[boost] Re: Lock unification

From: Michael Glassford (glassfordm_at_[hidden])
Date: 2004-07-15 14:11:44

Eric Niebler wrote:

> Peter Dimov wrote:
> 
>>
>>
>> The "unified" timed lock case is
>>
>> {
>>     scoped_lock l( m, false );
>>
>>     if( l.timed_lock( t ) )
>>     {
>>         // do stuff
>>     }
>>     else
>>     {
>>         // what do we do here? presumably either:
>>
>>         // setup for unbounded wait
>>         l.lock();
>>         // do stuff
>>
>>         // or signal failure
>>     }
>> }
>>
>> The scoped_timed_lock use case also needs to eventually contain an if( 
>> l )
>> statement; you can't do much if you don't know whether your timed lock
>> succeeded. One might also argue that a separate scoped_timed_lock makes
>> accidentally omitting the if() statement more likely and harder to spot.
>>
>> But it's hard to tell without real use cases.
>>
> 
> 
> I continue to believe that a better interface for a unified lock 
> involves descriptively named helper functions:

I like it better too, but...

> scoped_lock l1( m );                 // lock unconditionally
> scoped_lock l1 = defer_lock( m );    // don't lock
> scoped_lock l2 = try_lock( m );      // try lock
> scoped_lock l3 = timed_lock( m, t ); // timed lock
> 
> It's just as clear as you could hope for, and it's extensible. I feel 
> that the single lock c'tor with a bool arg is less readable and rather 
> inflexible.
> 
> Earlier I posted code demonstrating how this interface could be made to 
> work.

I believe there was (nearly?) consensus that we want to make locks 
movable at some point. My conclusion was that at that point this syntax 
becomes possible, too. I think Peter said or implied much the same 
thing, maybe before I did.

As I mentioned in another recent reply, the unified lock won't be in the 
upcoming release; making locks movable may happen before the next 
release after that, making this syntax possible in that release also.

In your opinion, should a unified lock then have no locking 
constructors, or should it have only the one already proposed?

Mike