Re: [boost] [thread] RFC standard proposed mutex, read-write mutex, condition

From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2007-08-25 16:10:02

On Aug 25, 2007, at 2:52 PM, Yuval Ronen wrote:

> I find the lock argument cumbersome. In a way it's redundant to  
> passing
> the mutex, and for no good reason - just for hinting, subtle or no
> subtle. Where else in C++ a user is hinted in such a way? It's without
> precedent, and plain weird in my eyes. And if we add to this the fact
> that it's only a partial compile-time check, then it makes things even
> weirder. The user, for example, can confuse and think that he is fully
> covered by the compile-time check, and continue with his guards down,
> passing it a non-owning unique_lock. If we can't do a complete job,  
> it's
> better to not do it at all, at least this way our users are forced  
> to be
> aware of what's happening.

We have to have a wait which takes at the very least a mutex, if not a  
lock.  This is to support the legal "dynamic" binding mutex binding.   
We could possibly achieve the same functionality through your  
set_mutex suggestion, but that interface seems error prone to me, as  
it then takes two calls to wait instead of one:

condition<mutex> cv;
...
while (true)
{
     Data d = get_data();
     unique_lock<mutex> lk = d.get_lock();
     d.process();
     while (d.not_ready_to_pass_on())
     {
         cv.set_mutex(lk.mutex());
         cv.wait();
     }
     stream << d;
}

as opposed to just:

     while (d.not_ready_to_pass_on())
         cv.wait(lk);

One possibility would be to have both wait(lock) and set_mutex(mutex)/ 
wait().  But this doesn't allow anything that wait(lock) alone allows.

-Howard