[boost] Toward metaprogrammable tuples

From: David Abrahams (dave_at_[hidden])
Date: 2002-11-21 10:28:24

I'd like to start a dialogue about how to bridge compile-time and
runtime metaprogramming. To start with, I have some simple problems to
solve:

1. I have an MPL type sequence S and a metafunction (class) F. I want
   to build a tuple type where the type of each tuple element is given
   by F(e) for each e in S.

   I think this is as simple as defining push_front<> for tuple cons
   elements and using fold_backward<> to build the tuple.

2. I have a runtime iterator range over some values of a uniform type
   X, and a type sequence which describes conversion functors from
   X->T1, X->T2,... X->TN. I also have a tuple type with element types
   T1', T2',... TN'. Now I want to construct the tuple with the
   results of calling these functors on the corresponding elements of
   the type sequence. Note that Ti might be a reference type, so
   two-phase construction isn't an option.

3. Well, I have already solved this problem (see
   boost/python/detail/def_helper.hpp), but I'd like to ask once again
   for consideration of an inherits-from-tail tuple implementation in
   the context of metaprogramming with tuples. The aforementioned file
   demonstrates why I think it would be useful. It is extracting from
   a tuple the value of the first type which fulfills some
   compile-time predicate. I believe that's a very common pattern. On
   the other hand, I do see some downsides to using a
   public-inheritance-from-tail implementation, so maybe we should
   just package the tuple_extract algorithm as a separate library and
   be done with it ;-)

-- 
                       David Abrahams
   dave_at_[hidden] * http://www.boost-consulting.com
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