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From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-07-09 08:35:02

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;

- 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;

- 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.

I don't have much experience with try locks, though. Pretty much the only
example that comes to mind is Howard's lock_both:

    if( l2.try_lock() ) return;

    if( l1.try_lock() ) return;

(I hope I got that right) and I don't see how it can be improved by the if()

But I may be missing some important use cases.

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