Hacker News

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 10 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 9 hours ago [ - ]

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

zrm 6 hours ago [ - ]

That's assuming you were reading it without writing to it. There are three common cases when that isn't true.

The first is that you have a fixed buffer large enough for the maximum message size even though the typical ones aren't that big. You most often write 1% of the buffer and read it back, the other 99% is never accessed.

The second is that you always write the entire contents before reading it but the compiler may not be able to see that.

And the third is that you have a code path where that variable is simply not used.

You would then have the compiler emitting instructions to write zeros that are either overwritten before being read or are never read at all.

Moreover, zero initializing the data doesn't actually remove the bugs when that isn't the case. Consider the first case when you mess up. You have a fixed buffer used to store variable length messages. For the first message the buffer is now zeros instead of uninitialized, but for every subsequent message the remainder of the buffer still contains the remainder of the previous message and subjects you to information disclosure or data modification if you're reading back a different amount than was written in the associated call.

Now consider the second or third case. You unintentionally read from a variable before assigning to it. You get zeros instead of uninitialized memory, but if you weren't expecting zeros, well, the UID field is now 0.

cryptonector 7 hours ago [ - ]

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