Thread

1. wahern · 1789747013 · focus · HN ↗

   > When both conditions are met, the loop body is replaced with a call to std::this_thread::yield(). This gives execution of the loop the forward-progress semantics it previously lacked.

   That's the epitome of the hidden code downside that Linus and many others dislike about C++. For constructors and destructors it's somewhat unavoidable and not so random, though Rust does better at limiting the blast radius of non-local code, at least in the drop case.

   If they didn't want to adopt the C11 rule, the C++ committee should've explored a rule that required the compiler to emit a diagnostic or error for trivial loops (whether as defined by C11 or otherwise), requiring the programmer to explicitly insert ::yield or similar. No hidden code, and less opportunity for the compiler to do surprising things.

   The C committee has been rigorously enumerating UB cases in the standard and addressing each case in turn, often by requiring a diagnostic, error, or by turning it into implemention defined behavior. But inserting code like that would be unthinkable.

   1. rfgplk · 1789750722 · focus · HN ↗

      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.

      1. mitxela · 1789777701 · focus · HN ↗

         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.

         1. dahart · 1789779523 · focus · HN ↗

            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.)

            1. mitxela · 1789785386 · focus · HN ↗

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

               1. cryptonector · 1789793319 · focus · HN ↗

                  That's not reason enough to have the compiler initialize.

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