Alan M. Carroll wrote:

The previous message reminded me of something I wanted to ask.

I wrote an extension for Boost.Tuple that, given a type T and a Boost N-tuple, computes the N+1 tuple (this turns out to be handy for my application). The basic definition looks like this:

template < typename R, typename T, int N = 0 > struct add_type { typedef typename add_type< typename R, typename T, length< typename T>::value >::type type; };

There's a bunch of specializations that handle the actual implementation that are obvious specializations for non-zero values of N.

It's used like this:

typedef typename add_type::type region

where "interval" is the type we want to prepend in to the tuple and "super::region" is the existing tuple. So if super::region is tuple then region is tuple.

Now, the actual style question: Should I have made the add_type be (stylistically) a Boost.MPL metafunction instead? E.g., a metafunction of two arguments (type, tuple) that produces the result tuple? Is that the common practice? Or is that only done when intending to use Boost.MPL? That would mean tthe usage (regardless of whether Boost.MPL itself was in use) would be

typedef typename add_type_mf::apply::type region;

Is that preferred (as in, a little more wordy but much clearer because of the convention)?

You need both a function and a metafuncion. In fusion, you have this functions: push_back(tup, x); push_front(tup, x); and its corresponding metafunction: meta::push_back::type meta::push_front::type If you haven't heard of fusion, here's an excerpt from the docs that I am writing right now: ''' Fusion is an implementation of Boost.Tuple library by Jaakko Jarvi with more sequence types (vectors, lists, sets, maps) iterators, views and algorithms (for_each, transform, etc.). The structure is somewhat modeled after MPL. It is named "fusion" because the library is the "fusion" of compile time metaprogramming with runtime programming. ''' It's (98%) feature complete now. All the important sructures are in place. I'll be asking for a formal boost review very soon now. Cheers, -- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net