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

[Mattijs Korpershoek]

Hi Laurent,

Thank you for the review.

On ven., janv. 03, 2025 at 16:53, Laurent Pinchart <laurent.pinchart at ideasonboard.com> wrote:

> Hi Mattijs,
>
> Thank you for the patch.
>
> 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.

>
>> [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 ?

>
>> ---
>>  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