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

Subject: Re: [boost] [variant] Basic rvalue and C++11 features support
From: Peter Dimov (lists_at_[hidden])
Date: 2013-01-11 09:18:20

Larry Evans wrote:
> I further jumped to the conclusion that *if* the variant had a
> null state, then it would not assume any of its bounded types
> were "in effect"; hence, would not call any destructors
> for any bounded type or perform any operations on any bounded
> type; hence, the pointer in recursive wrapper would not even
> need to be zeroed because the containing variant would not
> assume that it even had a recursive_wrapper as one of its
> bounded types.

You're quite correct. The problem does not stem from inadequate tagging 
though. The problem stems from the never-empty guarantee:

http://www.boost.org/doc/libs/1_52_0/doc/html/variant/design.html#variant.design.never-empty

If the variant could be empty, or equivalently, if the variant always 
contained a type that can be reliably default-constructed (without an 
exception) such as int, the problems with assignment (both copy- and move-) 
and with move construction do not occur, because the target can be left 
holding the int.

So, one option is to enable move only in this case.