Re: [Boost-users] [Concepts & EnableIf] Is void special?

Subject: Re: [Boost-users] [Concepts & EnableIf] Is void special?
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2012-03-28 13:09:48

On Wed, 28 Mar 2012, Robert Jones wrote:

> On Wed, Mar 28, 2012 at 4:24 PM, Jeremiah Willcock <jewillco_at_[hidden]> wrote:
> 
> It looks like the trick is to put the enable_if outside the BOOST_CONCEPT_REQUIRES, as in:
> 
> template <typename T>
> typename enable_if<
>           condition,
>           BOOST_CONCEPT_REQUIRES(concept, (void))>::type
> 
> Paragraph 4 of [dcl.fct] (in the latest draft) seems to say that the SFINAE error you are getting is required:
> only the specific type void, written in a non-dependent way, counts as a valid function parameter type.
> 
> 
> Well, I can't deny that it works! Seems to be reading quite a bit into the standard tho', assuming you're looking at
> 
>  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf
> 
> If I'm reading it right it is actually talking about function parameter lists, rather than function return types,
> and stating that
> void is not a valid parameter type except for f(void) to indicate an empty parameter list.

The internals of BOOST_CONCEPT_REQUIRES take the type that you give as the 
return type and use it as a function parameter type (that is why you need 
the parentheses around the outside).  That is where those rules in the 
standard get involved.

-- Jeremiah Willcock