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Subject: Re: [boost] [variant] Please vote for behavior (Was: Basic rvalue and C++11 features seupport)
From: Joel de Guzman (djowel_at_[hidden])
Date: 2013-01-21 19:25:58

On 1/22/13 8:13 AM, Jeffrey Lee Hellrung, Jr. wrote:
> On Mon, Jan 21, 2013 at 3:54 PM, Joel de Guzman <djowel_at_[hidden]> wrote:
>> On 1/22/13 7:49 AM, Jeffrey Lee Hellrung, Jr. wrote:
>>> On Mon, Jan 21, 2013 at 3:33 PM, Joel de Guzman <djowel_at_[hidden]> wrote:
>>> On 1/22/13 3:57 AM, Antony Polukhin wrote:
>>>> I've got some nice idea from discussion: nullable variant. If many
>>>>> people use or want to use a variant with a type, that represents empty
>>>>> state, we can create a separate nullable variant class. I think that
>>>>> nullable variant can be implemented more efficient that the current
>>>>> variant. For example we can simplify copy/move constructors and
>>>>> assignment operators, guarantee fast noexcept default construction,
>>>>> simplify metaprogamming and reduce compilation times. Maybe someone
>>>>> want to implement it?
>>>> I like it! If you implement it, I'll be your first user :-)
>>>> I don't really care much about this "never empty" guarantee and
>>>> I think it's not really worth the trouble.
>>> How is this different from variant< blank, ... > ?
>> Less work behind the scenes.
> You mean fewer LOC / simpler implementation (believable) or faster compiled
> code (I'd be skeptical)?


>> Lighter TMP load.
> Agreed, but I'm not sure how much the "never empty guarantee" accounts for
> the present TMP load.


> better noexcept
>> guarantees. Etc.
> I don't see this. I would think constructing blank has the same noexcept
> guarantees as assigning a pointer to null, and can equally well be
> propagated out to the variant interface. You might have a specific case in
> mind, though?

No. After thinking about it for a while, you are right.

> Do you presently use variant< blank, ... >? If not, is it because of the
> above reasons?

I *always* use variant< blank, ... > (or something similar). And that is
precisely why I don't care much about the never-empty guarantee.


Joel de Guzman

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