[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 11:11:23AM +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;
}
--------

$ 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