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

[Laurent Pinchart]

Hi Christian,

On Mon, May 30, 2022 at 11:20:06PM +0100, Christian Rauch via libcamera-devel wrote:
> Am 30.05.22 um 11:37 schrieb Laurent Pinchart:
> > On Mon, May 30, 2022 at 11:11:23AM +0100, Christian Rauch wrote:
> >> Am 30.05.22 um 10:03 schrieb 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.
> >>
> >> Isn't C++17 backwards compatible? My understanding is that every earlier
> >> C++ standard (e.g. C++11) should be able to compile with a C++17 (or
> >> later) compiler.
> >
> > Not quite I'm afraid.
> >
> > --------
> > #include <map>	/* This includes <optional> indirectly in libc++ */
> >
> > using namespace std;
> >
> > template<class T>
> > class optional
> > {
> > public:
> >         optional()
> >         {
> >         }
> > };
> >
> > int main()
> > {
> >         optional<int> opt;
> >
> >         return 0;
> > }
> 
> Right, this is clashing because of ambiguous symbols from different
> namespaces. You have a point there. I only thought in terms of API
> compatibility there.
> 
> Anyway, libcamera already requires C++17 and uses std::optional. I would
> like to push for a decision if libcamera:
> A) continues to use C++17 and std::optional
> B) reverts those changes and implements a custom "optional"
> 
> I am in favour of keeping the status quo of using std::optional. The
> reason is that I think that not many projects (if any) will be affected
> by these kinds of ambiguity issues above. Resorting to a custom
> solution, just to switch to std::optional in the public API later
> (when?), will cause more breakage issues. A custom temporary solution
> will also require someone to implement and maintain this, knowing that
> it will be replaced eventually anyway.
> 
> I simply think that the benefits outweigh the potential issues. Using
> std::optional, for example as control value return type, improves the
> descriptiveness of the API. While on the other side, there seem to be no
> projects struggling with the current C++17/std::optional situation.
> 
> Sorry for pushing for a solution, but I really would like to continue
> with my original patch set :-)

We may have trouble understanding each other. I'm fine with
std::optional being used in the public API for the time being. *If and
only if* we later realize C++17 is causing issues in some important use
cases, then we can switch to a custom utils::optional implementation.
The risk of having to do so seems low to me.

Tl;dr: More usage of std::optional in the public API is fine.

> > --------
> >
> > $ clang++ -W -Wall -stdlib=libc++ -std=c++14 -o version version.cpp
> > $ clang++ -W -Wall -stdlib=libc++ -std=c++17 -o version version.cpp
> > version.cpp:16:2: error: reference to 'optional' is ambiguous
> >         optional<int> opt;
> >         ^
> > version.cpp:6:7: note: candidate found by name lookup is 'optional'
> > class optional
> >       ^
> > /usr/include/c++/v1/optional:590:7: note: candidate found by name lookup is 'std::optional'
> > class optional
> >       ^
> > 1 error generated.
> >
> >>>> Given libcamera is still 0.0 it could be a nice opportunity to take.

-- 
Regards,

Laurent Pinchart