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Subject: Re: [boost] [variant] Please vote for behavior
From: Paul Smith (pl.smith.mail_at_[hidden])
Date: 2013-01-29 10:12:23

On Mon, Jan 28, 2013 at 8:27 PM, Joel de Guzman <djowel_at_[hidden]> wrote:
> On 1/29/13 1:40 AM, Paul Smith wrote:
>>> 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.
>> Okay, so replace "recursive_wrappers are not pointers" with
>> "recursive_wrappers are not iterators". How does it make it any
>> different? A singular value still doesn't fit in.
> The point is that iterators can have singular values. If iterators
> can have them, why can't any other object (have them)?

Why should any other object have them? Iterators are a generalization
of pointers and that's where they inherit their singular state from.
This has absolutely nothing to do with move-semantics, and that's
exactly why I say that such inferences, just like the NaN example, are
dangerously superficial. A moved-from iterator isn't singular just
like a default-constructed recursive_wrapper isn't. Different concepts
- different issues. The point I'm trying to make is that conceptually
recursive_wrappers don't have a value of their own. They have exactly
the same set of states as their underlying type. If this type has a
singular value, then and only then does a recursive wrapper around
this type has a singular value.

> IMO, ultimately,
> it's a matter of design. You may not agree with a recursive_wrapper
> being in a "singular" state after move, but that's just your preference.

Okay, I'm not sure what exactly we are disagreeing about anymore.

Do we agree that the move semantics we have in C++ are non-destructive
(I'm not asking whether you like it or not. At this point it's a fact
- not a preference)? If you agree with that, then you should
appreciate that it's like that for a reason, whether or not you agree
with that reason.

A moved-from object should remain in a valid state. You're suggesting
meeting this requirement by introducing a new (and yes, breaking)
state, let's bluntly call it the "invalid" state, and you don't see
what I'm talking about when I say that this is just missing the point?
Then there's really nothing more I can say...

> IMO, it's necessary for proxy-like objects that own and hold their
> subjects by pointer. It's not quite elegant, sure, but C++ is never
> elegant in many respects for the sake of high performance. I'd trade
> this quirk for the sake of efficiency any day.

That's a rant about how move semantics in C++ turned out (completely
intentionally) to be. This discussion isn't about that, it's about how
recursive_wrapper should behave under these semantics.

Paul Smith

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