Ulrich Eckhardt wrote:
On Friday 10 March 2006 17:14, Vaclav Vesely wrote:
When two optionals (one initialized and other not) are swaped, then the one value in uninitialized optional is constructed and the other one in initialized optional is destructed.
IMHO it's not necessary. It should be more effective to swap optional's m_storages without any values constructiona and destruction.
It could only be done if the object was moveable, i.e. depending on a type_trait. Looking at the Boost type_traits library, I wonder why this
doesn't exist - many things can be sped up using memcpy when is_pod is
... trait true,
but even things like vector/string/deque (list/map/set depending on whether their root node is stored by pointer) can be moved in memory without negative effects (yes, talking about typical implementations here, not guarantees).
I see now. But IMHO is_pod is not satisfactory condition for is_movable. For example the class struct X { X* me; // Can points to the instance }; seems to be POD, but it's not movable. The best implicit implamentation is probably the has_trivial_copy trait. For each other moveble type must be is_movable explicitly specialized. (BTW for compiler writers: compiler support for test a trait on all class members would be very useful for many such traits. And existing compilers with support of has_trivial_copy and similar traits must use this technique internally.) I think it worth it. For example I use optional<string> frequently. If you think that the is_movable trait is good enogh for Boost I would like write it including test and documentation. Regards, Vaclav