Re: [Boost-users] [parameters] Runtime enumeration of parameter list

From: Neil Hunt (boost_at_[hidden])
Date: 2005-12-10 20:26:28

Thanks David (and Joel),

Appreciate the time. That's helped me along the path.

As an aside, I think the docs in 
http://www.boost.org/libs/parameter/doc/html/#applying-sfinae-to-the-overload-set 
reference a superceded example.

I think "when the name argument is not convertible to const char*." should 
be "when the root_vertex argument is not convertible to int." as per the 
section above it.

Thanks again,
Neil

"David Abrahams" <dave_at_[hidden]> wrote in message 
news:u8xutrqem.fsf_at_boost-consulting.com...
> "Neil Hunt" <boost_at_[hidden]> writes:
>
>> I guess the main thrust of the question is "Does the parameters library
>> support runtime enumeration?"
>
> Not directly, but it seems like something we (sh/c)ould support,
> probably by making ArgumentPacks conforming Fusion sequences so you
> could use boost/spirit/fusion/algorithm/for_each.hpp to enumerate them.
> The problem is that I don't have expertise with Fusion-1 and AFAIK it's 
> not
> fully documented.  Specifically, I don't know how to make a conforming
> Fusion sequence.  Maybe Joel can help.  Joel?
>
>> or should I be looking at MPL?
>
> Wouldn't help you, at least not directly.  Well, here's one thing you
> could do (ALL_CAPS should be reserved for macros, so I'm going with
> lower_case):
>
>  // untested
>  BOOST_PARAMETER_KEYWORD(tag, person_window)
>  BOOST_PARAMETER_KEYWORD(tag, address_window)
>  BOOST_PARAMETER_KEYWORD(tag, employer_window)
>
>  typedef mpl::vector<
>     tag::person_window, tag::address_window, tag::employer_window
>  >
>  all_windows;
>
>  template <class ArgumentPack, class F>
>  struct process_arg
>  {
>      process_arg(ArgumentPack args, F f) : args(args), f(f) {}
>
>      template <class Keyword>
>      void operator()(Keyword k) const
>      {
>          f(k, args[k]);
>      }
>
>   private:
>      ArgumentPack const& args;
>      F const& f;
>  };
>
>  template <class Keywords, class ArgumentPack, class F>
>  for_each_arg(ArgumentPack const& args, F const& f)
>  {
>      mpl::for_each<Keywords>( process_arg<ArgumentPack,F>(args,f) );
>  }
>
>  Now you can for_each_arg(args, f) where f is some binary function
>  you'd like to be applied to each argument.  The first parameter is
>  the keyword (just in case you need it) and the second one is the
>  original argument passed.
>
> This won't cope well with missing default arguments, so it seems to me
> making our ArgumentPacks conforming fusion sequences is really the way
> to go.
>
> -- 
> Dave Abrahams
> Boost Consulting
> www.boost-consulting.com