logoalt Hacker News

gpderetta • last Tuesday at 2:24 PM • 1 reply • view on HN

bit_cast and reinterpret_cast do different things: one works at the value level, the second preserves address identity (and it is problematic from an aliasing point of view).

Not sure what any of this has to do with initialization though.

FWIW, the direct translation of your rust code is:

    constexpr char y = 2;
    constexpr bool x = std::bit_cast<bool>(y);
It fails on clang for y=2 and works for y=1, exactly like rust;

GCC produces UB for y=2, I don't know if it is a GCC bug or the standard actually allows this form of UB to be ignored at contexpr time.

What is the rust equivalent of reinterpret_cast and does it work at constexpr time?

edit: I guess it would be an unsafe dereference of a casted pointer. Does it propagate constants?

Replies

tialaramex • last Tuesday at 5:19 PM

Firstly, that's not a direct translation because you're making two variables and I made none at all. Rust's const is an actual constant, it's not an immutable variable. We have both, but they're different. The analogous Rust for your bit cast example would make two immutable variables that we promise have constant values, maybe:

    static y: u8 = 2;
    static x: bool = unsafe { core::mem::transmute(y) };
Of course this also won't compile, because the representation for 2 still isn't a boolean. If it did compile you'd also (by default) get angry warnings because it's bad style to give these lowercase names.

I also don't know if you found a GCC bug but it seems likely from your description. I can't see a way to have UB, a runtime phenomenon, at compile time in C++ as the committee imagines their language. Of course "UB? In my lexer?" is an example of how the ISO document doesn't understand intention, but I'd be surprised if the committee would resolve a DR with "That's fine, UB at compile time is intentional".

I understand that "these are different things" followed by bafflegab is how C++ gets here but the whole point of this sub-thread is that Rust didn't do that, so in Rust these aren't "different things". They're both transmutation, they don't emit CPU instructions because they happen in the type system and the type system evaporates at runtime.

So this is an impedance mismatch, you've got Roman numerals and you can't see why metric units are a good idea, and I've got the positional notation and so it's obvious to me. I am not going to be able to explain why this is a good idea in your notation, the brilliance vanishes during translation.

➕ show 1 reply