[cfe-commits] r143399 - in /cfe/trunk: lib/CodeGen/CGBlocks.cpp lib/CodeGen/CGExpr.cpp test/CodeGenCXX/block-rvalue-reference-capture.cpp

Douglas Gregor dgregor at apple.com
Wed Nov 2 14:31:39 PDT 2011

On Nov 2, 2011, at 12:48 PM, John McCall wrote:

> On Nov 2, 2011, at 7:59 AM, Douglas Gregor wrote:
>> On Nov 2, 2011, at 1:33 AM, John McCall wrote:
>>> On Oct 31, 2011, at 4:44 PM, Fariborz Jahanian wrote:
>>>> Author: fjahanian
>>>> Date: Mon Oct 31 18:44:33 2011
>>>> New Revision: 143399
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=143399&view=rev
>>>> Log:
>>>> Adds IRGen support for captured rvalue references in blocks.
>>>> In this case, temporary value is copied into block descriptor
>>>> as their own copy to work on. // rdar://9971124
>>> Sorry for missing this earlier, but I really don't think is the right
>>> thing to do.  Block captures of l-value references capture a
>>> reference;  making block captures of r-value references capture
>>> a copy is bizarrely inconsistent.
>> What do you propose? Capture the reference, even though it's almost surely going to refer to a temporary?
> Yes.  References are references.
> Among the things that are wrong with doing a copy:
> (1) It's inconsistent with the capture of l-value references.
> (2) In my experience, l-value references are about equally likely
>  to point into something stored on or owned by the stack, so it's
>  not like those are really any safer.
> (3) C++ programmers using blocks are certain to deal with both
>  reference types and therefore to be surprised by the inconsistency.
> (4) It has to be a copy, not a move, even though it's an r-value
>  reference and the programmer presumably actually cares.
> (5) Capturing a reference to a temporary or local is not actually
>  dangerous except in situations where the block is copied to the
>  heap, but there are plenty of APIs that don't need to do this.
> (6) Unlike lambdas, there's no way to override this behavior, so
>  this is actually a pretty strong expressivity constraint, and the
>  workarounds either involve extra moves or terrible things like
>  capturing a pointer to the temporary.

Okay, I agree. Let's go with capturing these as references. Thanks, John.

	- Doug

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