Subject: Re: [boost] Pervasive dependency upon mpl::bool_
From: Peter Dimov (lists_at_[hidden])
Date: 2014-06-03 05:39:42
John Maddock wrote:
> The idea of artificially splitting up type_traits seems pretty abhorrent
> to be honest.
Based on that description I suspect that I didn't explain myself clearly
enough, as I don't believe the refactoring is artificial at all.
A new module, core type traits, is created, with its contents corresponding
more or less to the standard type traits, minus the integral_constant
requirement, plus the requirement to not depend on anything non-core, and
hence, minus common_type.
This module contains, to take an example,
namespace boost
{
namespace core_type_traits
{
template<class T> struct is_pointer
{
BOOST_STATIC_CONSTANT( bool, value = false );
};
template<class T> struct is_pointer
{
BOOST_STATIC_CONSTANT( bool, value = true );
};
} // core_type_traits
} // boost
(workarounds omitted for brevity.)
A valid alternative implementation for it is:
namespace boost
{
namespace core_type_traits
{
using std::is_pointer;
}
}
boost::is_pointer is not removed from type traits. It is just implemented
as:
namespace boost
{
template<class T> struct is_pointer: public mpl::bool_<
core_type_traits::is_pointer<T>::value >
{
BOOST_MPL_AUX_LAMBDA_SUPPORT(1,is_pointer,(T))
};
BOOST_TT_AUX_TEMPLATE_ARITY_SPEC(1,is_pointer)
} // boost
That is, the MPL support is decoupled from the actual logic.
Currently, is_pointer happens to be implemented in exactly such a way,
except that the core part is called is_pointer_impl.
Now a type can specialize core_type_traits::is_pod so that ::value is true
and all existing uses of boost::is_pod will see mpl::true_.
I honestly don't see what do you consider artificial in such an approach.
It's basic layering.