread_id &) const;
};

class thread : private boost::noncopyable   // Exposition only
{
public:
   // construct/copy/destruct
   explicit thread(const boost::function0<void>&);
   ~thread();

   // modifier
   void join();

   // static
   static void sleep(const xtime&);
   static void yield();
};

Then every thread object would be joinable and every thread_id would be 
copiable.

Ben.