[Development] Proposing to officially allow C++20 types in Qt 6 ABIs

[Fabian Kosmale]

Hi,

I certainly would like to use vocabulary types like std::span in our API rather sooner than later. I have some questions regarding your
proposal, though. Did I understand you correctly that we
- would still guard any such method with an #if __cplusplus >= ...,
- would compile our shipped artifacts in C++20 mode on all platforms where this is possible
- and users still could compile their own code in C++17 mode on those platforms without issues?
- Lastly, in your proposal, platforms without C++20 support would still be able to build Qt, but lack the newer API? And if at some point they would gain a newer compiler, they would have to recompile Qt to get access to those methods, and linker errors otherwise?

If the above is correct (and it actually works), then I think we can do that. The only tricky part is the last point (you upgrade your compiler to support C++20, but your Qt still doesn't provide the C++20 methods you see in the headers), but I think this would be fine if it gets documented.

Regards,
Fabian

________________________________________
Von: Development <development-bounces at qt-project.org> im Auftrag von Marc Mutz <marc.mutz at qt.io>
Gesendet: Donnerstag, 27. Januar 2022 12:38
An: development at qt-project.org
Betreff: [Development] Proposing to officially allow C++20 types in Qt 6 ABIs

Hi,


C++20 brings several new library features that would be great to use in the Qt API, foremost among them are coroutines and std::span.


Yet, both of these are, in a sense, type-erasure techniques, and therefore most useful across ABI boundaries.


Traditionally, Qt has defined a set of minimum supported C++ compilers and whatever their lowest common denominator was, that's what  could be used in the Qt ABI, everything else, let's call it conditionally-supported, could only be used in inline API.


It is worth noting that on MSVC, inline API in exported classes already forms part of the ABI, and no-one seems to have cared.


I would therefore like to propose to soften our ABI guidelines such that we can officially use said conditionally-supported types also in the ABI. This would allow to create non-inline APIs that use std::span and coroutines without having to wait for all compilers to support them.


This is not such a big step: We already have that, de facto, in MSVC. The only difference would be that a C++20 project would require a C++20(or later)-build of Qt. The error upon failure to comply would be unspectacular: linker not finding a symbol in the Qt library.


Seeing as we have allowed C++latest types in our ABI de-facto in the past, I think it's time to allow it de-jure, too.

Discuss 🙂

Thanks,
Marc


Marc Mutz

Principal Software Engineer

The Qt Company
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