Re: [boost] [GSoC] [Boost.Hana] Formal review request

Subject: Re: [boost] [GSoC] [Boost.Hana] Formal review request
From: Eric Niebler (eniebler_at_[hidden])
Date: 2014-08-01 12:46:38

On 08/01/2014 08:22 AM, Robert Ramey wrote:
> b) settlement of "best practice in the real world" of implementation
> of this idea. I recommended Boost Concept Check.  Which isn't bad for
> a start, but isn't perfect either.  I ideally I would like to see
> this used where its a good fit and an alternative for cases where it
> isn't.

Boost Concept Check is horribly dated and very limited, IMO. For my work
on a new range library[*], I built a new concept check library for
C++11. You define a concept like:

namespace concepts
{
    struct BidirectionalIterator
      : refines<ForwardIterator>
    {
        template<typename I>
        auto requires_(I i) -> decltype(
            concepts::valid_expr(
                concepts::model_of<Derived>(
                    category_t<I>{},
                    std::bidirectional_iterator_tag{}),
                concepts::has_type<I &>(--i),
                concepts::has_type<I>(i--),
                concepts::same_type(*i, *i--)
            ));
    };
}

template<typename I>
using BidirectionalIterator =
    concepts::models<concepts::BidirectionalIterator>;

Then you can use it like a constexpr Boolean function in enable_if,
which can be conveniently wrapped in a simple macro like:

    template<typename I, CONCEPT_REQUIRES_(BidirectionalIterator<I>())>
    void advance(I i, iterator_difference_t<I> n)
    {
        // ...
    }

There's even a form that you can use on (non-template) member functions
without causing a hard error:

    template<typename I>
    struct wrapper
    {
        void next() { ... }

        CONCEPT_REQUIRES(BidirectionalIterator<I>())
        void prev() { ... }
    };

Types like concepts::BidirectionalIterator can be used like tags for the
sake of tag dispatching, a poor man's concept-based overloading.

I highly recommend working this way if your compiler is man enough. My
range effort would have been DOA without it.

And to answer the inevitable question, I'm not opposed to getting this
into Boost, but it's pretty far down my list of priorities right now. If
someone wanted to take this work and run with it, I'd be overjoyed.

Eric

[*] http://www.github.com/ericniebler/range-v3