Re: [boost] [move] Unifying move emulation code in boost

Subject: Re: [boost] [move] Unifying move emulation code in boost
From: David Abrahams (dave_at_[hidden])
Date: 2009-01-05 12:50:25

on Mon Jan 05 2009, "Daniel James" <daniel_james-AT-fmail.co.uk> wrote:

> 2009/1/4 Ion Gaztañaga <igaztanaga_at_[hidden]>:
>>
>> Howard's unique_ptr contribution has shown again the need for a unified move
>> emulation in Boost. We already have move emulation at least in 5 libraries:
>
> In the move library I was working on, I wasn't aiming for unified move
> emulation, instead it's meant to be flexible enough to support
> different move implementations. This would still be important when
> using libraries outside of boost, as they can't be expected to use our
> implementation.
>
>> -> Unordered: based on Adobe's move library. It's more advanced than the
>> previous one, but Adobe's own library says
>> (http://stlab.adobe.com/group__move__related.html) that a Movable type must
>> satisfy the requirements of concept Regular, which has copy constructor
>> (thus disabling move-only types!). I don't know if the move emulation itself
>> uses Regular requirements.
>
> Move only types are possible (see the attached example, it uses the
> library at http://svn.boost.org/svn/boost/sandbox/move, although I
> haven't updated the documentation yet. it's a similar technique to
> thread's). There is a problem - it doesn't support taking a const
> reference to a returned noncopyable type. I think your unique_ptr
> deals with this case correctly, but it is more complicated to
> implement. If it could be made easy that would be ideal.
>
> I'm probably not going to have any time to work on this in the near
> future, as I've got a few website and documentation tasks to do before
> the next release. I naively assumed that all I'd need to do was
> boostify the adobe library, but it ended up being a larger task. So if
> you want to take it on or do something different feel free.

Aside from contributions I've made to the Adobe move library, I also did
a bunch of additional work/thinking about this topic, and some
collaboration with Daniel on it.  I have some things that are worth
including, like:

           assign(x) = expr

that moves from expr if it is a movable rvalue and otherwise swaps if it
is a swappable rvalue.  Also I figured out how to build "perfect" moving
binary operations in C++03 (e.g. string+string), where there are no
ambiguities and it will always move if at least one argument is an
rvalue.

I can send what code I have if you want it.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com