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

From: Michael Glassford (glassfordm_at_[hidden])
Date: 2004-07-09 15:04:56

Peter Dimov wrote:
> Eric Niebler wrote:
> 
>>1) a reduction in interface complexity.
>>   - one lock class, instead of a scoped_lock, try_lock, timed_lock,
>>etc.
>>   - no bools or enums in constructors; instead, there are clearly
>>     named factory functions for customizing lock construction.
>>
>>2) You can initialize a lock in the condition of an "if" statement.
>>
>>3) You can return a lock from a function. (Benefit is debatable.)
> 
> 
> After giving it some thought:
> 
> - one lock class: tie;

This seems both popular and reasonable. I presume it would be templated 
on the mutex type?

> - no bools or enums in constructors: there is a bool argument, but it's (in
> my opinion) acceptable since there is a single constructor, and the argument
> participates directly in its postcondition:
> 
>     Lock( Mutex & m, bool l = true );
>     Post: locked() == l;

No argument here, either. I would mildly prefer a couple more 
constructors, as you know, but I do see understand your arguments for a 
single constructor and see the advantages of that design.

> - you can return a lock from a function: we can make the locks movable
> regardless of the interface, so it seems possible to ignore this bullet;

I agree.

> - if( lock l = try_lock( m ) ) { }
> 
> This is the main difference.
> 
> Declarations in conditions are a nice idiom. The typical use is
> 
> if( X x = f() )
> {
>     // do something with x
> }
> 
> Our example doesn't fit the pattern, however, as the "// do something" part
> typically does not reference the lock itself.
> 
> I don't have much experience with try locks, though. Pretty much the only
> example that comes to mind is Howard's lock_both:
> 
> for(;;)
> {
>     l1.lock();
>     if( l2.try_lock() ) return;
>     l1.unlock();
> 
> 
>     l2.lock();
>     if( l1.try_lock() ) return;
>     l2.unlock();
> }
> 
> (I hope I got that right) and I don't see how it can be improved by the if()
> idiom.

This idiom becomes possible when locks become transferable / returnable 
anyway, so I don't think we need to worry about it now, either.

Mike