Macro bytemuck::offset_of

macro_rules! offset_of {
    ($instance:expr, $Type:path, $field:tt) => { ... };
    ($Type:path, $field:tt) => { ... };
}

Find the offset in bytes of the given $field of $Type. Requires an already initialized $instance value to work with.

This is similar to the macro from memoffset, however it uses no unsafe code.

This macro has a 3-argument and 2-argument version.

• In the 3-arg version you specify an instance of the type, the type itself, and the field name.
• In the 2-arg version the macro will call the default method to make a temporary instance of the type for you.

The output of this macro is the byte offset of the field (as a usize). The calculations of the macro are fixed across the entire program, but if the type used is repr(Rust) then they're not fixed across compilations or compilers.

CAUTION: It is unsound to use this macro with a repr(packed) type. Currently this will give a warning, and in the future it will become a hard error.

• See rust-lang/rust#27060 for more info and for status updates.
• Once this issue is resolved, a future version of this crate will use raw_ref to correct the issue. For the duration of the 1.x version of this crate it will be an on-by-default cargo feature (to maintain minimum rust version support).

Examples

3-arg Usage

// enums can't derive default, and for this example we don't pick one
enum MyExampleEnum {
  A, B, C,
}

// so now our struct here doesn't have Default
#[repr(C)]
struct MyNotDefaultType {
  pub counter: i32,
  pub some_field: MyExampleEnum,
}

// but we provide an instance of the type and it's all good.
let val = MyNotDefaultType { counter: 5, some_field: MyExampleEnum::A };
assert_eq!(offset_of!(val, MyNotDefaultType, some_field), 4);

2-arg Usage

#[derive(Default)]
#[repr(C)]
struct Vertex {
  pub loc: [f32; 3],
  pub color: [f32; 3],
}
// if the type impls Default the macro can make its own default instance.
assert_eq!(offset_of!(Vertex, loc), 0);
assert_eq!(offset_of!(Vertex, color), 12);