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

Subject: Re: [boost] [variant] Please vote for behavior
From: Dave Abrahams (dave_at_[hidden])
Date: 2013-01-30 11:16:06

on Mon Jan 28 2013, Joel de Guzman <djowel-AT-gmail.com> wrote:

> On 1/28/13 12:18 AM, Dave Abrahams wrote:
>>
>> on Sat Jan 26 2013, Joel de Guzman <djowel-AT-gmail.com> 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.

Yes, it's like a singular iterator, but it's a lot more like a null
pointer.  A singular iterator is like (and may in fact be) an
uninitialized pointer, over which the program does not and cannot exert
any control.  A null pointer, on the other hand, has been initialized to
a known state, but one in which some operations (e.g. *p, ++p, --p,
p[0], ...) are not allowed.  Or you could think of it as being like the
a zero-valued primitive numeric type, which you can't use as a divisor.

> 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.

Does it break backward-compatibility, by changing the guarantees users
have about the possible values of certain variant types?  If so, the
documentation should make that clear as well.

-- 
Dave Abrahams
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