[withzombies]

Shouldn't the compilers be on the bleeding edge of the standards? What is the downside of switching to the newest standard when it's properly supported?

It's the type of dog fooding they should be doing! It's one reason why people care so much about self-hosted compilers, it's a demonstration of maturity of the language/compiler.

[cogman10]

There's a bootstrapping process that has to happen to compile the compiler. Moving up the language standard chain requires that compilers compiling the compiler need to also migrate up the chain.

So you can never be perfectly bleeding edge as it'd keep you from being able to build your compiler with an older compiler that doesn't support those bleeding edge features.

Imagine, for example, that you are debian and you want to prep for the next stable version. It's reasonable that for the next release you'd bootstrap with the prior releases toolset. That allows you to have a stable starting point.

[stabbles]

This is not the case. They are discussing the default value of `g++ -std=...`. That does not complicate bootstrapping as long as the C++ sources of GCC are compatible with older and newer versions of the C++ standard.

[cogman10]

> as long as the C++ sources of GCC are compatible with older and newer versions of the C++ standard.

I've worked on a number of pretty large projects. If the target for the source code changes it can be really hard to keep C++20 features from creeping in. It means that you either need to explicitly build targeting 11, or whoever does code reviews needs to have encyclopedic knowledge of whether or not a change leaked in a future feature.

It is "doable" but why would you do it when you can simply keep the compiler targeting 11 and let it do the code review for you.

[bluGill]

Compilers often allow things in 11 that technically are not there until some later standard. Or sometimes things they have always allowed finally got standardized in a later version. Setting your standard to 11 if that is what you want to target it a good first step but don't depend on it - the real tests is all the compilers you care to support compile your code.

Even if you only target 11, there may be advantages to setting a newer version anyway. Sometimes the standard finally allows some optimization that would work, or disallows something that was always error prone anyway. I would recommend you set your standard to the latest the compiler supports and fix any bugs. Solve your we have to support older standards problem by having your CI system build with an older compiler (and also the newest one). C++ is very good at compatibility so this will rarely be a problem.

[quietbritishjim]

> ... why would you do it when you can simply keep the compiler targeting 11 ...

It doesn't appear to me that the parent comment was implying otherwise.

The default is changing for any compilation that doesn't explicitly specify a standard version. I would have thought that the build process for a compiler is likely careful enough that it does explicitly specify a version.

[cogman10]

> It's the type of dog fooding they should be doing! It's one reason why people care so much about self-hosted compilers, it's a demonstration of maturity of the language/compiler.

I could be misreading this, but unless they have a different understanding of what it means to dog fooding than I do then it seems like the proposal is to use C++20 features in the compiler bootstraping.

[ziotom78]

I believe they are really referring to the default mode used by GCC when no standard is explicitly stated.

The email mentions that the last time they changed it was 5 years ago in GCC 11, and the link <https://gcc.gnu.org/projects/cxx-status.html#cxx17> indeed says

> C++17 mode is the default since GCC 11; it can be explicitly selected with the -std=c++17 command-line flag, or -std=gnu++17 to enable GNU extensions as well.

which does not imply a change in an obscure feature (bootstrapping) that would only affect a few users.

[eddd-ddde]

The answer is obvious, YES, specify your language version. Every single compiler invocation for production (for example ci builds) should explicitly select a version. Otherwise you are asking for trouble.

[stabbles]

Yeah, developers should specify what language and dialect a project is written in. In practice though, support for that in build systems is cumbersome.

For example in CMake the natural variable is CMAKE_CXX_STANDARD, but it's implemented backwards: if you set it to 14 but your compiler supports only C++11, they'll add -std=gnu++11. You have to also set CMAKE_CXX_STANDARD_REQUIRED to ON, which not man projects do. I don't think there's an easy way to say "this project requires C++14 or higher".

[menaerus]

There is - you simply build your code with -std=c++XY, and if your toolchain doesn't support it (which one btw doesn't support at least c++17?), it will simply error out, no? That should be a pretty strong signal that your code requires XY standard without having to go into the CMake territory. Even if you want to, I see nothing wrong with the way they are implemented. Two simple variables doing two simple things.

[rmu09]

Aren't they talking about the c++ dialect the compiler expects without any further -std=... arguments? How does that affect the bootstrapping process? This https://gcc.gnu.org/codingconventions.html should define what C/C++ standard is acceptable in the GCC.

[cogman10]

The way I read withzombies's comment (and it could be wrong) was they were talking about the language version of the compilers source. I assumed that from the "dogfooding" portion of the comment.

[maxlybbert]

Correct, this is a discussion of which language version the compiler should follow if the programmer doesn’t specify one. It’s not about which features are acceptable when implementing the compiler.

[kstrauser]

Counterpoint: you could write a C++ compiler in a non-C/C++ language such that the compiler’s implementation language doesn’t even have the notion of C++20.

A compiler is perfectly capable of compiling programs which use features that its own source does not.

[cxr]

That's not a counterpoint—at least not to anything in the comment that you're (nominally) "responding" to.

So why has it been posted it as a reply, and why label it a counterpoint?

