Re: [boost] [review] Fast track formal review of Forward library begins today

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-12-04 18:24:52

Dave,

thanks for your review and the positive vote.

David Abrahams wrote:
> on Mon Dec 03 2007, John Torjo <john.groups-AT-torjo.com> wrote:
> 
>>      * What is your evaluation of the design?
> 
> Looks good to me.
> 
>>      * What is your evaluation of the implementation?
> 
> Also looks good, although I don't understand the PP approach being
> used.  

It's quite simple, in fact: Looking at the bits of a counter with loop
unrolling applied (not to hit the limits of the PP lib for arities > 8).

> It certainly looks different from the very fast one that Paul
> Mensonides put together for me once (attached at bottom), 

Interesting! I never noticed that SEQ_PRODUCT macro actually takes an
arbitrary number of factors - pretty cool...

> but I'm willing to believe it's better.  

Well, I'm not :-). That is, not without at least taking a closer look.

Running a quick benchmark shows -to my very surprise- that the
performance difference is rather insignificant (at least with GCC/Darwin).
Even more surprisingly my code starts outperforming Paul's at max.arity>7.

I believe Paul's code can theoretically run a lot faster with a
preprocessor that is well-optimized for metaprogramming (not sure there
is one around) and it has the "wizardry bonus". Mine OTOH emits at least
a few newline characters - which I happen to like :-).

Anyway, I will benchmark with some more compilers and eventually change
things to use Paul's approach if a significant advantage becomes apparent.

> Comments would be helpful.

OK.

> 
> It should use compressed_pair to store the wrapped function object to
> take advantage of EBO.
>

The class only has a single member, so how would it be applicable?

We could just inherit the target function privately to exploit EBCO in 
cases when inheriting from 'forward<F>'...

>>      * What is your evaluation of the documentation?
> 
> Nice, but terse.  It would probably be helpful to many to add some
> more material explaining the forwarding problem.

Are you missing something in particular?

BTW: "Virgin guinea pig readers" out there? - Don't hesitate, you're 
always welcome...

Regards,
Tobias