Re: [PATCH 3/3] Do not use std::move when assigning an anonymous object to a unique_ptr.

John Baldwin
2016-11-23T23:31:23Z

On Wednesday, November 23, 2016 04:19:29 PM Simon Marchi wrote:
> On 2016-11-23 15:06, John Baldwin wrote:
> > Using std::move forces an extra copy of the object. These changes fix
> > -Wpessimizing-move warnings from clang.
>
> For those who, like me, do not quite understand what is happening here,
> I suggest the following read:
>
> https://www.ibm.com/developerworks/community/blogs/5894415f-be62-4bc0-81c5-3956e82276f3/entry/RVO_V_S_std_move?lang=en

My head also hurts. I think what clang is warning about is that the
std::move() in these lines breaks RVO for the function being called,
not the function that the modified line belongs to. That is:

foo = bar ();

Is able to do RVO if bar() does the right things for RVO to work.
However:

foo = std::move (bar ());

forces an extra copy of the object since the return value of bar
can't use the storge of 'foo' directly, it has to be copied into
an anonymous object (I think) for std::move to consume.

The commit log for the warning is here:

http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20150427/128053.html

I think these instances fall under the "using a move to create a new object
from a temporary object" case.

--
John Baldwin