[kstrauser]

Read them again a couple more times and it may become clear.

The prior post seemed to be claiming that this required any form of a bootstrapping process, when it does not.

[wat10000]

This particular compiler does require bootstrapping, and that's obviously what "the compiler" is referring to in that comment.

Building your compiler in another language doesn't help at all. In fact, it just makes it worse. Dogfooding C++20 in your compiler that isn't even built in C++ is obviously impossible.

[kstrauser]

It absolutely does not. There is no part of C++20 that requires the implementing compiler to be written in C++20.

My original point is that you can write a compiler for any language in any language.

[cxr]

You have lost the plot, and you are wrong.

[unclad5968]

Well there are still some c++20 items that aren't fully supported, at least according to cppref.

https://en.cppreference.com/w/cpp/compiler_support/20.html

[withzombies]

Yeah, I think it's because none of the compilers are obligated to support the standard and things get added that never get implemented.

A good example is the C++11 standard garbage collection! It was explicitly optional but afiak no one implemented it.

https://isocpp.org/wiki/faq/cpp11-library#gc-abi

[andsoitis]

> Shouldn't the compilers be on the bleeding edge of the standards? What is the downside of switching to the newest standard when it's properly supported?

C++ standards support and why C++23 and C++26 are not the default: https://gcc.gnu.org/projects/cxx-status.html

[ajross]

> What is the downside of switching to the newest standard when it's properly supported?

Backwards compatibility. Not all legal old syntax is necessarily legal new syntax[1], so there is the possibility that perfectly valid C++11 code exists in the wild that won't build with a new gcc.

[1] The big one is obviously new keywords[2]. In older C++, it's legal to have a variable named "requires" or "consteval", and now it's not. Obviously these aren't huge problems, but compatibility is important for legacy code, and there is a lot of legacy C++.

[2] Something where C++ and C standards writers have diverged in philosophy. C++ makes breaking changes all the time, where C really doesn't (new keywords are added in an underscored namespace and you have to use new headers to expose them with the official syntax). You can build a 1978 K&R program with "cc" at the command line of a freshly installed Debian Unstable in 2025 and it works[3], which is pretty amazing.

[3] Well, as long as it worked on a VAX. PDP-11 code is obviously likely to break due to word size issues.

[tcfhgj]

well, shouldn't not-up-to-date code use the corresponding compiler flag instead of someone starting a greenfield project, who might then write outdated code?

[ajross]

No? The "corresponding compiler flag" is a new feature. I mean, who told folks at Bell Labs in 1978 how the GCC --std= arguments would work in the coming decades? Legacy code is legacy, it doesn't know it needs to use the correct flags. When it was a greenfield project, it was the default!

Like, think about it: if you think the defaults should be good for greenfield projects, then greenfield projects won't be using the correct flags (because if they are, then the whole argument is specious anyway). And when C++34 shows up, they're going to be broken and we'll have this argument again.

Compatibility is hard. But IMHO C++ and gcc are doing this wrong and C is doing it much better.

[plorkyeran]

GCC's default has already changed once (to C++11). It did not cause any significant problems, and any software which is relying on the current value was created long after the flags to pick a standard version were added.

[menaerus]

> C++ makes breaking changes all the time,

Please don't spread misinformation. Breaking changes are actually almost inexistent with C++. The last one was with the COW std::string and std::list ~15 years ago with the big and major switch from C++03 to C++11. And heck, even then GCC wouldn't let your code break because it supported dual ABIs - you could mix C++03 and C++11 code and link them together.

So C++ actually tries really hard _not_ to break your code, and that is the philosophy behind a language adhering to something that is called backwards-compatibility, you know? Something many, such as Google, were opposing to and left the committee/language for that reason. I thank the C++ language for that.

Introducing new features or new keywords or making stricter implementation of existing ones, such as narrowing integral conversions, is not a breaking change.

[ajross]

> Introducing [...] new keywords [...] is not a breaking change.

This is some kind of semantic prestidigitation around a definition for "breaking" that I'm not following. Yes, obviously it is. New keywords were valid symbol names before they were keywords.

Makes me wonder if the "don't spread misinformation" quip was made in good faith.

[menaerus]

It was. And no, breaking change is not what you seem to imply it is. When talking about breaking changes, introducing new keywords is not what people usually think of. It's irrelevant.

[ajross]

Sigh. "Irrelevant" except in the sense that the code used to build and now it doesn't after a gcc upgrade, you mean. This is an attitude shared (c.f. "greenfield" comments elsewhere in this tree) among a bunch of people who do intensive development and maintenance on active products all the time.

That use case is probably less than 20% of all the C++ development cycles out there. This is a 40 year old language that the bulk of the industry has decided to abandon for new work. The large majority of people doing work on this code are doing minimal-change updates, and nonsense like this is how you end up with rules like "We have to deploy on Ubuntu 20.04 still because the AbandonWare 4.7 library doesn't work on later version".

And it's avoidable, but not if you run around lying to people and yourself about what a breaking change is. Again, look at how C does this. The C standard writers actually know that they're updating a legacy environment and care deeply about full backwards compatibility.

