Subject: Re: [boost] [TTI] Review
From: Edward Diener (eldiener_at_[hidden])
Date: 2011-07-17 18:29:07
On 7/17/2011 5:53 PM, 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.
What is IIUC ?
I did not add the nullary type metafunctions for "laziness" but just as
a syntactical convenience. I hope others realize that they can use them
or ignore them as they see fit and if they do slow down compile times
they should be ignored.
Eddie