Comment by pjmlp
1 day ago
Even if C++26 covers all scenarios needed for MOC, it is going to take ages until Qt can rely on it being available across all their customer platforms, unfortunely after a spike in the last decades regarding ISO C++ support, the compilers velocity has slowed down, while proprietary compilers have decided to freeze their support somewhere between C++14 and C++17.
Most likely the raise of AOT compiled alternatives, for scenarios where C and C++ were only being used due to being compiled languages, big names in C++ compiler world rather put money on their own alternatives, are the two main reasons of the slow down.
- https://en.cppreference.com/w/cpp/compiler_support.html
- https://en.cppreference.com/w/cpp/compiler_support/17.html
- https://en.cppreference.com/w/cpp/compiler_support/14.html
> Most likely the raise of AOT compiled alternatives, for scenarios where C and C++ were only being used due to being compiled languages, big names in C++ compiler world rather put money on their own alternatives, are the two main reasons of the slow down.
What alternatives?
Rust claws at market share but has problems with basic stuff like doubly linked lists, WIP gccrs and memory unsafety, Carbon looks like a meme with too few people working on it, Ada has a neglected dialect and a proprietary dialect, Go has garbage collection, Zig has not even reached 1.0, Swift is more of an application language than a systems language, Circle is dead.
What else is there?
Not that C++ doesn't have challenges.
> Rust claws at market share but has problems with basic stuff like doubly linked lists, WIP gccrs and memory unsafety,
Doubly linked lists aren’t that important today. Rust doesn’t need gccrs to be useful, and it gives much better memory safety than C++ without needing a GC. So I find your criticism of Rust seems a bit weak. Rust is a strong alternative to C++.
> Doubly linked lists aren’t that important today.
I guess Rust is just not a viable or good language if one cannot even implement easily something as basic as a doubly linked list. Doubly linked list is the bare minimum; if that is not easy to implement, more complex data structures will often be even worse.
Rust, contrary to public claims, often does worse on memory safety than C++, which is a sad state of affairs.
I fear my arguments are way stronger than yours, sorry. Please make it easier to implement data structures in Rust.
3 replies →
Go, Java (GraalVM/OpenJ9), .NET (Native AOT, .NET Native, IL2CPP, Mono AOT), Swift, Objective-C, D, Nim, Ada.
Many people that use C and C++, do it out of habit, any compiled language, even with AOT is capable to deliver for most scenarios in userspace code.
99% of user software written in compiled languages isn't systems programming juggling pointers and type casts.
Even when that is the case, all above languages have FFI features, no need to rewrite the whole thing.
You might have noticed that C and C++ are minority languages in the mobile OS duopoly, for starters, being used only for low level OS services, and game engines.
I think it is fine for C and C++ to be relegated for use caches where they make the most sense. I don't see the point of comparing them to niches, markets or cases where for instance garbage collection is fine.
And, I already mentioned Go, Swift and Ada.
Objective-C is not really on my radar, it's specific to Apple, and it doesn't seem like Apple is betting on it for the future.
D and Nim are languages with low momentum relative to others.
I am not sure the AOT approach will work. Maybe, maybe not. There are direct and indirect obstacles, I believe. D is probably interesting here; as I understand it, it supported both GC and no-GC. But, that meant that a lot of libraries used GC, and thus were not necessarily good options for applications that required no-GC. In some ways, a programming language can also be what people build with it and what its ecosystems are like.
>Ada has a neglected dialect and a proprietary dialect
What on earth are you on about? That’s not the case.
Ada 2022 has been released and Ada 202x is getting improvements. Those features are trickling into FSF GNAT (GCC).
Are you talking about the SPARK dialect, where GNATprove is open source?
It was indeed the SPARK dialect I am referring to.
I find the Ada community difficult to navigate, the existence of multiple dialects does not make it easier. Please do correct me if I am wrong, and I apologize for any mistakes that I might have made.
If it is not the case that the SPARK language or its proprietary compiler is ahead of what is available publicly, why does this page say that pattern matching is in production:
https://github.com/AdaCore/ada-spark-rfcs/blob/master/featur...
While as far as I can tell, it is not part of normal, not-SPARK Ada, and either is not available in open source compilers, or might require enabling an extension?
as for rust linked list
https://crates.io/crates/ghost-cell
That seems more an argument against Rust than one for, sorry. Library assistance, not part of the standard library, does not seem to be used much, not many examples.
> Use at your own risks[sic]!
>This is very much an Alpha quality release, at best.
Last update 1 year ago.
Unsafety.
Interesting research, however.
> What else is there?
There’s Nim, which is getting a major overhaul for v3 (cf. Nimony).
Where are you seeing proprietary compilers freezing support after C++17. C++20 support is almost green across the board (even Cray) and C++23 is getting there. As usual there is a lag, for ex. when nvcc will declare C++23 support and whole swath will turn green. Right now it only supports C++20 (but you can mix in C++23 files and link them together) etc.. I see most C++ projects make C++20 a minimum. (not counting someone who wants to keep running C++98).
Easy, go hunting their manuals for the compilers that aren't yet another clang fork, still a few around between TI, Microchip and co on embedded.
In general, not for Qt deployment scenarios, also take into consideration the surviving big iron UNIX, mainframes and micros still in business, again the compilers that are yet not clang forks.
The table is a non exaustive overview, however it also paints the picture that for those that care about real portable code, regular visits to the table are required before considering adopting any specific new feature, or the need to enforce style guides like "we use C++20, but feature X, Y, Z are forbidden due to lack of portablity".
But Qt doesn't care for microcontrollers, nor mainframes. The most exotic target they have is QNX, but "Qt needs C++XX" seems to be a pretty good motivator for QNX to update their compilers, which are gcc-based after all.
What are all the use cases for MOC?
I used to use Qt quite extensively years ago, and it seemed that all MOC was really bringing to the table was signal-slot connections which can certainly be done in much nicer ways (e.g. just use std::function to represent signals).
> I used to use Qt quite extensively years ago, and it seemed that all MOC was really bringing to the table was signal-slot connections which can certainly be done in much nicer ways (e.g. just use std::function to represent signals).
The linked article goes into detail on what are the use cases for MOC and what are the shortcomings of C++26 to completely replace MOC with C++ reflections.
If you read the article, the goal they are aiming for is replacing MOC with C++ reflections without requiring fundamental changes to how Qt works. There are many reputable and very stable signal&slots implementations in C++ that are both type safe and do not require reflection at all. Some of these projects exist for over a decade already, and required only C++14. But Qt's implementations of signals and slots require far more than that to have a drop-in replacement.
That is what the linked article is about: what moc provides and what the pure C++ equivalent (if any) could be.
If you use QML it does, like... everything? QML can't interact with native C++ at all, you must use the MOC to generate Q_PROPERTYs, signal/slots, the Q_OBJECT macro itself etc.