[menaerus]

Nonsense. Adding more keywords to a language is avoidable? Or making the compiler stricter in checking the integral promotions is avoidable? Sure, it is, but only if you stick compiling all of your code with C89. You're barking at the wrong door.

[dagmx]

This is about changing the default.

The issue with defaults is that people have projects that implicitly expect the default to be static.

So when the default changes, many projects break. This is maybe fine if it’s your own project but when it’s a few dependencies deep, it becomes more of an issue to fix.

[reactordev]

If you’re relying on defaults, and upgrade, that is entirely your fault. Don’t hold everyone in the world back because you didn’t want to codify your expectations.

[dietr1ch]

So it has the added benefit of having people learn how to set up their projects properly? Great.

[bluGill]

C++ is very good at compatibility. If your code breaks when the standard changes, odds are it was always broke and you just didn't know. C++ isn't perfect, but it is very good.

[wat10000]

On the other hand, if you didn't know your code was broken then it probably wasn't broken in a way that's catastrophic to whatever you use it for.

[nomel]

Do you have an example? Adding the `--std=<whatever you're using now here>` flag should work, which you should already be using anyways. Is the issue that you don't want to use that argument?

[MichaelZuo]

That sounds more like a problem of nonsensical assumptions… what possible expectation could there have been that GCC would never change this in the future?

[jayd16]

The assumption is along the lines of "this works so why should I ever think about it again if I don't have to?"

It's not an end user problem, anyway. The issue is the language didn't change in a backwards compatible way and also didn't require setting a language version.

[1718627440]

> What is the downside of switching to the newest standard when it's properly supported?

They are discussing in this email thread whether it is already properly supported.

> It's one reason why people care so much about self-hosted compilers

For self-hosting and bootstrapping you want the compiler to be compilable with an old version as possible.

[BeetleB]

> What is the downside of switching to the newest standard when it's properly supported?

"Properly supported" is the key here. Does GCC currently properly support C++23, for example? When I checked a few months ago, it didn't.

[jcelerier]

Where do you draw the line for properly supported? I've been using g++ in c++23 mode for quite some time now - even if every feature is not entirely implemented, the ones that work, work well and are a huge improvement

[tlb]

I draw the line where I can't expect the default gcc on most Linux and Mac systems to compile my code. And I don't want to force them to install a particular compiler. -std=c++20 seems to work pretty reliably these days.

We're starting to need caniuse.com for C++.

[cemdervis]

https://cppstat.dev

[tlb]

Aha, that's just what I wanted!

[cemdervis]

Glad I could help!

[pjmlp]

It already exists, https://en.cppreference.com/w/cpp.html

[binary132]

A lot of software, and thus build automation, will break due to certain features that become warnings or outright errors in new versions of C++. It may or may not be a lot of work to change that, and it may or may not even be possible in some cases. We would all like there to be unlimited developer time, but in real life software needs a maintainer.

[withzombies]

I'm not talking about software compiled by the compiler having a higher default.

Warnings becoming errors would be scoped to gcc itself only, and they can fix them as part of the upgrade.

[burnt-resistor]

Compatibility. This value has been lost, apparently, and so nothing in the future will be able to run anything else except modern things.

[hulitu]

> Shouldn't the compilers be on the bleeding edge of the standards? What is the downside of switching to the newest standard when it's properly supported?

cursing because the old program does not compile anymore No.

[withzombies]

No old programs wouldn't be able to compile anymore with the proposed change

[superkuh]

When a language changes significantly faster than release cycles (ie, rust being a different compiler every 3 months) it means that distros cannot self-host if they use rust code in their software. ie, with Debian's Apt now having rust code, and Debian's release cycle being 4 years for LTS, Debian's shipped rustc won't be able to compile Apt since nearly all rust devs are bleeding edge targeters. The entire language culture is built around this rapid improvement.

I love that C++ has a long enough time between changing targets to actually be useful and that it's culture is about stability and usefulness for users trying to compile things rather than just dev-side improvements uber alles.

[surajrmal]

The problem you mention is perhaps a sign that the model Debian uses is ill suited for development. Stable software is great but it need not impede progress and evolution. It's also possible to support older rust compiler versions if it's important - apt developers can do the work necessary to support 4yo lts compilers.

[mustache_kimono]

> Debian's shipped rustc won't be able to compile Apt since nearly all rust devs are bleeding edge targeters.

This is nonsense. Apt devs can target a rustc release and that release can be the same release that ships with Debian? Moreover, since those apt devs may have some say in the matter, they can choose to update the compiler in Debian!

> The entire language culture is built around this rapid improvement.

... Because this is a cultural argument about how some people really enjoy having their codebase be 6 years behind the latest language standard, not about any actual practical problem.

And I can understand how someone may not be eager to learn C++20's concepts or to add them immediately to a code base, but upgrades to your minimum Rust version don't really feel like that. It's much more like "Wow that's a nifty feature, I immediately understand and I'd like to use in the std lib. That's a great alternative to [much more complex thing...]" See, for example, OnceLock added at 1.70.0: https://doc.rust-lang.org/std/sync/struct.OnceLock.html