Format

Types which derive Archive generate an archived version of the type where:

  • Member types are replaced with their archived counterparts
  • Enums have #[repr(N)] where N is u8, u16, u32, u64, or u128, choosing the smallest possible type that can represent all of the variants.

For example, a struct like:


#![allow(unused)]
fn main() {
struct Example {
    a: u32,
    b: String,
    c: Box<(u32, String)>,
}
}

Would have the archived counterpart:


#![allow(unused)]
fn main() {
struct ArchivedExample {
    a: u32,
    b: ArchivedString,
    c: ArchivedBox<(u32, ArchivedString)>,
}
}

With the strict feature, these structs are additionally annotated with #[repr(C)] for guaranteed portability and stability.

In most cases, the strict feature will not be necessary and can reduce the space efficiency of archived types. Make sure you understand your use case carefully and read the crate documentation for details on the strict feature.

rkyv provides Archive implementations for common core and std types by default. In general they follow the same format as derived implementations, but may differ in some cases. For example, ArchivedString performs a small string optimization which helps reduce memory use.

Object order

rkyv lays out subobjects in depth-first order from the leaves to the root. This means that the root object is stored at the end of the buffer, not the beginning. For example, this tree:

  a
 / \
b   c
   / \
  d   e

would be laid out like this in the buffer:

b d e c a

from this serialization order:

a -> b
a -> c -> d
a -> c -> e
a -> c
a

This deterministic layout means that you don't need to store the position of the root object in most cases. As long as your buffer ends right at the end of your root object, you can use archived_root with your buffer.