[Development] Raising the minimum to C++20

[Vladimir Minenko]

Hello all,

on this occasion, I would like to call the Qt Development community for conscious and pragmatic decisions when it comes to changes in the "minimum C++ standard”.

For some reason, Qt became known to do these switches on some “surprising" basis. I recall well as a colleague was leaving the office in frustration some day in 2016 and saying, “I now need a break”, just because he installed Qt 5.7.0 (C++11) and realized that his (large) project does not compile anymore, out of sudden and without any notice in advance. We cannot change how things happened in 5.7 or 6.0 (C++17), but we can make it better in the future.

On Dec 21st last year, I sent an email to this list with the subject "About the timeline and phases to support C++20 with and in Qt”:

https://www.mail-archive.com/development@qt-project.org/msg42196.html

This email also refers to related issues on Qt Bug Reports to the roadmap and plans with C++20:

“…1. Use C++20 code with Qt - https://bugreports.qt.io/browse/QTBUG-109360
2. C++20 is required for the development of Qt itself -
https://bugreports.qt.io/browse/QTBUG-109361
3. C++20 is mandatory for users of Qt -
https://bugreports.qt.io/browse/QTBUG-109362
4. Long term: C++23, Qt7, and unsorted  -
https://bugreports.qt.io/browse/QTBUG-109363
…”

It would be great to have a coordinated and well-communicated migration this time. Please consider commenting or even co-working on these issues.

Just to leave a note for clarification for one topic mentioned below on this thread. A more conservative approach to mandating C++ standards is not only driven by the support of such “exotic” platforms like QNX or VxWorks, which might be slow following changes in desktop compilers. There are many regular Qt users who do not have the luxury of switching compilers or libc in their projects as often as they would like to. Plus, I got to know some really big Qt fans in embedded development using embedded Linux who, unfortunately, got stuck on some older BSPs. They feel being ignored in their concerns by developments in Qt, with Qt 6.0 and C++17 in particular, but they still need to get their jobs done and get products on the market. So please do not forget these folks!

Please correct me if I’m wrong, but I think, Qt can still use new compiler versions even if not mandating the latest C++ standard.

Greetings!

--
Vladimir

On 3. May 2023, at 08:58, Sune Vuorela <nospam at vuorela.dk> wrote:

On 2023-05-03, Thiago Macieira <thiago.macieira at intel.com> wrote:
13, while Ubuntu 22.04 and Debian 11 (current stable) have GCC 11. Debian will
probably release its next stable before Qt 6.7, though whether it'll still
upgrade from GCC 12 to 13 I don't know. When we released Qt 6.0, our minimum

Debian 12 "Bookworm" is planned for june 10th, and will be with gcc 12.

/Sune

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