Re: [boost] [variant] Basic rvalue and C++11 features support

Subject: Re: [boost] [variant] Basic rvalue and C++11 features support
From: Joel de Guzman (djowel_at_[hidden])
Date: 2013-01-10 18:29:41

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?
Nevin gives a compelling counter argument with NaN in that a NaN is a
valid state for a floating point value yet you can't do any operations
on it. I would think that such a state can be acceptable for an already
moved-from object. Well, because, it just makes sense! And IMO, it would
be foolish to let this optimization opportunity pass.

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