Re: [boost] [thread] Are thread ids unique during an applications life time?

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2008-05-28 13:10:55

On May 28, 2008, at 6:42 AM, Johan Torp wrote:

> I also expect re-use will mean users have to re-implement  
> "unique_thread_id"
> and some really difficult-to-fix bugs for people who do not think  
> about it.
> OTOH, I see some value in that std::thread::id are the same as  
> window's
> thread ids when you're debugging and post-mortem debugging.

std::thread::id has one bit of functionality that may reduce the need  
for "unique_thread_id":  It has a notion of an id that is guaranteed  
to be "not any thread".  For example:

std::thread::id id1;
std::thread::id id2 = std::this_thread::get_id();
assert(id1 != id2);

The assert is guaranteed to pass.

So when you know that a thread is terminating, or is otherwise no  
longer valid, one can reset thread id's to not-a-thread:

id2 = std::thread::id();

I've used this (for example) in implementing recursive mutexes:

class recursive_mutex
{
     unsigned state_;
     std::thread::id id_;
     ...
public:
     recursive_mutex() : id_() ...  // id_ guaranteed not to compare  
equal to any call to get_id()
     ...
     void unlock()
     {
          ...
          if (decrease_lock_count() == 0)
              id_ = std::thread::id();  // id_ guaranteed not to  
compare equal to any call to get_id()
          ...
     }
     ...
};

-Howard