On May 5, 2005, at 8:46 AM, Thorsten Ottosen wrote:
Thorsten Ottosen ha escrito:
What do you think of this? IMHO this can solve the problem of making std swapping algorithms work with ptr_containers, a problem for which (if I'm not wrong) no satisfying solution has been designed.
you're right that no satisfiable solution has been reached.
http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1771.html
explains a different way to fix standard algorithms by mandating moving is used if available.
my understanding of this, based on discussion with Howard, is that we can actually make an iterator with a proxy-reference for pointer containers that works.
Could you elaborate on this? I'm interested on how to apply this tecnhique for my own purposes. Thank you,
>>>>
well, first of all, it requires the algorithm to move elements around if it can (so nothing will work until C++0x).
secondly, your reference-type of the iterator is a proxy that can be moved where moving it actually moves the pointer underneith.
maybe there is more to it, but Howard assured my that the algorithms in question would never do a lvalue to rvalue conversion (from the iterators reference to its value type)
Clarification: I'm not positive of the definition of "algorithms in question", so I'll try to supply one. I believe you're referring to algorithms only allowed to use Swappable, and not MoveConstructible or MoveAssignable. N1771 lists those algorithms as: swap_ranges iter_swap reverse random_shuffle partition next_permutation prev_permutation Notably absent from this list is rotate, specifically mentioned in Joaquín's proposal. This algorithm, along with several others, requires Swappable, MoveConstructible and MoveAssignable. Such an algorithm may call unqualified swap, but is also allowed to move construct and move assign not only from one iterator reference to another, but to and from the iterator's value_type (typically a local temporary). For example from the insertion_sort example in N1771:
value_type tmp(std::move(*j)); // move construct to temporary for (BidirectionalIterator k = i; k != first && tmp < *--k; --j) *j = std::move(*k); // move assign from reference to reference *j = std::move(tmp); // move assign from temporary to reference
Such capability is considered critical for the typical "gcd" implementations of rotate. If you would like to rig up a self contained test case for me to try out on a full implementation of N1770/N1771 I would be happy to do so and report back with the results. -Howard