Re: [boost] New dependency report

Subject: Re: [boost] New dependency report
From: Peter Dimov (lists_at_[hidden])
Date: 2014-06-06 17:40:01

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.

> 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<bool, b>;

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.