[PATCH] libcamera: base: Don't redefine __nodiscard

[Mattijs Korpershoek]

Hi Laurent,

On lun., janv. 06, 2025 at 10:50, Laurent Pinchart <laurent.pinchart at ideasonboard.com> wrote:

> Hi Mattijs,
>
> On Fri, Jan 03, 2025 at 05:30:50PM +0100, Mattijs Korpershoek wrote:
>> On ven., janv. 03, 2025 at 16:53, Laurent Pinchart wrote:
>> > On Fri, Jan 03, 2025 at 03:02:50PM +0100, Mattijs Korpershoek wrote:
>> >> Some libc implementations (like bionic) already define the __nodiscard
>> >> macro [1].
>> >> 
>> >> Building with bionic results in compiler error since libcamera redefines
>> >> the __nodiscard macro.
>> >> 
>> >> Don't redefine __nodiscard if it's already defined.
>> >
>> > bionic defines it as __attribute__((__warn_unused_result__)) which
>> > (hopefully) does the exact same thing as [[nodiscard]]. Someone else
>> > could define it differently though. I suppose the risk is low ?
>> 
>> My gut feeling is that the risk is pretty low.
>
> Our guts agree.
>
>> >> [1] https://android-review.googlesource.com/c/platform/bionic/+/3254860
>> >> 
>> >> Signed-off-by: Mattijs Korpershoek <mkorpershoek at baylibre.com>
>> >> ---
>> >> Hi, it's been a while. I've found a (trivial) build issue when
>> >> building against recent bionic versions.
>> >> 
>> >> I hope this is the right fix. Thanks for your review!
>> >
>> > It's been nearly four years since we introduced __nodiscard. I wonder if
>> > we could require applications to use C++17 or newer now, and use
>> > [[nodiscard]] directly.
>> 
>> I've done a quick sed to use [[nodiscard]] directly and seems to build
>> fine as well on android-15.0.0_r7 (AP4A.241205.013).
>> 
>> Given that meson.build has cpp_std=c++17 and that [[nodiscard]] has been
>> introduced in C++17, should I send a v2 to get rid of __nodiscard and
>> compiler.h ?
>
> The question is whether or not anyone still uses libcamera in a project
> compiled with C++14. I have tried compiling simple-cam with
> cpp_std=c++14 and it failed due to usage of C++17 features in multiple
> locations in the libcamera public headers. It should be safe to assume
> nobody uses libcamera with C++14 anymore. Dropping compiler.h therefore
> seems best to me.
>
> It would be nice if you could send a v2.

Thanks, done here:

https://lists.libcamera.org/pipermail/libcamera-devel/2025-January/047937.html

>
>> >> ---
>> >>  include/libcamera/base/compiler.h | 2 ++
>> >>  1 file changed, 2 insertions(+)
>> >> 
>> >> diff --git a/include/libcamera/base/compiler.h b/include/libcamera/base/compiler.h
>> >> index fda8fdfdc543f86c5554e38ef790c00d72d60389..946e20d835b216446e4099b6ab6865638d9aaf26 100644
>> >> --- a/include/libcamera/base/compiler.h
>> >> +++ b/include/libcamera/base/compiler.h
>> >> @@ -7,8 +7,10 @@
>> >>  
>> >>  #pragma once
>> >>  
>> >> +#ifndef __nodiscard
>> >>  #if __cplusplus >= 201703L
>> >>  #define __nodiscard		[[nodiscard]]
>> >>  #else
>> >>  #define __nodiscard
>> >>  #endif
>> >> +#endif
>> >> 
>> >> ---
>> >> base-commit: 35ed4b91291d9f3d08e4b51acfb51163e65df8f8
>> >> change-id: 20250103-nodiscard-redef-9158e8fdc3f5
>
> -- 
> Regards,
>
> Laurent Pinchart