"Undefined behavior" in a C++ context is a legalese feature of the ISO standard. The standard defines, very precisely, the behavior of a compliant C++ compiler. In places where behavior cannot be specified (for logical or practical reasons), the behavior is marked as "undefined".

In absence of a standard effectively every language construct is 'undefined behavior'.

Again, look at Python for a vivid example of this. (We were discussing just this is a neighboring thread: https://news.ycombinator.com/item?id=26826158)

I have zero reason to believe that Rust won't meet Python's fate.

The Rust people don't have a standard and don't understand why they need one; in fact, their lack of standards is somehow touted as a benefit. Apparently, people think that if there is no standard for "defined" and "undefined" behavior that everything is "defined" by default.

(They're absolutely wrong, of course; it's actually the opposite.)

What you seem to be describing is "unspecified behaviour" or "implementation defined behaviour". If your program contains undefined behaviour the compiler makes no guarantees about how your program will behave.

For more see: https://blog.regehr.org/archives/213 and https://raphlinus.github.io/programming/rust/2018/08/17/unde... .

If there is one implementation as the definition then by default all behaviour is defined by the implementation, and hence parent is correct.

No. If we accept your argument then this would be correct:

> In absence of a standard effectively every language construct is 'implementation defined behaviour'.

But the parent said:

> In absence of a standard effectively every language construct is 'undefined behavior'.

Undefined behaviour != implementation defined behaviour. Both of these things exist in C and are separate. You can rely on implementation defined behaviour giving some consistent result on a given, compiler and hardware. You cannot rely on the behaviour of your program if you hit undefined behaviour.

I didn't state 'implementation defined behavior' which encompasses more than the Rust implementation. The Rust implementation defines the behavior, for example signed int overflow.

The "Rust Book" says signed integer overflow to be two's complement in release builds and panic in debug builds: https://doc.rust-lang.org/book/ch03-02-data-types.html

That's clear enough for programmers to rely on. It is nonsense to argue that this is equivalent to C's "the compiler may do anything it wants" just because the "Rust Book" is not called the "Rust Standard" or not "formal" enough.

this magical thinking about standards documents is one of the primary incentives to not have one