[Boost-users] [parameter] A few usage questions

From: John Maddock (john_at_[hidden])
Date: 2007-05-27 07:38:12

I have a few usage questions for the Boost.Parameter library:

1) If I want to have a function called "policy" that's declared using 
BOOST_PARAMETER_FUNCTION can I access the types of the arguments both from 
within the functions body and for the functions return type:

BOOST_PARAMETER_FUNCTION(
  (return-type-computed-from-arguments),
  policy,
  tag, // 3. namespace of tag types
  (optional // optional parameters, with defaults
    (domain_error, *, throw_on_error)
  )
  (deduced
    (optional
      (precision, *, boost::mpl::int_<53>())
    )
  )
)
{
   // what type are the arguments?
}

I realise I can do this with an argument pack, but then either my users will 
have to call the function with an extra set of parenthesis, or I will have 
to write a bunch of forwarding functions myself - which itself generates a 
"what the is the return type" problem.

>From within the body of the BOOST_PARAMETER_FUNCTION generated function it 
appears that I can use the name "Args" to refer to the type of the argument 
pack, and maybe in the return type as well?  But this isn't documented, so 
can I rely on it?

2) All the arguments I want to pass are actually going to end up being used 
in meta-programs: for symbolic values, I can make each one a unique enum 
type, and use the type of the argument (even though it's actually passed as 
a value) to generate my metacode.  But I have one optional argument where I 
require the value to be an arbitrary compile time constant, the only 
interface I can come up with at present is something like:

   policy(_digits10 = mpl::int_<10>());

But I'd really like to be able to write:

   policy(_digits10 = 12);

and have the "12" converted to mpl::int_<12> "by magic", but I can't think 
of any magic that could be applied here... anyone any ideas?

Thanks, John.