5 ms·

> I haven't anything like it in other languages Is that different from compiling one lib with --std=c++11 and another with --std=c++17?

heftig 5y ago
If the application is using C++17, wouldn't the headers of the C++11 lib then be compiled as C++17, potentially breaking things? PS: The other way around is more obviously broken, with the C++17 lib headers getting compiled using C++11.
- pjmlp 5y ago
  Or you make use of the preprocessor or if constexpr, and then have the specific code for each language version.
  - int_19h 5y ago
    The maintainers would have to do it for every new language version that breaks them, though. The edition system keeps old stuff working without any effort.
    - pjmlp 5y ago
      I don't want to start this discussion thread yet again, but I am a firm believer that edition system only appears to work right now because: 1 - Rust is still quite young and doesn't have 30 years of accumulated editions 2 - There is still only one major Rust compiler 3 - Editions are designed only to work when compiling the whole project, including 3rd party dependencies, from source code within a single build 4 - So far the editions don't have semantic breaking changes across editions, where behaviour changes across the edition border 5 - There is no plan to ever have editions work across ABIs So "The edition system keeps old stuff working without any effort." might not be true when Rust achieves an adoption scale similar to C and C++, in about 20 years, with several accumulated editions, and a couple of compilers in use. I might be proven wrong, but that is how I see it today.
      - tialaramex 5y ago
        > 1 - Rust is still quite young and doesn't have 30 years of accumulated editions Rust 1.0 was six years ago. When do we start the clock on six years of C++ evolving while also having "old stuff working without any effort" ? C++ 98 to C++ 03 was five years, and that introduces almost no features at all. C++ 11 was eight years later but it's notoriously incompatible to prior versions of C++. C++ 14 is only three years, C++ 17 has breaking changes, as does C++ 20... I think C++ 20 should have taken Epochs (yes even at considerable cost to other new features like Concepts) for this reason. I think ten years from now even if Rust has found it can't achieve everything it wants to via editions, this feature will be generally considered to be a good idea, like generics or string literals. Something you need a specific rationale for not including in your general purpose language. I might be proven wrong too about how far this can go, but I feel like the Rust 2018 and 2021 editions already prove the value of the idea.
        pjmlp 5y ago
        For starters epochs could never work in ecosystems that value binary libraries, regardless of possible ABI issues, which is also a reason why Apple went to such an effort to define an ABI alongside language versions. Lets say you have a noexcept function compiled in C++20 that calls a throws() function, compiled in C++03, which actually ends up throwing, linked together. What is the runtime supposed to do now? To which semantics does it follow now? Call std::unexpected() as it is supposed to do pre-C++14 or call std::terminate() as it should do in C++20? What about the user defined handler that was configured for such scenario? Which of them gets called, or do both get called, in which order? Maybe I am doing it more complex that it is, but I see several scenarios from point of view where multiple compilers, binary libraries and semantic changes come into play, editions turn just into another way to define language versions, because they don't cover all possible uses cases how a language might evolve. Anyway, history will tell how things work out in the end.
        tialaramex 5y ago
        > For starters epochs could never work in ecosystems that value binary libraries Not solving everybody's problem is not the same thing as not solving anybody's problem. You present subtle considerations which, ignoring the fact that they assume Epochs don't exist in C++ 20 yet ask what Epochs should have done in C++ 20 - might have taken up committee time if Epochs had advanced, and which now can only be answered in the vaguest way, they should definitely have decided on a coherent strategy for resolving such problems. It is true that for any conceivable change, Hyrum's Law applies, and so it would apply to Epochs just as it does for Rust's editions. Mara's "competition" for writing Rust that gives different results when re-formatted provides examples of the sort of stuff Hyrum's Law invariably breaks. Nobody should be writing non-toy programs like that and in Rust it seems like nobody is. It's a sad fact that too often C++ programs are written in a very fragile way and many of them can and do break with the least provocation. You can't magically rewrite all those programs, but you can make fragile techniques like SFINAE unattractive to propagate into new programs, and I argue Epochs would have allowed C++ to begin the much harder part of Stroustrup's ever-evolving quest to ship a good programming language - not adding yet more kitchen sinks but removing parts of the language that in hindsight were a bad idea and revisiting old design decisions in the light of what has been learned.
  - tialaramex 5y ago
    It's certainly true that if C++ library maintainers are up for the ever-growing maintenance burden, they can all individually deliver the same promise Rust gets out of the box. This is in practice what the maintainers of the three standard libraries have to do, perhaps one or more of them will offer their opinion about that experience?
orra 5y ago
