3 ms·

The idiomatic way to do this in Rust is to use functions like .to_le_bytes(), so you have the u32 (or whatever) on one end and raw bytes (something like [u8; 4]

The idiomatic way to do this in Rust is to use functions like .to_le_bytes(), so you have the u32 (or whatever) on one end and raw bytes (something like [u8; 4]) on the other. It can get slightly tedious if you're doing it by hand, but it's impossible to accidentally forget. If you're doing this kind of thing at scale, like dealing with TrueType fonts (another bastion of big-endian), it's common to reach for derive macros, which automate a great deal of the tedium.

nly 5y ago
Who decides what methods to add to the bytes type/abstraction? If I have a 3 byte big endian integer can I access it easily in rust without resorting to shifts? In C++ I could probably create a fairly convincing big_uint24_t type and use it in a packed struct and there would be no inconsistencies with how it's used with respect to the more common varieties
- raphlinus 5y ago
  In Rust, [u8; N] and &[u8] are both primitive types, and not abstractions. It's possible to create an abstraction around either (the former even more so now with const generics), but that's not necessary. It's also possible to use "extension traits" to add methods, even to existing and built-in types[1]. I'm not sure about a 3 byte big endian integer. I mean, that's going to compile down to some combination of shifting and masking operations anyway, isn't it? I suspect that if you have some oddball binary format that needs, this it will be possible to write some code to marshal it, that compiles down to the best possible asm. Godbolt is your friend here :) [1]: https://rust-lang.github.io/rfcs/0445-extension-trait-conventions.html https://rust-lang.github.io/rfcs/0445-extension-trait-conven...
  - nly 5y ago
    I agree then that in Rust you could make something consistent. I think there's no need for explicit shifts. You need to memcpy anyway to deal with alignment issues, so you may as well just copy in to the last 3 bytes of a zero-initialized, big endian, 32bit uint. https://gcc.godbolt.org/z/jEnsW8WfE https://gcc.godbolt.org/z/jEnsW8WfE
    - raphlinus 5y ago
      That's just constant folding. Here's what it looks like when you actually need to go to memory: https://gcc.godbolt.org/z/9qGqh6M1E https://gcc.godbolt.org/z/9qGqh6M1E And I think we're on the same page, it should be possible to get similar results in Rust.

nly 5y ago

Who decides what methods to add to the bytes type/abstraction? If I have a 3 byte big endian integer can I access it easily in rust without resorting to shifts? In C++ I could probably create a fairly convincing big_uint24_t type and use it in a packed struct and there would be no inconsistencies with how it's used with respect to the more common varieties

raphlinus 5y ago
In Rust, [u8; N] and &[u8] are both primitive types, and not abstractions. It's possible to create an abstraction around either (the former even more so now with const generics), but that's not necessary. It's also possible to use "extension traits" to add methods, even to existing and built-in types[1]. I'm not sure about a 3 byte big endian integer. I mean, that's going to compile down to some combination of shifting and masking operations anyway, isn't it? I suspect that if you have some oddball binary format that needs, this it will be possible to write some code to marshal it, that compiles down to the best possible asm. Godbolt is your friend here :) [1]: https://rust-lang.github.io/rfcs/0445-extension-trait-conventions.html https://rust-lang.github.io/rfcs/0445-extension-trait-conven...
- nly 5y ago
  I agree then that in Rust you could make something consistent. I think there's no need for explicit shifts. You need to memcpy anyway to deal with alignment issues, so you may as well just copy in to the last 3 bytes of a zero-initialized, big endian, 32bit uint. https://gcc.godbolt.org/z/jEnsW8WfE https://gcc.godbolt.org/z/jEnsW8WfE
  - raphlinus 5y ago
    That's just constant folding. Here's what it looks like when you actually need to go to memory: https://gcc.godbolt.org/z/9qGqh6M1E https://gcc.godbolt.org/z/9qGqh6M1E And I think we're on the same page, it should be possible to get similar results in Rust.