Re: [Libreoffice-commits] use init lists for property sequences

On 02/24/2015 12:32 PM, Bjoern Michaelsen wrote:
> On Tue, Feb 24, 2015 at 08:13:56AM +0100, Stephan Bergmann wrote:
> > Rather than allowing to move out of the temporary pair, better avoid
> > its creation in the first place:
> >
> >             for(auto const & aEntry : vInit)
>
> Eh, true. But actually, I wanted both: Iterating over the pairs in the
> initializer list by reference and still using the move-ctor from there.

You can't move out of an initializer_list (it only offers const access 
to its elements).

> > That use of std::move is actually a pessimization, as it prevents RVO.
>
> How so? The RVO would be to do the ::std::move(). So this is just making that
> (needlessly) explicit, isnt it? At least that is what I read from
>
>   http://stackoverflow.com/questions/17473753/c11-return-value-optimization-or-move

No; see e.g. Item 25 of Meyers' "Effective Modern C++" for details.  (In 
short, RVO only applies if what's returned is a local object, not a 
reference to a local object.  While a compiler would be required to 
internally do the equivalent of adding the std::move if it decided 
against applying RVO in the no-explicit-std::move version, the 
explicit-std::move version prevents it from applying RVO.)


"And check out Scott Meyers' helpful 'Effective Modern C++,' 
<http://shop.oreilly.com/product/0636920033707.do>, if only to make you 
realize that C++ is still a baroque pile of gotchas." 
[<http://lists.freedesktop.org/archives/libreoffice/2014-September/063326.html>]