On 1/8/2013 1:11 PM, Dave Abrahams wrote:
on Mon Jan 07 2013, Eric Niebler <eric-AT-boostpro.com> wrote:
On 1/7/2013 12:14 PM, Paul Smith wrote:
A recursive_wrapper is not a pointer. It's a value-like wrapper that is assumed to always contain a valid object. The move constructor should leave the moved-from recursive_wrapper in a valid state, which precludes nullifying it.
Paul, I think you are confused about move semantics. Joel and Hartmut are right. Nobody should ever be seeing a moved-from object.
No, Paul is right on the money. The moved-from state is part of the object's invariant, and objects in that state may become exposed in several ways (e.g. use std::remove on a sequence of them). If you want a given class to have an empty moved-from state, that's fine... but you can't claim the object is invalid after a valid operation is used on it. You have to document the client-visible consequences of that state as part of the object's behavior, etc.
Yes, I'd already figured out that Paul knows what he's talking about, and I apparently don't, so I've shut up. :-) This discussion has been fascinating, and I've learned a lot. Thanks.
Move constructors and assignment operators exist precisely to enable this kind of optimization.
True, but irrelevant. :-)
I admit that I find this somewhat disappointing. I'm now readjusting my expectations of move semantics. -- Eric Niebler BoostPro Computing http://www.boostpro.com