Making compatible with C++ 20

[jerabaul29]

I wanted to use the format C++ 20-style on strings:

https://en.cppreference.com/w/cpp/utility/format/format

But got an error message. I think this may indicate that the makefile makes use of an older C++.

• Is that the right diagnostic?
• Would it be possible to make the makefiles C++ 20 compatible?

[jerabaul29]

OK, just checked, I have a quite old compiler that does not support C++20, that may be the reason. Still keeping open just to check if others are able to use C++20 features.

[siebenschlaefer]

Not all compilers are ready for C++20, some LTS versions of popular Linux distributions are not even ready for C++17. That's why there's a line CXX_STANDARD 14 in the file CMakeLists.txt.
If your compiler supports C++20 and you have a fairly recent version of CMake then you can just modify that line.
Does that help?

[jerabaul29]

This is perfect, many thanks :) . Do you think it would be possible to add a line or two of explanations about this on the readme of this repo? :) .

[siebenschlaefer]

Good idea!
I would love to have that documented somewhere.
Even better if it would contain the reasons for that decision so that we can upgrade to C++17 or later at some point in the future.

[arcuru]

The downside of this is for mentors. A common workflow is for the mentors to download and run the exercise locally, so:

1. Their local environment would need to support it, and not everyone wants (or can) run a bleeding edge compiler.
2. They'd have to recognize the problem and change the CMakeLists locally, as the CMake file is not uploaded AFAIK.

Also, it makes the mentoring job a lot harder. C++ has had some pretty large changes in usability and best practices from C++14 to C++20, so the solutions that they would suggest would differ depending on which version you were using, and there's not a consistent way to know the version just looking at the code.

There's lots of existing discussion from when I updated things to C++14 See #189 and #292

And to be clear I'm not saying "Don't do this". I am saying that you should probably label your version at the top of your solution file if you expect a mentor to look at it.

You are welcome to add that info to the documentation - https://github.com/exercism/cpp/tree/main/docs

[jerabaul29]

Would it be reasonable to have a small 'config' file in each project, that gets automatically uploaded at each submission, which would contains this kind of information? :)

[arcuru]

Coming back to this, in the past we've said that our minimum supported version should be the default installs on the oldest Ubuntu LTS version. If that still makes sense, we can now update the default to C++17, and updating to C++20 could be done when Ubuntu 18.04 goes out of support in April 2023.

[siebenschlaefer]

@patricksjackson Tying "our" C++ to the oldest Ubuntu LTS sounds somewhat reasonable. But in practice we've already transitioned to C++17, that's what we're using in out CMakeLists.txt files (e.g. see this one from hello-world).

[arcuru]

Oh man, I guess I've really not been paying attention. Thanks @siebenschlaefer

[sophec]

It is worth noting that as of June 2022, the only STL to support std::format is clang 14, which disables it by default because it's incomplete still.

[mattnewport]

Recent versions of GCC now have pretty much complete support for C++20, both core language and libraries. Is there anything holding back updating to C++20 at this point? It would be nice to be able to use ranges in solutions.

[vaeng]

@siebenschlaefer @mattnewport

When I look at the table for compiler compatibility , I see a lot of overlaps with C++20 and the latest clang (12) and GCC (10.5) builds that are available on Ubuntu 20.04.

Many people are looking for std::format support, which is still incomplete in these versions, as far as I can tell.
I suggest we transfer to C++20 when LTS for Ubtunu 20.04 ends in April 2025.

[mattnewport]

@vaeng a year seems a long time to wait given that there are a number of other C++20 features available in GCC 10 that would be quite useful - ranges and span are the two that I see a lot of use for. Those would make for cleaner solutions to quite a lot of exercises, more so than format support in my opinion. There are some other niceties like contains() for associative containers, the spaceship operator, etc. but ranges are really the big one that could change how a lot of exercises can be solved for the better.

[vaeng]

@mattnewport
I'm with you about ranges and spans. I also understand the hesitation to leave an LTS behind before it's due.

Since yesterday there is Ubuntu 24.04 LTS and therefore the future compiler milestone.
I could see the argument to skip the last year with two LTS versions, but I'm very unsure about the actual version distribution with exercism users. What do you think @siebenschlaefer ?

[mattnewport]

@vaeng

Ok, I realize now that I can set CXX_STANDARD 23 in CMakeLists.txt and add it to my config.json under files -> solution and submit that and it will mostly work, barring some missing features in whatever compiler version exercism is using (ranges::fold_left() is not available for example).

This actually addresses most of what I want as it means I can submit solutions to exercises using ranges and even some C++23 features like the monadic functions on optional.

I do think ranges makes C++ much more approachable for newcomers to the language (being able to just write sort(vec) rather than sort(begin(vec), end(vec)) defers a whole lot of questions to later in the learning process) so I think there would be value in moving the defaults to C++20 and I would probably mention the simpler C++20 solutions in some mentorship discussions but being able to use most of C++23 in my own solutions is good enough for now.