Subject: Re: [boost] New dependency report
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2014-06-15 06:07:44
Le 13/06/14 20:27, Peter Dimov a écrit :
> Eric Niebler wrote:
>> On 06/13/2014 10:48 AM, Peter Dimov wrote:
>> > Eric Niebler wrote:
>> >> I think in the C++11 world, boost type_traits should *all* inherit
>> from >> std::true_type. That's easily accomplished by aliasing
>> mpl::true_ to >> std::true_type. It fixes everything.
>> > I can't help but note that you're focusing on the wrong aspect of
>> the > problem. In the C++11 world, we have type traits in std and
>> none of this > matters. Use of Boost type traits is motivated by the
>> need to support > C++03.
>> I'm focusing on a larger set of problems. And use use of mpl isn't
>> restricted to C++03 code. Nor is boost.type_traits for that matter;
>> it's used in code that must be portable, like boost libraries.
> That's exactly what I said. Boost type traits are used in code that
> needs to compile under C++03.
> I agree with you that ::tag as a requirement for the integral constant
> concept is a terrible idea. I do not agree with your C++11-based
> arguments, neither do I agree that type traits should inherit from
> std::true_type. Type traits should NOT inherit from anything.
> The only requirement for a type trait (or a metaprogramming predicate,
> in general) should be to have a ::value with a value of 0 or 1.
> Nothing more. No ::tags. No required base classes. No mandatory
> dependencies of any kind.
What should be and what it is a quite different concern. The current
documentation says that a boolean type traits inherits from
boost::true_type or boost::false_type.
> If you want to dispatch, type traits with a ::value == 0 should be
> _convertible_ to false_type, and type traits with a ::value == 1
> should be convertible to true_type, but they should not be required to
> derive from them. This convertibility can be achieved from the
> true_type/false_type side.
I don't see how.
> The base classes in the standard are a mistake. The base classes in
> Boost are a mistake.
You are surely right, but the status is that these traits inherits. I
think that the issue is not a major one and that compatibility between
the Boost type traits and the standard ones is more important.
Just my 2 cts,
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk