Re: [boost] New dependency report

Subject: Re: [boost] New dependency report
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2014-06-08 17:53:41

El 08/06/2014 8:23, Vicente J. Botet Escriba escribió:

> Which library needs bool_ in Core?
>
> IMO, what we need is integral_constant in Core and import it to Boost.
>
> //boost/core/intergral_constant.hpp
> namespace  boost {
> namespace  core {
>    template <..> struct integral_constant {... };
> }
> }
>
> //boost/type_traits/intergral_constant.hpp
> namespace  boost {
>    using  core::integral_constant;
> }
>
> Note that the current implementation of integral_constant using
> mpl::integral_c is not documented, so the changes are backward
> compatible (Of course, the user or Boost code that is using the
> implementation detail would be broken).
>
> This doesn't needs a change in MPL at all.

I think you are pointing to the right solution. I'm strongly against 
using MPL::bool_ as a basic building block for traits. If possible we 
should have a boost::core::integral_type and 
boost::core::true_type/false_type. I would even define them directly 
into the boost namespace in Core. TypeTraits and MPL should depend on 
the core integral_constant.

MPL integral_c has additional features so it could derive from core 
integral_constant and add the needed features. Ideally, TypeTraits 
should not depend on MPL, but this could break a lot of code, so we need 
to maintain interoperability without adding an explicit dependency.

So to refine Vicente's idea, how about defining boost::integral_constant 
boost::true_type / boost::false_type in core, with mpl's requirements:

http://www.boost.org/doc/libs/1_55_0/libs/mpl/doc/refmanual/integral-constant.html

then inject it into MPL and type_traits namespaces in each library.

This way, when defining our own trait, we only depend on Core but we are 
still interoperable with MPL/TypeTraits. TypeTraits would not be MPL 
dependant. MPL could depend on TypeTraits and Core.

Best,

Ion