On the richness of C++

[Jason House]

Walter Bright Wrote:

> Jason House wrote:
> > For each delegate, the job variable gets captured.  Each delegate will have
> > access to the job variable.  In cases of defered delegate execution,
> > changes to the job variable will affect what the delegate does.
> > 
> > In my example, what happens if runthread is really access to a non-blocking
> > thread pool that queues excess commands.  When the thread pool gets around
> > to running the delegate passed in, what will be in "job"?  If this is done
> > after the loop finishes, the value inside job will be the final value from
> > opApply (or worse, corrupt).
> > 
> > The net effect is that a queue of 50 jobs may have the final job executed 50
> > times, and the first 49 jobs are never run.  This does not seem like
> > correct behavior to me.
> 
> This is just the usual distinction between value and reference 
> semantics. D does closures by reference. If you want a value, it's 
> simple enough to make a copy. I think D's method is more powerful, 
> because it's a lot harder to make by-value closures into by-reference 
> than the other way around.
> 
> I also believe D's method is inherently faster, because it isn't 
> necessary to copy the values around (and there can be an arbitrary 
> number of them, and can be arbitrarily large structs).

The only thing I'm contesting is your statement that closures eliminate the need for a bind library.  While it's true they reduce the need for a bind library, they don't eliminate it.