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


thanks for your review and the positive vote.

David Abrahams wrote:
> on Mon Dec 03 2007, John Torjo <> 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.


> 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...


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