Re: [boost] New dependency report

On 06/06/2014 22:40, Peter Dimov wrote:

John Maddock wrote:

...

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_ { };

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_.

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.

For me it seems there are relatively few uses of type_traits that don't end up requiring either mpl::if_ or mpl::bool_ anyway?

...

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.

...

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.

Ah, good point, clearly we can't do that then. John.