Re: [boost] [variant] Basic rvalue and C++11 features support

Subject: Re: [boost] [variant] Basic rvalue and C++11 features support
From: Paul Smith (pl.smith.mail_at_[hidden])
Date: 2013-01-08 13:30:14

On Tue, Jan 8, 2013 at 8:09 PM, Peter Dimov <lists_at_[hidden]> wrote:
> Paul Smith wrote:
>>
>> Sure. My point was that "destructible and assignable" are not always
>> enough as far as the library goes.
>
>
> Yes, I agree. To expand on that, the requirements defined by the standard
> library such as CopyConstructible, LessThanComparable, and so on, do not
> have preconditions. So if a movable type claims to conform to these
> requirements, it can't impose a precondition that the operations only work
> on values that haven't been moved from. And the same holds for requirements
> defined by libraries other than the standard library.
>
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That's exactly my point.
If you were doing something like this (i.e. nullifying rec_wrap) in
your own code, I wouldn't be all that concerned because you should
know the requirements from your own types. But variant is a generic
library, which is used by different people that have different
requirements. Once we decide that moved-from recursive_variants need
to be less-than comparable for the STL's sake, we essentially admit
that the moved-from state *is* visible. And why stop here? Why just
guarantee comparability? Different users might have other equally
legitimate requirements just as well. The only way to make everyone
happy is to guarantee that a moved-from recursive variant is a valid
variant.

--
Paul Smith