Re: [boost] [type_traits] Rewrite and dependency free version

Subject: Re: [boost] [type_traits] Rewrite and dependency free version
From: John Maddock (boost.regex_at_[hidden])
Date: 2015-01-20 08:14:52

>>> Wasn't this macro a VC6 workaround?
>>>
>>> As a first step we could just add an empty macro and see if anything
>>> today actually needs it.
>>
>> That's what I was planning, these macros are all basically superfluous
>> now.
>
> I am seeing in MPL empty_base.hpp:
>
> #include <boost/type_traits/detail/bool_trait_def.hpp>
>
> but this file does not exist in type_traits Version2.

I've added back "do nothing much" versions of those missing files.  A 
lot more should build now.

However, there are a couple of show-stoppers I don't know how to solve:

1) boost::integral_constant is no longer a model of mpl's 
integral_constant: 
http://www.boost.org/doc/libs/1_57_0/libs/mpl/doc/refmanual/integral-constant.html

As a result all of function_types lib is broken.  I guess one could 
argue that if you want to do arithmetic on integral_constant's then one 
should use MPL all the way through.  But it's a big breaking change.

2) There are few places (well, myself in multiprecision lib) where folks 
do dispatch on multiple conditions:

do_something(mpl::true_ const&,  mpl::true_ const&){}
do_something(mpl::true_ const&, mpl::false_ const&){}
template <bool B>
do_something(mpl::true_ const&, mpl::bool_<B> const& {/*catch all case*/}

Then:

do_something(trait1<T>(), trait2<T>());

The problem here is that the final catch all case can not be found 
unless trait2 actually inherits from mpl::bool_ rather than being merely 
convertible to it.

John.