Proposal: @offsetOf instead of @byteOffsetOf

[hvenev]

I believe that the builtin used for getting the byte offset of a struct field should be called @offsetOf rather than @byteOffsetOf. Here are a few reasons why:

• It is consistent with @sizeOf / @bitSizeOf. @alignOf is also not called @byteAlignOf.
• offsetOf is common in other programming languages. The C macro is called offsetof. There are also a few Rust libraries that provide macros named offset_of.
• Bit-packed structs are rare, especially dealing with pointers to individual fields.

I also believe @offsetOf should fail when the struct field is not aligned to a byte boundary. The most obvious reason is to prevent bugs where @byteOffsetOf is used without checking if the offset is actually a whole number of bytes. Having this check as a part of @offsetOf will make subtly incorrect code invalid and will make correct code cleaner.

Using @byteOffsetOf on non-byte-aligned bitfields is rarely useful, and I'd imagine that all such uses will be made more clear by doing @bitOffetOf(T, field) / 8 instead. Even if we keep the current @byteOffsetOf for backwards compatibility, I believe it should be phased out.

A quick grep through the standard library revealed that the only uses of @byteOffsetOf are assertions about the layout of extern structs. This can also be done with the proposed @offsetOf with the added advantage that if we were to make the structs packed, we'd automatically make sure that the fields are byte-aligned.

[faraazahmad]

I'm not too familiar with Zig yet so forgive my ignorance, but what was the reason @byteOffsetOf and @offsetOf were named so?

[hvenev]

@offsetOf does not exist.

[hvenev]

I've implemented the non-breaking version of this proposal at #7903.

[hvenev]

I've updated the proposal.

[andrewrk]

See also #8642