On Wed, 3 Jun 2009, David Abrahams wrote:
on Tue May 26 2009, Jeremiah Willcock <jewillco-AT-osl.iu.edu> wrote:
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?
Yes, and it's documented. Have you read through the documentation, or do you mean something else?
Are you talking about section 3.2.1? I also need to get the types of the resulting ArgumentPacks.
Oh!
Hmm, and it looks like the comma operator is completely missing from the reference docs. https://svn.boost.org/trac/boost/ticket/3128
I looked at that ticket, and it doesn't mention the return type thing.
I am currently using many internal data structures of Boost.Parameter (empty_arg_list and such).
That doesn't sound like a public interface to me, but maybe it should be.
Yes, that is the issue. I could use the operators in 3.2.1 but need a way to get the type of the result.
Makes sense. Please open a Trac ticket for that feature.
OK -- https://svn.boost.org/trac/boost/ticket/3129
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?
Nope, that's the way we recommend.
OK. I guess I would prefer an explicit metafunction for that.
Patches welcome :-)
Does something like this work (tested only in another context): struct bgl_parameter_not_found_type {}; template <typename ArgPack, typename KeywordType> struct parameter_exists : boost::mpl::not_< boost::is_same< typename boost::parameter::binding< ArgPack, KeywordType, bgl_parameter_not_found_type >::type, bgl_parameter_not_found_type> > {};
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.
Ouch. It would be nice to have a fix for that one.
Can you open a ticket?
I already did -- https://svn.boost.org/trac/boost/ticket/3114
Yes -- I am starting with keeping the old external interfaces (only) and translating the implementation to use Boost.Parameter internally. This means that users will not see any changes and yet the internals of BGL can use Boost.Parameter default computation and such. Later, once that's working, the external interfaces can be switched.
All I can say is, open the necessary tickets. We always intended that Boost.Parameter be usable for Boost.Graph (although we never imagined you'd take this route to get there ;-))
Sure, but you understand that interface compatibility is important. It would be better to have a single implementation that works with both Boost.Parameter and with the old interface (externally), while being able to use Boost.Parameter syntax internally.
One more question -- I have a situation where I'd like to declare an extra variable only when a certain parameter is not specified, and the type of that variable may not even be valid when the parameter is specified.
I solved this another way, so never mind on that. A couple more questions: 1. Is there a way to create a function that uses Boost.Parameter syntax to declare named parameters and then passes an ArgumentPack on to some other function rather than decoding that ArgumentPack itself? That way the same back-end could be used for both functions with actual (vararg) named parameters and the old-style interface. Note that there are some non-const reference parameters used in BGL. 2. Does Boost.Parameter prevent unrecognized keywords from being passed to functions? Can a direct user of ArgumentPacks use this capability? -- Jeremiah Willcock