Proper way to offset `*const anyopaque`?

Help
1

I have a pointer to whatever, *const anyopaque. I know that it points to a struct, and I have an offset of a field in this struct. What’s the proper way to get a pointer to field?

I can do

const field_ptr: *const anyopaque = @ptrFromInt(
    @intFromPtr(value) + offset,
);

but that round-trips a pointer through integer, which, in general, can’t always work (eg, on CHERI), and have poor aliasing implications. Is there a more precise way to offset anyopaque?

2

No idea what the implications of this are wrt aliasing, but you could try using pointer arithmetic:

const field_ptr: *const anyopaque = @as([*]const u8, @ptrCast(value)) + offset;
2 Likes
3

:exploding_head:

I didn’t realize Zig supports pointer arithmetic for [*] pointers, thanks!

4

It didn’t in the past, you’re not remembering something wrong, just an outdated truth :^)

5

Out of curiosity, why do you have an offset to the field, and not the knowledge of which field it is?

6

Context is Things Zig comptime Won't Do

I am building a print function that can print any Zig type not via monomoprising for every type, but by working of *const anyopaque and RTTI information, which includes field names&offsets