Re: [boost] Pervasive dependency upon mpl::bool_

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.