Re: [boost] [contract] first attempt to N3351 concepts

Subject: Re: [boost] [contract] first attempt to N3351 concepts
From: Matt Calabrese (rivorus_at_[hidden])
Date: 2012-10-10 14:19:53

On Wed, Oct 10, 2012 at 4:51 AM, Andrzej Krzemienski <akrzemi1_at_[hidden]>wrote:

> Even if you decide to go with pseudo-signatures, one novel thing from N3351
> is worth adapting: treating every concept as a predicate, so that the
> programmer can check himself whether a certain set of types models the
> concept:
>
>   static_assert( ForwardIterator<int*>, "ouch" );
>
> and so that you can combine concept requirements with any boolean
> meta-function:
>
>   template <class T> requires Copyable<T> && sizeof(T) < 8 ...

The functionality of retrieving a boolean value is supported in
Boost.Generic, but you can't just use the concept directly as a
value/MPL-style integral constant because accessing the concept as, for
instance, Copyable<T> has to access the concept map. Making it an MPL
integral constant type would conflict. The way you do it in Boost.Generic
is any of a number of ways. First, if you want a boolean value you can use
the macros BOOST_GENERIC_CONCEPT_IS_MODELED_BY
and BOOST_GENERIC_CONCEPT_IS_MODELED_BY_C.

If you want to assert, which is generally more common, you can #include
<boost/generic/assert.hpp> and do any of the following:

// Generates a static_assert, alerting the user if the concept is not
modeled and all of the individual reasons why it's not.
BOOST_GENERIC_ASSERT( YourConcept< T > );

// Generates a static_assert, alerting the user if the concept is not
modeled, giving a user-provided error message.
Boost_GENERIC_ASSERT_MSG( YourConcepts, "Your message." );

// Generates a static_assert, alerting the user if the concept is modeled.
BOOST_GENERIC_ASSERT_NOT( YourConcept< T > );

// Generates a static_assert, alerting the user if the concept is modeled,
giving a user-provided error message.
BOOST_GENERIC_ASSERT_NOT_MSG( YourConcept< T >, "Your message." );

If you look through the Boost.Generic std_concept/concepts and
std_concept/support_concepts tests in the sandbox you can see examples. I'm
working on detailed documentation right now while I update some other
functionality in the library, but there's a ton of stuff to do at the
moment.

-- 
-Matt Calabrese