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

Subject: Re: [boost] [variant] Basic rvalue and C++11 features support
From: Larry Evans (cppljevans_at_[hidden])
Date: 2013-01-11 10:10:08

On 01/11/13 08:59, Peter Dimov wrote:
> Larry Evans wrote:
>> Are you suggesting that move is enabled only if the current tag (the
>> variant<*>::which() result) indicates the current type is default
>> constructable?
> 
> No, I'm suggesting that move (in the presence of recursive_wrappers) can
> be enabled only when there is at least one type that is
> nothrow-default-constructible, regardless of whether it's current.
> 
> typedef variant<int, recursive_wrapper<foo>> V;
> 
> V v1( std::move(v2) );
> 
> This move-constructs v1 from v2 and leaves int() into v2.
> 
Thanks for the clarification.

However, that seems to involve a lot of checking because you'd have
to check possibly all the bounded types to find one that
is nothrow-default-constructible.  Wouldn't it be simpler
to define variant to have, implicitly, a nothrow-default-constructible
type something like the special_type<nothing_id> I mentioned
in my previous post or like the boost::blank which Andrey just
mentioned in his post?

-regards,
Larry