[boost] [parameter] Testing for existence of a parameter, building ArgumentPacks, more lazy return types

Subject: [boost] [parameter] Testing for existence of a parameter, building ArgumentPacks, more lazy return types
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2009-05-23 23:50:13

I have a few questions about the parameter library.  Please note that I am 
using argument packs directly, since I am creating them from another data 
structure.

1. Is there a way to build Boost.Parameter ArgumentPacks from a program? 
I am currently using many internal data structures of Boost.Parameter 
(empty_arg_list and such).  Is there another interface to use?

2. Is there a way to tell at compile-time whether a particular parameter 
exists in an ArgumentPack?  I currently have a metafunction that uses a 
special default type and tests for that; is there a more "official" way?

3. I have a case where the default type for a named parameter cannot be 
instantiated in some cases where that parameter is given explicitly.  The 
operator||() lazy defaults don't seem to work for that because they get 
the result_type member from my default right away, even if it won't be 
used.  I currently have a hack to work around this using my 
parameter_exists test and some metaprogramming.

The reason I am working on this low of a level is that I am playing with 
converting Boost.Graph named parameter structures into ArgumentPacks to 
have access to Boost.Parameter's nicer capabilities and syntax.  I thus 
need to build all of the argument data structures and such within another 
function and then access them later.  Thank you for your help.

-- Jeremiah Willcock