On Feb 14, 2005, at 7:06 AM, John Maddock wrote:
The following simple patch to tuple_basic.hpp allows tuples to be constructed from std::pairs (there is already an assignment operator, but the copy constructor is missing for some reason, and is required for TR1 conformity).
RCS file: /cvsroot/boost/boost/boost/tuple/detail/tuple_basic.hpp,v retrieving revision 1.28 diff -r1.28 tuple_basic.hpp 627a628,636
template <class T, class U> tuple(const std::pair<T, U>& p) : inherited(p.first, p.second, detail::cnull(), detail::cnull(), detail::cnull(), detail::cnull(), detail::cnull(), detail::cnull(), detail::cnull(), detail::cnull()) { BOOST_STATIC_ASSERT(length<tuple>::value == 2);// check_length = 2 }
Is it acceptable to add this?
I think we could do better, but it would be stretching TR1 (which I think is good). I think any compatible tuple-like type should be convertible to tuple (pair, array, maybe complex?, anything else that declares itself tuple-like and implements the tuple interface). Furthermore, I think is_convertible should answer correctly, even for negative cases (for those pair that are not convertible to a 2-tuple). E.g.: pair<short, long> p(0, 1); tuple<long, short> t1(p); array<int, 2> a = {1, 2}; tuple<long, short> t2(a); static_assert(is_convertible<array<int, 2>, tuple<long, short> >::value, "array<int, 2> is not convertible to tuple<long, short>"); static_assert(is_convertible<pair<short, long>, tuple<long, short>
::value, "pair<short, long> is not convertible to tuple<long, short>"); static_assert(!is_convertible<pair<short*, long>, tuple<long, short> ::value, "pair<short*, long> is convertible to tuple<long, short>");
Such a converting constructor can be done generically, instead of once for each tuple-like type, if we bring back the (sorely missed) is_tuple_like trait. Actually, now that I think more about it, I'm not positive is_tuple_like is really needed (but my proof of concept currently uses it). template <class P> tuple(const P& p, typename Metrowerks::restrict_to<__tuple_convertible<P, tuple>::value>::type* = 0); Where __tuple_convertible does a size check, and an element by element is_convertible check. -Howard