Re: [boost] [TTI] Review

Subject: Re: [boost] [TTI] Review
From: lcaminiti (lorcaminiti_at_[hidden])
Date: 2011-07-18 11:22:34

Dave Abrahams wrote:
> 
> on Sun Jul 17 2011, Edward Diener <eldiener-AT-tropicsoft.com> wrote:
> 
>> I think the nullary type syntax is cleaner syntactically and easier to
>> use. There may be a better solution for this sort of thing, but I am
>> looking for simplicity.
>>
>> The whole idea, syntactically, of the nullary type metafunctions is
>> that one passes around the metafunction itself rather than its nested
>> type, so there is much less of the ::type in the syntax. It does
>> involve using a separate set of boost::tti::mf_xxx metafunctions and
>> passing metafunctions as data, but I feel that the syntactical gain is
>> worth it.
>>
>> I really will update my documentation so that I will compare the macro
>> metafunction and nullary type metafunction usage together when dealing
>> with all of the cases of using the non-composite macro metafunctions.
> 
> I will admit to not knowing any of the background here, so what I'm
> about to say may be totally irrelevant but...
> 
> One can build a completely lazy MPL if you say that everything has to be
> a metafunction: all types are wrapped in nullary metafunctions before
> they are used, so you don't reach in and grab the nested ::type until
> you are ready to get the final result of your computation.  Vesa
> Karvonen was the first one to do this IIUC.  Unfortunately my
> experiments have revealed that if you rely on this idiom everywhere you
> get horrible compile-time performance.
> 
> FWIW-ly y'rs,
> 

Does this mean that if I use the nullary metafunciton macros to gain
syntactical convenience, I will pay in compile-time performance?

Again, this should be clarified and _quantified_ in the docs so as a user I
can make an informed choice when you offer multiple choices for the same
functionality. For example, the TTI authors should please measure and
compare compile-times within and without nullary metafunctions for some
version of MVSC and GCC and add it to the docs (and please do the same of
MTFC).

Thanks,
--Lorenzo

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