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 an hour ago | parent [-]

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 an hour ago | parent [-]

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.