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

Subject: Re: [boost] [variant] Please vote for behavior
From: Dave Abrahams (dave_at_[hidden])
Date: 2013-02-01 17:57:11

on Fri Feb 01 2013, Paul Smith <pl.smith.mail-AT-gmail.com> wrote:

> On Fri, Feb 1, 2013 at 7:15 PM, Dave Abrahams <dave_at_[hidden]> wrote:
>>
>> on Fri Feb 01 2013, Paul Smith <pl.smith.mail-AT-gmail.com> wrote:
>>>
>>> Okay, I'm truely confused about what you're saying then.
>>
>>> Either the standard library requires moved-from user-types to keep
>>> meeting all their requirements, or it only requires them to remain
>>> destructible and assignable.
>>
>> Not knowing if there's specific text about this, I can't say for sure.
>> If, as I think is likely, what it requires of them is not explicitly
>> called out, then it is implied by other requirements.  In other words,
>> the library has the freedom to perform any operations on such objects as
>> it may perform on any other object in the same context.  For example, in
>> a call to sort(first, last), it is free to compare two objects in the
>> range [first,last) even if one or both of them has been moved-from, so
>> the objects passed to such a call must, technically, preserve
>> less-than-comparability of moved-from objects.
>>
>> In practice, though, IIUC implementations will only actually use
>> assignment and destruction on moved-from objects.
>
> This was my understanding.
>
>>
>>> The standard says the former.
>>
>> If you say so.
>
> Table 20, [moveconstructible]:
>
>   rv (the moved-from object) must still meet the requirements of
>   the library component that is using it. The operations listed in
>   those requirements must work as specified whether rv has
>   been moved from or not.
>
> How else should I read it? When I say that "this is what the library
> requires" I mean exactly that - what it technically requires, not what
> it will or will not use in practice.

I think you read it correctly.  However, be aware that the text in
question is in a non-normative note, which means that it doesn't change
the meaning of the standard.  Those facts are implied by the other text;
in particular, the lack of any special and explicit permission for
moved-from objects to stop meeting their requirements.

>> No, I only disavow these parts of what you attributed to me:
>>
>> "it's not something the standard actually guarantees in general, though,
>> and the requirements are still much stricter, perhaps superfluously
>> so.
>
> I'm not sure what you're disavowing then. You don't say that the
> standard guarantees that destructibility and assignability are enough,
> but you do say that this is all it will need in practice. That's
> called superfluous in my book. 

That's a judgement that you're willing to make, but I'm not.

-- 
Dave Abrahams
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