David Abrahams wrote: > That's not (I think) what Alexander had in mind: IIUC he was talking > about a wrapper over tuples such that: > > tuple_sequence<some_tuple> > > is an MPL sequence. Of course, a better solution would be to > specialize begin/end so that any Boost tuple is *itself* an MPL > sequence.
I am a bit uneasy about any proposal making adding MPL sequence semantics to an otherwise-typical value type. If such proposals are implemented, I see a significant ambiguity problem arising in generic code: given a type T that is an MPL sequence, should we treat it as a sequence or as a value type? In particular, if we were to implement boost::tuple as an MPL sequence, the following code would seem to me quite unclear insofar as its intended behavior: typedef mpl::list<int, std::string> some_types; typedef boost::tuple<int, std::string> some_tuple; typedef boost::variant<some_types> v1; // ok, unambiguous typedef boost::variant<some_tuple> v2; // ambiguity!? It seems clear that objects of type v1 should behave equivalently to boost::variant<int, std::string>. However, if boost::tuple were an MPL sequence, it seems less clear whether objects of type v2 should behave likewise or instead as a variant holding a 2-tuple. Thus, this question is of particular importance for boost::variant. But also, I imagine, it is important for any other type implementing a pseudo-variadic template interface as I intend for the final release of variant (which is coming, by the way). Input? Thanks, Eric _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
