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Subject: Re: [boost] [Review.Contract] Vicente's review
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2012-09-03 21:01:49

On Sun, Sep 2, 2012 at 11:14 PM, Vicente J. Botet Escriba
<vicente.botet_at_[hidden]> wrote:
> Le 03/09/12 00:14, Lorenzo Caminiti a écrit :
>> On Sun, Sep 2, 2012 at 2:39 PM, Vicente J. Botet Escriba
>> <vicente.botet_at_[hidden]> wrote:
>>> Hi,
>>> here is my short review.
>>> My vote is YES subject to these condition.
>> Thanks a lot for the review.
>>> * Removal of the Concepts part.
>>> The use of Boost.ConceptCheck to emulate C++ concepts proposal is in my
>>> opinion erroneous. Boost.Concept.Check is used to assert the actual
>>> parameter satisfy statically the concept conditions, while if I'm not
>>> wrong
>>> the C++ Concepts proposal use the requires clause to state the conditions
>>> of
>>> a possible instantiations. Using the same grammar with different
>>> semantics
>>> is confusing. In a last resort, if this feature is preserved, the
>>> requires
>>> keyword must be changed by another one.
>> If that were to be the final decision, I'll remove the interface to
>> Boost.ConceptCheck or I can use any (alphanumeric) mane rather than
>> requires (e.g., where).
>> However, I can use the requires clausule to do multiple things:
>> 1. Check concepts defined using Boost.ConceptCheck (current, I'd leave
>> it as is).
>> 2. Check static_assert (under consideration, to be moved here from
>> within pre/post/inv).
>> 3. Check Boost.Generic-like concepts implemented using C++11
>> expression SFINAE (for future... this will be *a lot* of work...).
>> I should be able to support of all those because I can distinguish 2
>> using the pp (starts with static_assert)
> I will really prefer the static_assert goes outside requires, e.g. in a new
> *check* clause. This clause could also contain the ConceptCheck part. In
> this way it is clear that the requires part will disable the instantiation
> if not satisfied, and the check part will report a compile-time error if not
> satisfied.
>> and I can distinguish between
>> 1 and 2 using the compiler for example using a (hidden) tagging base
>> class to all concepts defined for 3.
> I'm not sure this distinction is possible. For function templates, the
> requires part associated to ConceptCheck should be included in the function
> definition, while the one associated to enable_if/SFINAE c++ proposal
> concepts emulation should be part of the C++ declaration. For template
> classes, ConceptCheck goes to the members part, while Concepts goes to the
> template parameter part.
> As you can see the semantics are different, so a specific keyword would help
> to identify the difference.

Can you provide an example of how all of this would ideally look like?
You can also pick a "keyword" other than requires that makes sense to
you and then indicate where to use requires and where to use the other
keyword in the example. Maybe you start from the static_assert example
I gave:

    template< typename To, typename From >
        requires // new concepts??
            Convertible<From, To> // disable this declaration using SFINAE??
        unless // static_assert and Boost.ConceptCheck??
            static_assert(sizeof(To) >= sizeof(From), "destination too
small"), // generate compiler-error??
            boost::Copyable<From> // generate compiler-error??
    To* memcopy ( To* to, From* from )
        precondition {
            to; // pointer not null
            from; // pointer not null
        // ...

Or any other example will do but I want to fully understand what your
idea is. Thanks :)

>> IMO, I'd stick with requires, leave 1 as is, maybe add 2 for now, and
>> consider 3 for future.
>>> * static assertions are removed from the pre/post conditions. They can be
>>> moved to a specific "static assertions" section at the same level than
>>> the
>>> requires clause. This section could also include the concept check
>>> included
>>> now in the requires clause.
>> OK (but I'd personally bundle all into requires).
>>> * The library extends the EDSL to use directly C++11 features, in
>>> particular
>>> move semantics.
>>> Now that since the C++11 standard is released, more and more people are
>>> using C++11 specific features. I, in particular, will not use Contract if
>>> I
>>> can not make use of C++11 features provided by the compiler.
>> I'd prefer to add C++11 in future releases because the lib is usable
>> as is for C++03 and adding the C++11 features will take time but it
>> won't break the interface. Of course, I'll comply with whatever the
>> final decision is.
>>> * The contract broken handlers are noexcept, so that adding contracts
>>> don't
>>> change the original prototype.
>> They will be when C++11 support is added (again, I'd prefer later but
>> I'm OK otherwise even if that will delay a possible release).
> If you are allowed to release without C++11 support, it will be great if the
> documentation states clearly that the contract broken handlers can not
> throw. Otherwise, when moving to C++11, the C++03 usage will be broken. Even
> if you don't take care of C++11 features for the functions declaration, you
> could request that the contract broken handlers are noexcept using the Boost
> macro BOOST_NOEXCEPT, which is empty in C++03.

OK, makes sense.


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