Re: [Boost-users] [boost] TTI library updated in the sandbox

Subject: Re: [Boost-users] [boost] TTI library updated in the sandbox
From: Edward Diener (eldiener_at_[hidden])
Date: 2011-02-06 17:44:02

On 2/6/2011 4:14 PM, Dave Abrahams wrote:
> Couple o' things:
>
> On Sun, Feb 6, 2011 at 12:39 PM, Edward Diener<eldiener_at_[hidden]>  wrote:
>> Features of the update include support for the older gcc 3.4.2 and 3.4.5,
>> finer-grained introspection for the basic macro metafunctions, documentation
>> examples, better and tighter design, and better use of Boost MPL methodology
>> ( metafunctions are now passed as lambda expressions and metafunction
>> globbing has been removed from the two metafunctions where it previously
>> existed ). You can view the changes in the History section of the
>> documentation.
>
> FWIW, passing lambda expressions instead of metafunction classes is a
> trade-off: it's more expressive, but will lengthen compile times.  For
> the implementation of a boost library, it's probably better to err on
> the side of speed.

I could create metafunction classes for use instead of having the 
end-user use placeholder expressions. Is that what you are suggesting 
for better compiler speed ? It should be easy enough to do. I was 
influenced by your MPL book and how easy it is for the end-user to use 
placeholder expressions.

>
>> The TTI library is based on the type_traits_ext portion of the Concept
>> Traits Library, with improvements and additions, and also lifts
>> functionality, for the purposes of orthogonality, from Boost.MPL regarding
>> introspection of types and templates.
>
> Orthogonality in what sense?  Duplicating functionality seems like the
> opposite of orthogonality.

I am certainly willing and comfortable removing from the TTI library the 
TTI_HAS_TYPE and TTI_HAS_TEMPLATE metafunction macros, which just 
duplicates the functionality of the corresponding 
BOOST_MPL_HAS_XXX_TRAIT_DEF and BOOST_MPL_HAS_XXX_TEMPLATE_DEF 
respectively. But let me first argue for why I included them and what I 
mean when I say orthogonality:

1) The functionality is complete in a single library for what the 
library wishes to do. In this case it is to introspect the inner 
elements of a type. I have functionality to introspect member data and 
functions and static data and functions, and some new functionality to 
introspect types ( checking for the actual type ) and class templates ( 
specifying the exact template parameters ), so it seemed natural for me 
that I should just as well also provide functionality in general for 
types and templates, even if it was to provide the exact same 
functionality which is in Boost MPL. This is more a situation of 
completeness than orthogonality, but I felt that providing all of the 
functionality the library promises was more important than duplicating 
the MPL functionality. And of course the duplication code-wise is just 
minimal since I am not recreating the macros which constitute 
BOOST_MPL_HAS_XXX_TRAIT_DEF and BOOST_MPL_HAS_XXX_TEMPLATE_DEF but just 
using the MPL macros internally and delegating to their already existing 
functionality.

2) For the nullary type metafunctions in my library I have corresponding 
type and template functionality, to be used where the types are nullary 
metafunctions. It would have felt odd, in the sense of the nullary type 
metafunctions being orthogonal in functionality to the macro 
metafunctions, not to also have the same type and template functionality 
in the macro metafunctions.

3) I wanted the naming I used to be consistent with the other names in 
the library. So instead of BOOST_MPL_HAS_XXX_TEMPLATE_DEF I have 
TTI_HAS_TEMPLATE, instead of BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF I have 
TTI_TRAIT_HAS_TEMPLATE, instead of BOOST_MPL_HAS_XXX_TRAIT_DEF I have 
TTI_HAS_TYPE and instead of BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF I have 
TTI_TRAIT_HAS_TYPE. This sort of ease of use, because the naming is 
consistent, is important to me when I think about design for an 
end-user. Of course the documentation can tell the end-user to use the 
already existing MPL macros to get type and template introspection 
instead of providing my own set that does the exact same thing and just 
reuses the MPL functionality. But I felt it more natural to provide my own.

I hope you know that there is no attempt being made to create the 
TTI_HAS_TYPE and TTI_HAS_TEMPLATE metafunction macros without 
acknowledging that I am just lifting this from the MPL library. It is in 
the description when I post on this NG and it is in the documentation in 
the Acknowledgments section. I would be glad to put it anywhere else. At 
the same time I can remove it easily enough from the TTI library but I 
think in that case it would be less complete as a compile-time 
introspection library if I did.