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

[Laurent Pinchart]

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.

> >> ---
> >>  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