I don't know what guarantees your typical C++ compiler gives you that those can link together? Regardless, at the very least, you would need to write the headers to be interoperable.
masklinn 5y ago
Aside from the header issue, it doesn't allow for or support backwards-incompatible changes. Because editions are opt-in (and library-level source metadata) the language itself can be modified in non-backwards-compatible ways. So for instance a C++ with editions could make ctors `explicit` by default, or it could entirely change the automatic member generation (by removing it for instance). As long as the ABI and API remain compatible, that's fine.
- neandrake 5y ago
  It’s probably worth noting that the backwards compatibility breaking changes I think only applies to the language and not the standard library. I’m unable to look up the details right now but I believe there’s a trait function defined in stdlib which is deprecated, supersede, but can’t be removed even as part of an Edition, or it would break older code.
  - masklinn 5y ago
    That is correct, editions are mostly about language-level syntactic changes, the APIs have to be compatible between editions. The only place where that isn't the case is the standard prelude (the "builtins" you don't have to import).
    - tialaramex 5y ago
      The ability to replace the prelude does mean that you can imagine if somebody invents a much better thing than, say, Rust's iterators Rust 2050 can have a prelude that brings in std::better::Iterator instead of std::iter::Iterator and then most coders will end up using the new better iterators, just because that's the kind you get out of the box. Anybody who literally refers to std::iter::Iterator gets the old ones of course as does any library code from prior editions, but the documentation could lead those few people in the right direction. And presumably std::better::Iterator politely implements std::iter::IntoIterator because why not. I would be interested to understand if they're allowed to replace the macros. The standard macros aren't actually from the prelude, but instead if you aren't no_std you get all the standard macros anyway. Are they allowed to change those in a future edition? Or not?
pornel 5y ago
Yes, because C versions are frozen in time, and editions aren't. Today Rust added brand new features to Rust 2015 and Rust 2018, and will continue to expand them forever (every new feature lands in all editions whenever possible). Rust editions are closer to source code parsing modes. More like enabling trigraphs in C or "use strict" in JS. Additionally, textual header inclusion in C makes mixing versions tricky. Rust has properly isolated crates, and tracks edition per AST node (so that even cross-crate macros work correctly with mixed editions).
- alerighi 5y ago
  C compilers usually backport features in older standards as well (I know, they are breaking the standard). One example are C++ style comments, that are in the standard only from C99, but basically every compiler supports them even in ANSI/C90 mode. By the way the difference is that Rust is not a standard, thus is easier to evolve (the process is much shorter). On the other side, the fact that a language changes slowly it's something good in a way, it means that you don't have to continue to change the way you do things, and update older projects. That to everyone that has to maintain code for decades it's important. And every serious software project (not hobby stuff) does stay in production decades really. I don't use Rust, or even C++, for that reason.
  - nicoburns 5y ago
    > On the other side, the fact that a language changes slowly it's something good in a way, it means that you don't have to continue to change the way you do things, and update older projects. Isn’t the point of Rust editions that this is also true for Rust? Don’t want to update to a new edition? Then… don’t. The old ones are maintained.
  - lfairy 5y ago
    > By the way the difference is that Rust is not a standard, thus is easier to evolve (the process is much shorter). Another thing is ABI. C and C++ are ABI-stable, which means that many historic mistakes (intmax_t, std::regex, polymorphic allocators) are impossible to fix. Rust only promises source compatibility, not ABI compatibility, so it has a lot more freedom to tweak its design. https://thephd.dev/binary-banshees-digital-demons-abi-c-c++-help-me-god-please https://thephd.dev/binary-banshees-digital-demons-abi-c-c++-...
EdSchouten 5y ago
Yes, in that in C++ it would break if one library used a modern language feature as part of its public header files.
- chc 5y ago
  Surely the difference there is that Rust doesn't have header files.
  - masklinn 5y ago
    Not necessarily, you could imagine a system where the header file specifies its edition.
    - cpeterso 5y ago
      A header file can check #if __STDC_VERSION__ or __cplusplus versions to make some code conditionally available for, say, C11 or C++11. #if __STDC_VERSION__ >= 201112L // C11 feature #endif #if __cplusplus >= 201103L // C++11 feature #endif
    - dralley 5y ago
      But don't header files generally get concatenated together due to the way "dumb" textual includes work?
- hiccuphippo 5y ago
  Can't you compile libraries to object files independently with whatever version of c++ they require and then link them together?
  - MaulingMonkey 5y ago
    The problem is a C++ library's headers must be compiled with the settings, context, and flags of every downstream thing that depends on them, rather than separately. Which isn't such a big problem if your C++ library exposes a minimal C-like API from it's headers, with most of the meat of the library hidden away in source files, but might be a very big problem if your C++ library is a miserable little pile of ~~secrets~~ templates, a la boost.

heftig 5y ago

If the application is using C++17, wouldn't the headers of the C++11 lib then be compiled as C++17, potentially breaking things? PS: The other way around is more obviously broken, with the C++17 lib headers getting compiled using C++11.

pjmlp 5y ago
Or you make use of the preprocessor or if constexpr, and then have the specific code for each language version.
- int_19h 5y ago
  The maintainers would have to do it for every new language version that breaks them, though. The edition system keeps old stuff working without any effort.
  - pjmlp 5y ago
    I don't want to start this discussion thread yet again, but I am a firm believer that edition system only appears to work right now because: 1 - Rust is still quite young and doesn't have 30 years of accumulated editions 2 - There is still only one major Rust compiler 3 - Editions are designed only to work when compiling the whole project, including 3rd party dependencies, from source code within a single build 4 - So far the editions don't have semantic breaking changes across editions, where behaviour changes across the edition border 5 - There is no plan to ever have editions work across ABIs So "The edition system keeps old stuff working without any effort." might not be true when Rust achieves an adoption scale similar to C and C++, in about 20 years, with several accumulated editions, and a couple of compilers in use. I might be proven wrong, but that is how I see it today.
    - tialaramex 5y ago
      > 1 - Rust is still quite young and doesn't have 30 years of accumulated editions Rust 1.0 was six years ago. When do we start the clock on six years of C++ evolving while also having "old stuff working without any effort" ? C++ 98 to C++ 03 was five years, and that introduces almost no features at all. C++ 11 was eight years later but it's notoriously incompatible to prior versions of C++. C++ 14 is only three years, C++ 17 has breaking changes, as does C++ 20... I think C++ 20 should have taken Epochs (yes even at considerable cost to other new features like Concepts) for this reason. I think ten years from now even if Rust has found it can't achieve everything it wants to via editions, this feature will be generally considered to be a good idea, like generics or string literals. Something you need a specific rationale for not including in your general purpose language. I might be proven wrong too about how far this can go, but I feel like the Rust 2018 and 2021 editions already prove the value of the idea.
      - pjmlp 5y ago
        For starters epochs could never work in ecosystems that value binary libraries, regardless of possible ABI issues, which is also a reason why Apple went to such an effort to define an ABI alongside language versions. Lets say you have a noexcept function compiled in C++20 that calls a throws() function, compiled in C++03, which actually ends up throwing, linked together. What is the runtime supposed to do now? To which semantics does it follow now? Call std::unexpected() as it is supposed to do pre-C++14 or call std::terminate() as it should do in C++20? What about the user defined handler that was configured for such scenario? Which of them gets called, or do both get called, in which order? Maybe I am doing it more complex that it is, but I see several scenarios from point of view where multiple compilers, binary libraries and semantic changes come into play, editions turn just into another way to define language versions, because they don't cover all possible uses cases how a language might evolve. Anyway, history will tell how things work out in the end.
        tialaramex 5y ago
        > For starters epochs could never work in ecosystems that value binary libraries Not solving everybody's problem is not the same thing as not solving anybody's problem. You present subtle considerations which, ignoring the fact that they assume Epochs don't exist in C++ 20 yet ask what Epochs should have done in C++ 20 - might have taken up committee time if Epochs had advanced, and which now can only be answered in the vaguest way, they should definitely have decided on a coherent strategy for resolving such problems. It is true that for any conceivable change, Hyrum's Law applies, and so it would apply to Epochs just as it does for Rust's editions. Mara's "competition" for writing Rust that gives different results when re-formatted provides examples of the sort of stuff Hyrum's Law invariably breaks. Nobody should be writing non-toy programs like that and in Rust it seems like nobody is. It's a sad fact that too often C++ programs are written in a very fragile way and many of them can and do break with the least provocation. You can't magically rewrite all those programs, but you can make fragile techniques like SFINAE unattractive to propagate into new programs, and I argue Epochs would have allowed C++ to begin the much harder part of Stroustrup's ever-evolving quest to ship a good programming language - not adding yet more kitchen sinks but removing parts of the language that in hindsight were a bad idea and revisiting old design decisions in the light of what has been learned.
- tialaramex 5y ago
  It's certainly true that if C++ library maintainers are up for the ever-growing maintenance burden, they can all individually deliver the same promise Rust gets out of the box. This is in practice what the maintainers of the three standard libraries have to do, perhaps one or more of them will offer their opinion about that experience?

orra 5y ago

I don't know what guarantees your typical C++ compiler gives you that those can link together? Regardless, at the very least, you would need to write the headers to be interoperable.

masklinn 5y ago

Aside from the header issue, it doesn't allow for or support backwards-incompatible changes. Because editions are opt-in (and library-level source metadata) the language itself can be modified in non-backwards-compatible ways. So for instance a C++ with editions could make ctors `explicit` by default, or it could entirely change the automatic member generation (by removing it for instance). As long as the ABI and API remain compatible, that's fine.

neandrake 5y ago
It’s probably worth noting that the backwards compatibility breaking changes I think only applies to the language and not the standard library. I’m unable to look up the details right now but I believe there’s a trait function defined in stdlib which is deprecated, supersede, but can’t be removed even as part of an Edition, or it would break older code.
- masklinn 5y ago
  That is correct, editions are mostly about language-level syntactic changes, the APIs have to be compatible between editions. The only place where that isn't the case is the standard prelude (the "builtins" you don't have to import).
  - tialaramex 5y ago
    The ability to replace the prelude does mean that you can imagine if somebody invents a much better thing than, say, Rust's iterators Rust 2050 can have a prelude that brings in std::better::Iterator instead of std::iter::Iterator and then most coders will end up using the new better iterators, just because that's the kind you get out of the box. Anybody who literally refers to std::iter::Iterator gets the old ones of course as does any library code from prior editions, but the documentation could lead those few people in the right direction. And presumably std::better::Iterator politely implements std::iter::IntoIterator because why not. I would be interested to understand if they're allowed to replace the macros. The standard macros aren't actually from the prelude, but instead if you aren't no_std you get all the standard macros anyway. Are they allowed to change those in a future edition? Or not?

pornel 5y ago

Yes, because C versions are frozen in time, and editions aren't. Today Rust added brand new features to Rust 2015 and Rust 2018, and will continue to expand them forever (every new feature lands in all editions whenever possible). Rust editions are closer to source code parsing modes. More like enabling trigraphs in C or "use strict" in JS. Additionally, textual header inclusion in C makes mixing versions tricky. Rust has properly isolated crates, and tracks edition per AST node (so that even cross-crate macros work correctly with mixed editions).

alerighi 5y ago
C compilers usually backport features in older standards as well (I know, they are breaking the standard). One example are C++ style comments, that are in the standard only from C99, but basically every compiler supports them even in ANSI/C90 mode. By the way the difference is that Rust is not a standard, thus is easier to evolve (the process is much shorter). On the other side, the fact that a language changes slowly it's something good in a way, it means that you don't have to continue to change the way you do things, and update older projects. That to everyone that has to maintain code for decades it's important. And every serious software project (not hobby stuff) does stay in production decades really. I don't use Rust, or even C++, for that reason.
- nicoburns 5y ago
  > On the other side, the fact that a language changes slowly it's something good in a way, it means that you don't have to continue to change the way you do things, and update older projects. Isn’t the point of Rust editions that this is also true for Rust? Don’t want to update to a new edition? Then… don’t. The old ones are maintained.
- lfairy 5y ago
  > By the way the difference is that Rust is not a standard, thus is easier to evolve (the process is much shorter). Another thing is ABI. C and C++ are ABI-stable, which means that many historic mistakes (intmax_t, std::regex, polymorphic allocators) are impossible to fix. Rust only promises source compatibility, not ABI compatibility, so it has a lot more freedom to tweak its design. https://thephd.dev/binary-banshees-digital-demons-abi-c-c++-help-me-god-please https://thephd.dev/binary-banshees-digital-demons-abi-c-c++-...

EdSchouten 5y ago

Yes, in that in C++ it would break if one library used a modern language feature as part of its public header files.

chc 5y ago
Surely the difference there is that Rust doesn't have header files.
- masklinn 5y ago
  Not necessarily, you could imagine a system where the header file specifies its edition.
  - cpeterso 5y ago
    A header file can check #if __STDC_VERSION__ or __cplusplus versions to make some code conditionally available for, say, C11 or C++11. #if __STDC_VERSION__ >= 201112L // C11 feature #endif #if __cplusplus >= 201103L // C++11 feature #endif
  - dralley 5y ago
    But don't header files generally get concatenated together due to the way "dumb" textual includes work?
hiccuphippo 5y ago
Can't you compile libraries to object files independently with whatever version of c++ they require and then link them together?
- MaulingMonkey 5y ago
  The problem is a C++ library's headers must be compiled with the settings, context, and flags of every downstream thing that depends on them, rather than separately. Which isn't such a big problem if your C++ library exposes a minimal C-like API from it's headers, with most of the meat of the library hidden away in source files, but might be a very big problem if your C++ library is a miserable little pile of ~~secrets~~ templates, a la boost.