[adambatkin]

Something that Rust got _really_ right: Editions. And not just that they exist, but that they are specified per module, and you can mix and match modules with different Editions within a bigger project. This lets a language make backwards incompatible changes, and projects can adopt the new features piecemeal.

If such a thing came to C++, there would obviously be limitations around module boundaries, when different modules used a different Edition. But perhaps this could be a way forward that could allow both camps to have their cake and eat it too.

Imagine a world where the main difference between Python 2 and 3 was the frontend syntax parser, and each module could specifically which syntax ("Edition") it used...

[CrendKing]

But Edition can exist only because Rust intrinsically has the concept of package, which naturally defines the boundary. C++ has nothing. How do you denote a.cpp be of cpp_2017 edition which b.cpp be cpp_2026? Some per-file comment line at top of each file?

C++ is a mess in that it has too much historic baggage while trying to adapt to a fiercely changing landscape. Like the article says, it has to make drastic changes to keep up, but such changes will probably kill 80% of its target audiences. I think putting C++ in maintenance mode and keep it as a "legacy" language is the way to go. It is time to either switch to Rust, or pick one of its successor languages and put effort into it.

[umanwizard]

Rust doesn't have the concept of package. (Cargo does, but Cargo is a different thing from Rust, and it's entirely possible to use Rust without Cargo).

Rust has the concept of _crate_, which is very close to the concept of compilation unit in C++. You build a crate by invoking `rustc` with a particular set of arguments, just as you build a compilation unit by invoking `g++` or `clang++` with a particular set of arguments.

One of these arguments defines the edition, for Rust, just like it could for C++.

[ynik]

That only works for C++ code using C++20 modules (i.e. for approximately nothing). With textual includes, you need to be able to switch back and forth the edition within a single compilation unit.

[humanrebar]

It's not clear that modules alone will solve One Definition Rule issues that you're describing. It's actually more likely that programs will have different object files building against different Built Module Interfaces for the same module interface. Especially for widely used modules like the standard std one.

But! We'll be able to see all the extra parsing happen so in theory you could track down the incompatibilities and do something about them.

[bluGill]

Modules are starting to come out. They have some growing pains, but they are now ready for early adopters and are looking like they will be good. I'm still in wait and see mode (I'm not an early adopter), but so far everything just looks like growing pains that will be solved and then they will take off.

[calibas]

At the current rate, we'll have full module support for all of the most popular C++ libraries sometime around Apr 7th, 2618.

https://arewemodulesyet.org/

[bluGill]

I expect modules to follow a S curve of growth. Starting in about 2 years projects will start to adopt in mass, and over the next 5-10 years there will be fast growth and then (in about 12 years!) on a few stragglers will not use modules. They are not free to adopt but there appear to be a lot of long term savings from paying the price.

[einpoklum]

I'll mention that library maintainers/authors can't even _consider_ modules unless they set C++20 as a requirement. Many/most popular libraries will not do that anytime soon. I maintain a moderately-popular library and my requirement is C++11... now, to be fair, I started it back in 2016-2017; but still, I won't even consider requiring C++20 until C++17-and-earlier application code is close to disappearing.

[epage]

Mixing editions in a file happens in Rust with the macro system. You write a macro to generate code in your edition and the generation happens in the callers crate, no matter what edition it is.

[hypeatei]

> I think putting C++ in maintenance mode and keep it as a "legacy" language is the way to go

I agree but also understand this is absolutely wishful thinking. There is so much inertia and natural resistance to change that C++ will be around for the next century barring nuclear armageddon.

[actionfromafar]

I don't think even that would suffice. :)

[adgjlsfhk1]

Cobol's still around. Just because a language exists doesn't mean that we have to keep releasing updated specifications and compiler versions rather than moving all those resources to better languages.

[AnimalMuppet]

COBOL's most recent standard was released in 2023, which rather ruins your point.

[tialaramex]

I think the existence of COBOL-2023 actually suggests that it's not merely possible that in effect C++ 26 is the last C++ but that maybe C++ 17 was (in the same sense) already the last C++ and we just didn't know it.

After all doubtless COBOL's proponents did not regard COBOL-85 as the last COBOL - from their point of view COBOL-2002 was just a somewhat delayed further revision of the language that people had previously overlooked, surely now things were back on track. But in practice yeah, by the time of COBOL-2002 that's a dead language.

[pjmlp]

Fully agree, because for the use cases of being a safer C, and keeping stuff like LLVM and GCC running, that is already good enough.

From my point of view C++26 is going to be the last one that actually matters, because too many are looking forward to whatever reflection support it can provide, otherwise that would be C++23.

