Re: [boost] [phoenix3] at_c

Subject: Re: [boost] [phoenix3] at_c
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2010-07-29 03:34:09

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
>> {
>> Â  namespace phoenix
>> Â  {
>> Â  Â  template<int N>
>> Â  Â  struct at_impl
>> Â  Â  {
>> Â  Â  Â  template<class Signature>
>> Â  Â  Â  struct result;
>>
>> Â  Â  Â  template<class This, class Arg>
>> Â  Â  Â  struct result<This(Arg)>
>> Â  Â  Â  {
>> Â  Â  Â  Â  typedef typename boost::fusion::result_of::at_c<
>> Â  Â  Â  Â  Â  typename boost::remove_reference<Arg>::type
>> Â  Â  Â  Â  , N
>> Â  Â  Â  Â  >::type type;
>> Â  Â  Â  };
>>
>> Â  Â  Â  template<class Arg>
>> Â  Â  Â  typename boost::result_of<at_impl<N>(Arg&)>::type
>> Â  Â  Â  operator()(Arg&arg) const
>> Â  Â  Â  {
>> Â  Â  Â  Â  return fusion::at_c<N>(arg);
>> Â  Â  Â  }
>> Â  Â  };
>> Â  } // 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

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.