On 1/30/13 7:23 AM, Larry Evans wrote:
This discussion might be facilitated if Joel et al (sorry Joel, I don't mean to pick on you, I just mean the group arguing for introducing this "singular" post-move state) simply said "yes, we understand we're making a breaking change (by possibly introducing an additional state to variant that violates the never-empty guarantee), but we still think it's the most practical approach to introduce efficient move semantics to variant". I can jive with that but I think Paul's concerned that you (again, as a representative of the platform you're taking) don't appreciate that this is a breaking change to variant.
No, Jeff, that is wrong. We are not violating the semantics to variant. It's not about variant. It's about recursive_wrapper. I think people are confused with this. The variant's never-empty guarantee still holds.
The page:
http://www.boost.org/doc/libs/1_52_0/doc/html/variant/design.html#variant.de...
says:
variant may be viewed precisely as a union of exactly its bounded types
but having a singular-valued recursive_wrapper violates this view, because there's no way you can dereference a singular-valued recursive_wrapper to get at one of the bounded types.
That is actually a good point. I didn't think of it that way, but yeah, I can appreciate that. So, I think I am inclined to agree with Jeff now. Regards, -- Joel de Guzman http://www.boostpro.com http://boost-spirit.com