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Subject: Re: [boost] [variant] Basic rvalue and C++11 features support
From: Joel de Guzman (djowel_at_[hidden])
Date: 2013-01-11 01:23:42

On 1/11/13 11:38 AM, Rob Stewart wrote:
> On Jan 10, 2013, at 6:29 PM, Joel de Guzman <djowel_at_[hidden]> wrote:
>> On 1/9/13 11:03 PM, Paul Smith wrote:
>>> On Tue, Jan 8, 2013 at 8:48 PM, Peter Dimov <lists_at_[hidden]> wrote:
>>>> Paul Smith wrote:
>>>>> The only way to make everyone happy is to guarantee that a moved-from recursive
>>>>> variant is a valid variant.
>>>> Well, the performance hit from `new T` is making a lot of people unhappy, so
>>>> technically, this is not true. :-)
>>> Unfortunately, that's probably not the worst thing about it. The fact that it can
>>> also throw is the one that has the more subtle and far reaching consequences.
>> And that is utterly disappointing. This "conservative" move makes all proxy-like
>> objects with pointer ownership very inefficient! While we are advocating pass by
>> value!
>> So... can anyone finally give a good definition of what "valid" means?
> Validity depends upon the class in question. A smart pointer can be null and still
> valid. In your case, the class has the semantics that it always references a value.
> Changing that, to permit a null state, may be acceptable or it may have unacceptable
> side effects. Perhaps a new type, with the null state, can be added and used in some of
> the current contexts.
> That your tests passed after your changes suggests that Spirit might well use such a
> type without problem.

The problem Paul mentions is very unlikely to happen in real code. I agree
with him that it is more of a paranoia. In 99.9999999% of the cases, the
moved-from object will be destructed right away. Such a paranoid conservative
move will penalize the 99.9999999% of cases, IMO.


Joel de Guzman

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