There is also the whole issue that past C++17, all compilers seem like a swiss cheese in language support for the two following language revisions.

[bluGill]

> I think putting C++ in maintenance mode and keep it as a "legacy" language is the way to go

That is not possible. The the following function in C++ std::vector<something> doSomething(std::string); Simple enough, memory safe (at least the interface, who knows what happens inside), performant, but how do you call that function from anything else? If you want to use anything else with C++ it needs to speak C++ and the means vector and string needs to interoperate.

[zozbot234]

You can interoperate via C ABI and just not use the C++ standard types across modules - which is the sane thing to do. Every other language that supports FFI via C linkage does this, only C++ insists on this craziness.

[galangalalgol]

Also I wouldn't start by rewriting the thing that calls do_something, I'd start by rewriting do_something. Calling into rust from c++ using something like zngur lets you define rust types in c++ and then call idiomatic rust. You can't do it in the opposite direction because you cannot safely represent all c++ types in rust, because some of them aren't safe.

[bluGill]

I have millions of lines of C++. do_something exists and is used but a lot of those lines and works well. I have a new feature that needs to call do_something. I'm not rewriting any code. My current code base was a rewrite of previous code into C++ started before rust existed), and it costs a nearly a billion dollars! I cannot go to my bosses and say that expensive rewrite that is only now starting to pay off because of how much better our code is needs to be scrapped. Maybe in 20 years we can ask for another billion (adjust for inflation) to rewrite again, but today either I write C++, or I interoperate with existing C++ with minimal effort.

I'm working on interoperation with existing C++. It is a hard problem and so far every answer I've found means all of our new features still needs to be written in C++ but now I'm putting in a framework where that code could be used by non-C++. I hope in 5 years that framework is in place by enough that early adopters can write something other than C++ - only time will tell though.

[galangalalgol]

Yeah that use case is harder, but I'm involved in a similar one. Our approach is to split off new work as a separate process when possible and do it entirely in rust. You can call into c++ from rust, it just means more unsafe code in rust wrapping the c++ that has to change when you or your great grandchild finally do get around to writing do_something in rust. I am super aware of how daunting it is, especially if your customer base isn't advocating for the switch. Which most don't care until they get pwned and then they come with lawyers. Autocxx has proven a painful way to go. The chrome team has had some input to stuff and seem to be making it better.

[bluGill]

Sure I can do that - but my example C++ function is fully memory safe (other than don't go off the end of the vector which static rules can enforce by banning []). If I make a C wrapper I just lost all the memory safety and now I'm at higher risk. Plus the effort to build that wrapper is not zero (though there are some generators that help)

[tsimionescu]

How about going off the end of the vector with an iterator, or modifying the vector while iterating it, or adding to the vector from two different threads or reading from one thread while another is modifying it or [...].

There is nothing memory safe whatsoever about std::vector<something> and std::string. Sure, they give you access to their allocated length, so they're better than something[] and char* (which often also know the size of their allocations, but refuse to tell you).

[bluGill]

> going off the end of the vector with an iterator,

The point of an iterator is to make it hard to do that. You can, but it is easy to not do that.

> modifying the vector while iterating it

Annoying, but in practice I've not found it hard to avoid.

> adding to the vector from two different threads or reading from one thread while another is modifying it

Rust doesn't help here - they stop you from doing this, but if threads are your answer rust will just say no (or force you into unsafe). Threads are hard, generally it is best to avoid this in the first place, but in the places where you need to modify data from threads Rust won't help.

[pjmlp]

Only if using a hardened runtime with bounds checking enabled, without any calls to c_str().

[SkiFire13]

> And not just that they exist, but that they are specified per module

Nitpick: editions are specified per crate, not per module.

---

Also note that editions allow to make mostly syntactic changes (add/remove syntax or change the meaning of existing ones), however it is greatly limited in what can be changed in the standard library because ultimately that is a crate dependency shared by all other crates.

[wink]

My C++ knowledge is pretty weak in this regard but couldn't you link different compilation units together just like you link shared libraries? I mean it sounds like a nightmare from a layout-my-code perspective, but dumb analogy: foo/a/* is compiled as C++11 code and foo/b/ is compiled as C++20 code and foo/bin/ uses both? (Not fun to use.. but possible?)

Is that an ABI thing? I thought all versions up to and including C++23 were ABI compatible.

[zozbot234]

How does foo/bin use both when foo/a/* and foo/b/ use ABI-incompatible versions of stdlib types, perhaps in their public interfaces? This can easily lead to breakage in interop across foo/a/* and foo/b/ .

