Re: [boost] [contract] static_assert in contracts or not?

Subject: Re: [boost] [contract] static_assert in contracts or not?
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2012-09-03 21:07:49

On Mon, Sep 3, 2012 at 12:48 AM, Andrzej Krzemienski <akrzemi1_at_[hidden]> wrote:
>>
>> Based on your and Vicente's input, I'll remove static_assert from contracts
>> (pre, post, inv, etc).
>>
>> Is any predicate programmable with static_assert also programmable with
>> Boost.ConceptCheck? I guess so if the Boost.ConceptCheck concept can do the
>> static assertion using Boost.MPL or Boost.StaticAssert. If that it true
>> then no need for me to provide static_assert at all, just use
>> Boost.ConceptCheck. Otherwise, I'll allow static_assert together with
>> Boost.ConceptCheck concepts in the requires clausle. How does that sound?
>>
>
> I do not know if Boost.ConceptCheck offers the capability of verifying any
> boolean predicate. I know that concepts proposal (N2081) offered a special
> concept True for that purpose (not sure if it is definable in
> Boost.ConceptCheck). As a side note, in the new attempt at concept design
> described in N3331,

OK, I need to study N3331.

> any concept is a boolean predicate by definition; thus
> you can constraint template like this:
>
>   template <typename T>
>   requires InputIterator<T> && sizeof(T) > sizeof(void*)
>   ...
>
> I believe that whatever the answer to your question is, your idea to enable
> static asserts along concept checking adds some value (let others judge ho
> much): with concept checks you can only eliminate your template (or
> template specialization) from overload resolution set and see what happens:
> perhaps another overload will be picked and the compilation will succeed.
> With static assertions, on the other hand, your template is _always_ always
> preferred only to trigger compilation error the second after: this is often
> useful when you want to provide clear error messages and you know there
> will be no other overload. For instance, if I write a binary number
> literal, and I want to specify at compile-time that my literal can only
> accept 0s or 1s, I would never use concepts or sfinae tricks:
>
>   template <char... C> requires Only0sAnd1s<C...>::value
>   constexpr unsigned operator "" _B();
>
> Instead, I would prefer to write:
>
>   template <char... C>
>   constexpr unsigned operator "" _B()
>   {
>     static_assert(
>       Only0sAnd1s<C...>::value,
>       "only 0s and 1s allowed in binary literal"
>     );
>     return ...
>   }

Thanks,
--Lorenzo