[boost] Re: Boost.Threads: Do we need all those mutexes?

From: Michael Glassford (glassfordm_at_[hidden])
Date: 2004-07-06 09:00:13

Peter Dimov wrote:

> Michael Glassford wrote:
> 
>>To me, it just feels like the wrong way to be notified that a lock
>>promotion failed so you have to do something else--a try_promote()
>>seems much better for this. However, if people really wanted a throwing
>>promote(), I'd add it.
> 
> 
> The point is that the
> 
>     scoped_write_lock wr( rd );
> 
> syntax, if supported, should never "succeed" without giving you a write
> lock.

OK, that makes sense. I was still thinking in terms of the 
non-constructor promotion model.

>     scoped_try_write_lock wr( rd );
> 
> can do that, but not scoped_write_lock.
> 
> That said, can you show an example that demonstrates how a 'try_promote' can
> be used? I'm curious.

I was still thinking in terms of the non-constructor promotion model 
here as well:

void f(read_write_mutex& m)
{
     read_write_mutex::try_read_write_lock l(m, read_lock);

     //do "read stuff"

     if (!l.try_promote())
     {
         l.unlock();
         l.write_lock();
         //redo some of "read stuff"
     }

    //do write stuff
}

Translating this to the constructor model of promotion would have to 
look something like this, I guess:

void f(read_write_mutex& m)
{
     read_write_mutex::try_read_lock r(m);

     //do "read stuff"

     read_write_mutex::try_write_lock w(r, /*maybe a parameter here to 
use try_promote() instead of promote()*/);

     if (!w.locked())
     {
         r.unlock();
         w.lock();
         //redo some of "read stuff"
     }

    //do write stuff
}