[boost] Pervasive dependency upon mpl::bool_

Subject: [boost] Pervasive dependency upon mpl::bool_
From: Peter Dimov (lists_at_[hidden])
Date: 2014-06-01 16:40:19

We have lots of headers that depend on mpl::bool_ in order to define traits 
classes as deriving from mpl::true_ or mpl::false_.

I don't think that this dependency is necessary even for MPL 
interoperability. The normal way of

template<class T> struct trait
{
    BOOST_STATIC_CONSTANT( bool, value = false );
};

should work just as well. I think.

As one example, there's detail/blank.hpp, which starts with

#include "boost/mpl/bool.hpp"
#include "boost/type_traits/is_empty.hpp"
#include "boost/type_traits/is_pod.hpp"
#include "boost/type_traits/is_stateless.hpp"

It doesn't really need these includes.

namespace boost {

struct blank
{
};

// type traits specializations
//

template <>
struct is_pod< blank >
    : mpl::true_
{
};

This can be written as

template<class T> struct is_pod;

template<> struct is_pod< blank >
{
    BOOST_STATIC_CONSTANT( bool, value = true );
};

which requires no includes.

The problem is, though, that all type traits do derive from mpl::true_ or 
mpl::false_, and I'm not sure if rewriting blank.hpp in the above manner is 
actually correct. Should type trait specializations always derive from 
mpl::true_ or mpl::false_? Will I break something if I specialize a type 
trait to not derive from mpl::bool_? Or is a nested ::value enough? Who 
knows. :-)

But really, if a header, whose entire purpose is to define an empty struct, 
can't get by without including type traits and mpl, we've lost the 
dependency game before it's even started.