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

Subject: Re: [boost] [variant] Please vote for behavior
From: Joel de Guzman (djowel_at_[hidden])
Date: 2013-01-28 11:11:02

On 1/28/13 10:10 PM, Paul Smith wrote:
> On Mon, Jan 28, 2013 at 2:02 AM, Joel de Guzman <djowel_at_[hidden]> wrote:
>> I think we are OK!
>>
>> Come to think of it, the situation is a lot like a "singular" iterator:
>>
>>    "Iterators can also have singular values that are not associated with
>>    any sequence. [snip] Results of most expressions are undefined for
>>    singular values; the only exceptions are destroying an iterator that
>>    holds a singular value, the assignment of a non-singular value to an
>>    iterator that holds a singular value, and, for iterators that satisfy
>>    the DefaultConstructible requirements, using a value-initialized iterator
>>    as the source of a copy or move operation."
>>
>> Indeed, for a singular valued iterator, i, you can assign to i and
>> destruct i, place i in a container, etc. You just cannot dereference
>> i, access its underlying value through ->, compare i with another
>> iterator, etc.
>>
>> The same is true with a nulled recursive_wrapper.
>>
>> Being honest about this is just a matter of documentation. I do not
>> see any problem at all with having a "singular" recursive_wrapper
>> value.
>
> Then you're still thinking in terms of destructive move. The reason
> why you don't see a problem with a "singular valued" recursive_wrapper
> (that's what we call it now? :-) is that you don't think anyone will
> try to use it. And, c'mon, it's not just a matter of documentation -
> we're not trying to weasel our way out of this. Adding a new and
> entirely different state just for the sake of move construction is a
> polite way of ignoring the possiblity that moved-from objects will be
> used. It might win you a case in court, but it's completely missing
> the point.
>
> Besides, drawing from iterators to recursive_wrappers is dangerous.
> They're completely different things. A recursive_wrapper's value *is*
> its underlying object's value - it's not a pointer. Talking about
> "singular values" under this concept doesn't make sense.
>
> If you want a recursive_wrapper that has an empty state, that's fine.
> But it's a different concept that deserves a different type.

Oh man, here we go again. No I am not convinced. Sorry. The standard
only mentions assign and destroy as necessary, and Dave confirms that,
as did people like Stanley Lippman (C++ Primer 5th Edition). I am also
not convinced that drawing from objects with "singular" values is wrong,
regardless if it's a pointer or not. I think it is you who's missing
the point because Iterators are likewise *not* pointers.

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