[libcamera-devel] Usage of C++17 in the public API (was "[PATCH] cam: convert to libfmt")

[Laurent Pinchart]

On Mon, May 30, 2022 at 09:47:25AM +0100, Eric Curtin via libcamera-devel wrote:
> On Sat, 28 May 2022 at 00:19, Christian Rauch wrote:
> > Am 22.05.22 um 11:52 schrieb Laurent Pinchart via libcamera-devel:
> > > On Sun, May 22, 2022 at 11:08:27AM +0100, Kieran Bingham wrote:
> > >> Quoting Laurent Pinchart (2022-05-10 14:04:57)
> > >>> On Tue, May 10, 2022 at 01:59:37PM +0100, Eric Curtin via libcamera-devel wrote:
> > >>>> On Tue, 10 May 2022 at 11:10, Kieran Bingham via libcamera-devel wrote:
> > >>>>> Quoting Naushir Patuck (2022-05-10 10:49:14)
> > >>>>>> On Tue, 10 May 2022 at 08:16, Tomi Valkeinen via libcamera-devel wrote:
> > >
> > > [snip]
> > >
> > >>>>>> For what it's worth, I absolutely loathe formatting in std iostream, and
> > >>>>>> libfmt is a wonderful alternative.
> > >>>>>> It also is close enough to the C++20 std::format implementation that
> > >>>>>> eventual porting would be low effort.  So I am all for this change :)
> > >>>
> > >>> It's going to take a while before we can switch to C++20, but I'm
> > >>> looking forward to it (and before that to some of the C++17 features
> > >>> too, it would be nice to use std::optional in the public API).
> > >>
> > >> So, I've just seen as Christian said - we already do!
> > >>
> > >> It's used by color space ..
> > >
> > > Oops...
> > >
> > >> So we're faced with either reverting, or rolling back on the color-space
> > >> change, or pushing forwards with C++17 on the public-api.
> > >
> > > Switching to C++17 worries me as it's fairly recent and may not be
> > > easily available for all users. std::optional could easily rbe replaced
> > > with a custom implementation in utils::optional if needed. I'm tempted
> > > to leave std::optional in for now, and accept new users of std::optional
> > > but no other C++17 APIs. Switching to utils::optional for ColorSpace
> > > would cover new users of std::optional so we're not creating any new
> > > liability.
> >
> > I think that C++17 is quite old by now (nearly 5 years) and all major
> > compilers support it for multiple versions and years now. Do you know of
> > an example, where requiring C++17 would hinder the adaptation of
> > libcamera? Do any of the target system not support this standard?

The main blocker used to tbe Chromium, which used C++14. It seems to
have moved to C++17 now, and we have also dropped the plan for direct
integration of libcamera support in Chromium (and Firefox) as they
should go through PipeWire and the XDG camera portal.

> > I think the use of std::optional to express invalid values is quite
> > significant compared to the previous solution of using default
> > constructed values. Reimplementing this in libcamera would be a
> > solution, but it just adds development and maintenance costs for no
> > obvious reason (to me).

We can use std::optional (as it's there already). *If* C++17 support
ends up being an issue in the future, requiring us to move back to
C++14, then it will be easy to make a custom implementation, but only if
necessary (we've done that with std::span already, which is only
available in C++20).

> > >> Maybe this deserves it's own thread...
> > >
> > > Done :-)
> > >
> > >>>> I am all in for this change also, personally I would have changed to
> > >>>> printf for now
> > >>>> to have one less dependency (also an easy port to C++20 std::format). The
> > >>>> dependency list is getting large, I noticed a build of mine failing
> > >>>> recently because
> > >>>> I didn't have libyaml.
> > >>>
> > >>> I've posted a patch that falls back to a subproject in that case. Would
> > >>> you be able to give it a try ?
> > >>>
> > >>>> But std::format and libfmt are quite fast and anything is better than
> > >>>> streams so +1.
> > >>>>
> > >>>>> I've never used (yet) {fmt}, but I've only heard good things about
> > >>>>> performance, and of course it's headed into the standard, so I also
> > >>>>> think there is some good merit to be found in this development.
> 
> Only speaking for the red hat distros, every distro that has a package
> built for libcamera or could have in future has at least C++17,
> including newly released LTS release RHEL9.

That's good to know. The other question is whether all C++ applications
that may want to use libcamera can be compiled with C++17 or newer.

> Given libcamera is still 0.0 it could be a nice opportunity to take.

-- 
Regards,

Laurent Pinchart