Re: [boost] [phoenix3] at_c

Subject: Re: [boost] [phoenix3] at_c
From: Denis Taniguchi (taniguchi_at_[hidden])
Date: 2010-07-29 09:14:46

On Thu, 2010-07-29 at 09:34 +0200, Thomas Heller wrote:
> On Thu, Jul 29, 2010 at 1:54 AM, Joel de Guzman
> <joel_at_[hidden]> wrote:
> > On 7/29/10 3:51 AM, Denis Taniguchi wrote:
> >>
> >> Hi,
> >>
> >> I think that a reasonable implementation of at_c for the new phoenix3
> >> could be:
> >>
> >> #pragma once
> >>
> >> // boost headers
> >> #include<boost/fusion/include/at.hpp>
> >> #include<boost/phoenix/function.hpp>
> >> #include<boost/type_traits/remove_reference.hpp>
> >> #include<boost/type_traits/remove_const.hpp>
> >>
> >> // std headers
> >> #include<algorithm>
> >> #include<iostream>
> >>
> >> namespace boost
> >> {
> >>   namespace phoenix
> >>   {
> >>     template<int N>
> >>     struct at_impl
> >>     {
> >>       template<class Signature>
> >>       struct result;
> >>
> >>       template<class This, class Arg>
> >>       struct result<This(Arg)>
> >>       {
> >>         typedef typename boost::fusion::result_of::at_c<
> >>           typename boost::remove_reference<Arg>::type
> >>         , N
> >>         >::type type;
> >>       };
> >>
> >>       template<class Arg>
> >>       typename boost::result_of<at_impl<N>(Arg&)>::type
> >>       operator()(Arg&arg) const
> >>       {
> >>         return fusion::at_c<N>(arg);
> >>       }
> >>     };
> >>   } // namespace phoenix
> >> } // namespace boost
> >>
> >> If that piece of code is valid (I have tested but I'm not sure if that's
> >> the right way to do it) how would boost::phoenix::at_c be implemented?
> >> In phoenix2 at_impl was a eval so at_c was a template inline function.
> >> Best regards,
> >
> > I'm not sure why phoenix2 implemented it using the extension
> > mechanism instead of a plain phoenix function. In phoenix3,
> > I suggest making it a phoenix function. Oh, and I also suggest
> > dealing with const/non-consts and returning references as well.
> 
> I implemented phoenix::at_c the same way as it was done with phoenix2.
> Have a look here:
> https://svn.boost.org/trac/boost/browser/sandbox/SOC/2010/phoenix3/boost/phoenix/fusion/at.hpp
> 
That's exactly what I needed. Thanks again Thomas.

> I think the reason why it was (and is done) this way, is to supply N
> as a template parameter, not a parameter to the lazy function.
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