Re: [boost] New dependency report

Le 06/06/14 23:40, Peter Dimov a écrit :

John Maddock wrote:

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I'm tied of saying this.... but if mpl::bool_ and int_ were moved out of MPL full, either to core, or to some mpl_core, then there's no need to split type_traits anymore.

The question is: what is the minimal conforming implementation of a type trait? Is it

template<class T> struct something { bool const value = false; }

or is it, instead,

template<class T> struct something: mpl::false_ { };

The answer as for the documentation * for TypeTraits trais is template <class T> struct is_abstract : public /|true_type http://www.boost.org/doc/libs/1_55_0/libs/type_traits/doc/html/boost_typetra...-or-false_type http://www.boost.org/doc/libs/1_55_0/libs/type_traits/doc/html/boost_typetra...|/ {}; I interpret these are the ones on boost namesapce. * for MPL traits is template< typename T1 , typename T2 > structless http://www.boost.org/doc/libs/1_55_0/libs/mpl/doc/refmanual/less.html { typedef/unspecified/ type; }; Returns a true-valuedIntegral Constant http://www.boost.org/doc/libs/1_55_0/libs/mpl/doc/refmanual/integral-constan... ifT1 is less thanT2. Here an integral constant has some constraints not satisfied by boost::integral_constant, as tag, next, prior. I think that TypeTraits shouldn't require the MPL constraints. In order to be compatible with C++11 we should provide the same interface than std::integral_constant.

For C++11, the answer to this question determines whether the std type traits are conforming type traits for us. In other words, whether "using std::is_pointer;" is a conforming implementation of boost::is_pointer.

For C++03, the answer determines whether I can specialize a type trait for my type without including the header that defines mpl::true_ and mpl::false_. Well, you could include a header that defines boost::true_type and boost::false_type ;-) Or you could define the traits defining every_thing integral_constant defines.

I prefer the first definition for reasons already stated. This doesn't mean however that going with the second, along with a lean and mean mpl::bool_, will not going to be a considerable improvement as well. Far from it.

I prefer to conform to the standard traits as much as possible. The moving from Boost to the standard is easier. The porting of C++11 libraries to Boost for non-conforming C++11 compilers also.

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Indeed I suspect there's a huge amount of obfuscated code inherited from mpl's broken-compiler-workarounds that can now be removed: not only do we not support those compilers any more, but given that they haven't been tested in heaven knows how long they're unlikely to compile *any* of modern Boost anyway.

That's certainly true. If we remove the obsolete workarounds from MPL and TypeTraits things will start looking much better.

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In fact in C++11 it makes sense to me have:

namespace boost{ namespace mpl{

template <bool b> using bool_ = std::integral_constant;

Interesting. This makes the std type traits conform to my second definition above, so that "using std::is_member_pointer;" (say) is a legitimate implementation of boost::is_member_pointer even when derivation from mpl::bool_ is required.

I'm not sure that this is a conforming mpl::bool_ though. It almost is. But mpl integral constants need to have a nested ::tag (integral_c_tag), and std::integral_constant doesn't have one. Maybe this requirement can be dropped. Maybe it can't. I don't know.

We agree here. The preceding bool_ definition is no equivalent to the MPL current definition. The question is whether we need tag dispatching for the result of a trait. IMHO not. mpl::bool_could be defined template <bool b> struct bool_ : std::integral_constant { using tag integral_c_tag; using type bool_<b>; }; IMO, the next and prev meta-functions can be implemented outside the class but this is another history. Vicente