Re: [boost] [variant] Please vote for behavior

Subject: Re: [boost] [variant] Please vote for behavior
From: Joel de Guzman (djowel_at_[hidden])
Date: 2013-01-30 21:36:44

On 1/31/13 9:31 AM, Dave Abrahams wrote:
>
> on Wed Jan 30 2013, Joel de Guzman <djowel-AT-gmail.com> wrote:
>
>> On 1/30/13 1:25 AM, Jeffrey Lee Hellrung, Jr. wrote:
>>> On Tue, Jan 29, 2013 at 7:12 AM, Paul Smith <pl.smith.mail_at_[hidden]> 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.
>> A variant holding a singular-valued recursive_wrapper is similar to
>> a variant holding a singular valued iterator. The variant is not empty.
>
> But Joel, IIUC a variant never actually, from the abstract interface
> point-of-view, contains a recursive_wrapper anyway.  Isn't the
> recursive_wrapper just a detail required to make it possible for a
> variant type to (logically) contain itself?  A visitor never sees the
> recursive_wrapper, for example, and the never-empty guarantee doesn't
> says that the variant always contains one of the types it can logically
> contain.

Agreed. If you read my later reply, I am now agreeing with Jeff.
I was wrong. Jeff is right. I see that now.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com