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

From: alnsn_at_[hidden]
Date: 2004-07-09 11:44:44

Peter Dimov wrote:
> - 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;
> 
> - 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.

This is a degenerate case of more general idea of passing lock reference
to functions inside condition:

if(lock_ref l = scoped_lock(m))
{
    // do something with l:
    action1(l);
    action2(l);
}

Apparently, action1 and action2 can't be called without prior obtaining
of a lock.
Plus, you get some sort of polymorphism because the functions can accept
other locks as well:

action1(scoped_try_lock(m1)); // lock_ref is const reference
action2(scoped_timed_lock(m2));

The only big problem I know of so far is unspecified order of calls in
the case of getting multiple locks (incorrect order leads to deadlock):

action3(scoped_lock(m1), scoped_lock(m2));

-- 
Alexander Nasonov