Re: [boost] [concept] "not" and "or" concept constraints (N2081)

Subject: Re: [boost] [concept] "not" and "or" concept constraints (N2081)
From: David Abrahams (dave_at_[hidden])
Date: 2010-05-28 12:02:16

At Fri, 28 May 2010 11:31:09 -0400,
Lorenzo Caminiti wrote:
> 
> Hello all,
> 
> How can I use Boost.ConceptCheck to implement "not" and "or" concept
> constraints as they are defined in N2081?

A. You can't, not with Boost.ConceptCheck.

B. Those are a really bad idea anyway.  It's been proven
   mathematically that the use of “or” constraints would lead to a
   combinatorial explosion at typechecking time.  

> (If this is obvious from the documentation, please just point me to the link.)
> 
> For example, how can I program the following code from N2081 using
> Boost.ConceptCheck:
> 
>     // "not" `!` concept constraint for some concept C (not C++ code).
>     template<Regular T> where !C<T> void f(T x) { ... }
> 
>     // "or" `||` concept constraint (not C++ code).
>     template<Regular T> where Integral<T> || Floating<T> T cos(T x) { ... }
> 
> If I understand it right, I can program the "and" concept constraints
> in Boost.ConceptCheck just by specifying multiple concepts as in:
> 
>     // "and" `&&` concept constraint.
>     template<typename T>
>         BOOST_CONCEPT_REQUIRES( ((boost::Integral<T>)) ((boost::
> Arithmetic<T>)),
>     T) f(T x) { ... }
> 
> But how do I use BOOST_CONCEPT_REQUIRES(), etc to program "not" and
> "or" concept constraints?

Boost.ConceptCheck generates errors aggressively, which means you
can't use BOOST_CONCEPT_REQUIRES in an overloading context where one
overload's requirements might not be satisfied.  For this use case you
need to use type traits, MPL, and enable_if.

-- 
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