[tomjakubowski]

> ABI-incompatible versions of stdlib types

libc++ and glibc++ both break ABI less frequently than new versions of C++ come out. As far as I'm aware, libc++ has never released a breaking change to its ABI.

[layer8]

How does Rust do it?

[steveklabnik]

There’s only ever one instance of the standard library when a program is compiled, so an and b cannot depend on different versions of it.

For normal libraries, an and b could depend on different versions, so this could be a problem. The name mangling scheme allows for a “disambiguator” to differentiate the two, I believe that the version is used here but the documentation for it does not say if there’s more than that.

[GolDDranks]

By linking both and not allowing mixing types, i.e. it considers types from a totally unrelated with types from b.

Also, Rust compiles the whole world at once, so any ABI breakage from mixing code from different compiler versions doesn't happen. (Editions are different thing from compiler versions, a single version of the compiler supports multiple editions.)

[steveklabnik]

Rust does not compile the whole world at once. Each crate is compiled separately, and then they’re all linked together at the end.

[GolDDranks]

Yeah, I know, but I mean that it's normal to link together crates compiled with the same compiler, unlike with C, where ABIs are stabler and binary dependencies are more common.

[bluGill]

What is the point? C++ is mostly ABI compatible (std::string broke between C++98 and C++11 in GNU - but we can ignore something from 13 years ago). The is very little valid C++11 code that won't build as C++23 without changes (I can't think of anything, but if something exists it is probably something really bad where in C++11 you shouldn't have done that).

Now there is the possibility that someone could come up with a new breaking syntax and want a C++26 marker. However nobody really wants that. In part because C++98 code rebuilt as C++11 often saw a significant runtime improvement. Even today C code built as C++23 probably runs faster than when compiled as C (the exceptions are rare - generally either the code doesn't compile as C++, or it compiles but runs wrong)

[Maxatar]

There are plenty of things between C++11 and C++23 that have been removed and hence won't compile:

Implicit capture of this in lambdas by copy.

std::iterator removed.

std::uncaught_exception() removed.

throw () exception specification removed.

std::strstream, std::istrstream, and std::ostrstream removed.

std::random_shuffle removed.

std::mem_fun and std::mem_fun_ref, std::bind1st and std::bind2nd removed.

There are numerous other things as well, but this is just off the top of my head.

[bluGill]

None of those things I've never used. Many of those are in bad practice for C++11.

[lmm]

Sure. But per your own other posts in this thread, you've got > 10 million lines of "legacy C++". Probably those bad practices are present in that code and not automatically fixable. So switching to compiling everything with a C++23 compiler is every bit as much not an option for you as switching to Rust, no?

[bluGill]

If I turn on C++23 and get a handful of errors over those million lines of code I will spend the week or two needed to rewrite just those areas of code. That is much easier than rewriting everything from scratch in rust. Even if we just wrap all needed C++ APIs in C so we can use rust that is a lot of effort before all our frameworks have the needed interfaces (this is however my current plan - it will just be a few years before I have enough wrappers in place that someone can think about Rust!)

Note too that we are reasonably good (some of us are experts) at C++ and not Rust. Like any other human when we first do Rust we will make a mess because we are trying to do things like C++ - I expect to see too much unsafe just to shut up Rust about things that work in C++ instead of figuring out how to do it in safe rust. (there will also be places where unsafe is really needed) I want to start slow with Rust so that as we learn what works we don't have too much awful code.

[j16sdiz]

cppreference say strstream is removed in C++26, not C++23.

I knew they are bad, but I don't think it should be removed.

[delduca]

std::shared_ptr::unique()

[wink]

There is no inherent point, I was just wondering, if it's possible, why people don't use such a homegrown module layout like Rust editions in C++.

I only ever worked in a couple of codebases where we had one standard for everything that was compiled and I suppose that's what 90% of people do, or link static libs, or shared libs, so externalize at an earlier step.

So purely a thought experiment.

[steveklabnik]

There was a similar proposal for C++, using rust’s original names: epochs. It stalled out.

[ykonstant]

They should call them 'eras'. Then they can explain that epochs did not lead to a new era in the language, but eras will mark the next epoch of C++.

[humanrebar]

The C++ profiles proposal is something like an "editions lite". It could evolve into more fully featured editions some day, though not without some significant tooling work to support prevention of severe memory and type safety issues across different projects linked into the same program.

[kccqzy]

That's irrelevant. Look, the C++ committee has decided yet again not to break ABI. That is to say, they have affirmed that they DO NOT want backwards incompatible changes. So suggesting a way to make backwards incompatible changes is of no interest to the C++ committee. They don't want it and they have said so more than once.