Re: [Boost-users] [parameter] Parameter-Enabled Class Templates:combining keyword and deduction for the same parameter

From: Joaquin M Lopez Munoz (joaquin_at_[hidden])
Date: 2007-07-28 04:00:19

Daniel Wallin <daniel <at> boost-consulting.com> writes:

> 
> JOAQUIN LOPEZ MU?Z wrote:
[...]
> > I've read this over and over and can't say whether you're correcting
> > Dave or me :( excuse my incompetence. So, given (in the context
> > of class template parameters)
> > 
> >   required<deduced<tag::T>,F>,
> > 
> > can Boost.Parameter match it to T<X> even if X does *not*
> > satisfy F? Thank you!
> 
> Yes, that is correct. However, is you use `parameters::match<>` to
> restrict a function template, SFINAE will kick in. For example:
> 
>   BOOST_PARAMETER_FUNCTION((void), f, tag,
>     (deduced
>       (required (x, (std::string)))
>     )
>   );
> 
>   f(x = 10); // no overload matches

OK, thanks for the info! Since it's only class templates that
I'm interested, the behavior is exactly what I need.

In the meantine, I've tried the lib and learnt the additional
fact that, for

  required<deduced<tag::T>,F>,

if T<X> is passed and T<X> (rather than X) satisfies F,
then Boost.Parameter returns X as the matched arg rather
than T<X>. Not that this poses any particular difficulty
with what I'm trying to use the lib for, but it's a
somewhat corner case I'd like to share with you.

BTW, Boost.Parameter is a pleasure to work with! I hope
to be able to show soon a B.P-powered lib where your
stuff makes it a joy, both for the lib writer and the user,
to deal with class templates with lots of parameters.

Best,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo