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Comment by rfgplk

17 hours ago

It's catastrophic actually. Like disastrously catastrophic. It started with C++20 mostly, and has only kept getting worse from then. See zero initializing variables by default (WHY?) compare/meta including half the STL and HARDCODING those symbols, std::initializer_list being in the std namespace (if you don't include <initializer_list> you literally can't use it, and there is no such thing as a __initializer_list or some internal symbol), the entire coroutine library where you MUST provide coroutine_handle, noop_coroutine, suspends et al (coroutines aren't that bad because they're not necessarily spaghetti).

<meta> is the single WORST OFFENDER, where they hardcode std::vector (literally std::vector in the std namespace) std::ranges std::allocator.

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rfgplk

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mitxela  10 hours ago

I can't find anything saying variables are zero initialized by default in C++20. But the reason to do so is obvious: many bugs are caused by the lack of this, and as long as you can opt out with "= void" or something, it's not violating C++ core principles.

  • dahart  9 hours ago

    They were saying the problematic philosophy started in C++ 20, not the variable initialization rule.

    Yes the reason is obvious, but it’s neither simple nor black and white. One huge problem is that this can cause serious performance regressions, and you have to change your code to opt out, e.g. add “[[indeterminate]]”. There are many, many cases in high performance computing where the intended & desired behavior is don’t touch my variables until I fill them.

    This is changing C++ core principles, there’s a new designation for the state of a variable: erroneous. It’s also subtle and weird, because you can still have well-defined behavior even with erroneous state. It does seem like this might be an experiment though, I don’t think this is the end of the story. (It seems they’re already talking some redesign of this idea.)

    • ack_complete  9 hours ago

      What I'm most annoyed at with the variable initialization change is that:

        - It's potentially a performance change in every single function, especially ones that have sizable fixed-size buffers
  - If you have regressions you have to spray [[indeterminate]] everywhere, because there is no coarser way of suppressing it.
  - While the language says unrecognized attributes are ignored, compilers frequently warn on unrecognized attributes. Clang, for instance, currently warns on [[indeterminate]].
  - There is no defined macro name for backwards compatibility.

      Which means that libraries are going have to all declare their own macros for [[indeterminate]] and pepper their code with it.

    • mitxela  7 hours ago

      Uninitialized variables were already UB to read, because some architectures have trap representations, even for integers. Every register on Itanium has one.

      2 replies →

  • nullc  4 hours ago

    Zero initializing also hides bugs.

    Say you have some code that should not be reading the initial state and is buggy if it does. Without zero-init, valgrind and msan will give you an immediate and false positive message that your code is wrong-- or forget dynamic analysis: the compiler can often statically tell you that the code will use an uninitialized variable. Zero initialize it and you lose that signal.

aw1621107  15 hours ago

> See zero initializing variables by default

Strictly speaking the standard only requires some pattern that is not tied to program state. Zero works for that, but so do other static patterns like 0xABAB... or the like.

> (WHY?)

The motivation section of the corresponding paper [0] might be interesting. tl;dr: it lets wrong code be wrong without suffering from (all) the consequences of full-blown UB.

[0]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p27...

  • dahart  9 hours ago

    I’d guess the concern is performance, not what initializer value is used. And performance is a valid concern that is discussed in the proposal, and a reason there’s an escape hatch. Still, it might cause some confusion.

  • fc417fc802  12 hours ago

    In other words, it's a sane default that you can opt out of on a case by case basis which is the way it should have been all along.