Re: [boost] [type_erasure] Review ends today July 27, 2012

Subject: Re: [boost] [type_erasure] Review ends today July 27, 2012
From: Joel de Guzman (joel_at_[hidden])
Date: 2012-08-01 08:34:39

>>>> Please state clearly whether you think this library should be accepted
>>>> as a Boost library.

Yes. I vote to accept.

>>>> Other questions you may want to consider:
>>>> 1. What is your evaluation of the design?

Outstanding.

>>>> 2. What is your evaluation of the implementation?

Haven't looked.

>>>> 3. What is your evaluation of the documentation?

I just skimmed through the docs to get what I need (see below). It
would be good to have examples at the bottom of each of the reference
sections like that of MPL, Fusion and the PPlib. Example:

   http://www.boost.org/doc/libs/1_50_0/libs/mpl/doc/refmanual/fold.html

At the bottom, we have an example.

>>>> 4. What is your evaluation of the potential usefulness of the library?

Extremely useful!

>>>> 5. Did you try to use the library?  With what compiler?  Did you have
>>>> any problems?

Yes. I tried to use it as a replacement for Boost.Function in Boost.Spirit.
It worked like a charm. My only difficulty was in trying to figure out
how to detect a default-initialized 'any'. With Boost.Function, you can use
an instance in a condition, e.g. if (f) ... This seems to be not the case
with 'any'. I'm pretty sure this is something simple to do, but I can't dig
it out in the docs. I ended up casting to void*. It turns out that it returns
0 (null) if any hasn't been initialized yet. I'm not sure about the cost of
that though (would that have to use RTTI? It seems that way as it says in
the any_cast docs: http://tinyurl.com/d8sx46m where it says that typeid_
is required.

I did a CT test on one of the largest examples in spirit. I compared
it against 1) the original implementation using Boost.Function and
2) A hand-coded implementation using virtual functions. Here are
the results:

TypeErasure:      01:14 secs
Boost.Function:   00:55 secs
Virtual Function: 00:52 secs

For that matter, I'm not ready to use TypeErasure yet for my purpose,
but I'm hoping that Steven can improve CT. It might also be possible
that I am using TypeErasure incorrectly when I resort to any_cast to
test for null any. Would this explain the CT slowdown? I am not sure.

I'd also suggest the addition of an easy to use Boost.Function
replacement in there that can be used out of the box with full
compliance.

I know this is merely scratching the surface, but I am sure I'll
be using TypeErasure's more powerful features.

>>>> 6. How much effort did you put into your evaluation?  A glance?  A
>>>> quick reading?  In-depth study?

A few hours of study.

>>>> 7. Are you knowledgeable about the problem domain?

I believe so.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com