Trait rkyv::ArchiveUnsized

source ·

pub trait ArchiveUnsized: Pointee {
    type Archived: ArchivePointee + Portable + ?Sized;

    // Required method
    fn archived_metadata(&self) -> ArchivedMetadata<Self>;
}

Expand description

A counterpart of Archive that’s suitable for unsized types.

Unlike Archive, types that implement ArchiveUnsized must be serialized separately from their owning object. For example, whereas an i32 might be laid out as part of a larger struct, a Box<i32> would serialize the i32 somewhere in the archive and the Box would point to it as part of the larger struct. Because of this, the equivalent Resolver type for ArchiveUnsized is always a usize representing the position of the serialized value.

ArchiveUnsized is automatically implemented for all types that implement Archive. Nothing special needs to be done to use them with types like Box, Rc, and Arc. It is also already implemented for slices and string slices, and the rkyv_dyn crate can be used to archive trait objects. Other unsized types must manually implement ArchiveUnsized.

§Examples

This example shows how to manually implement ArchiveUnsized for an unsized type. Special care must be taken to ensure that the types are laid out correctly.

use core::ops::{Deref, DerefMut};

use ptr_meta::Pointee;
use rkyv::{
    access_unchecked,
    primitive::ArchivedUsize,
    rancor::{Error, Fallible},
    ser::{Positional, Writer, WriterExt as _},
    to_bytes,
    traits::ArchivePointee,
    Archive, ArchiveUnsized, Archived, ArchivedMetadata, Portable, RelPtr,
    Serialize, SerializeUnsized,
};

// We're going to be dealing mostly with blocks that have a trailing slice
#[derive(Portable)]
#[repr(C)]
pub struct Block<H, T: ?Sized> {
    head: H,
    tail: T,
}

unsafe impl<H, T> Pointee for Block<H, [T]> {
    type Metadata = <[T] as Pointee>::Metadata;
}

// ArchivePointee is automatically derived for sized types because pointers
// to sized types don't need to store any extra information. Because we're
// making an unsized block, we need to define what metadata gets stored with
// our data pointer.
impl<H, T> ArchivePointee for Block<H, [T]> {
    // This is the extra data that needs to get stored for blocks with
    // trailing slices
    type ArchivedMetadata = <[T] as ArchivePointee>::ArchivedMetadata;

    // We need to be able to turn our archived metadata into regular
    // metadata for our type
    fn pointer_metadata(
        metadata: &Self::ArchivedMetadata,
    ) -> <Self as Pointee>::Metadata {
        metadata.to_native() as usize
    }
}

// We're implementing ArchiveUnsized for just Block<H, [T]>. We can still
// implement Archive for blocks with sized tails and they won't conflict.
impl<H: Archive, T: Archive> ArchiveUnsized for Block<H, [T]> {
    // We'll reuse our block type as our archived type.
    type Archived = Block<Archived<H>, [Archived<T>]>;

    // Here's where we make the metadata for our archived type.
    fn archived_metadata(&self) -> ArchivedMetadata<Self> {
        // Because the metadata for our `ArchivedBlock` is the metadata of
        // the trailing slice, we just need to return that archived
        // metadata.
        self.tail.archived_metadata()
    }
}

// The bounds we use on our serializer type indicate that we need basic
// serializer capabilities, and then whatever capabilities our head and tail
// types need to serialize themselves.
impl<H, T, S> SerializeUnsized<S> for Block<H, [T]>
where
    H: Serialize<S>,
    T: Serialize<S>,
    S: Fallible + Writer + ?Sized,
{
    // This is where we construct our unsized type in the serializer
    fn serialize_unsized(
        &self,
        serializer: &mut S,
    ) -> Result<usize, S::Error> {
        // First, we serialize the head and all the tails. This will make
        // sure that when we finally build our block, we don't accidentally
        // mess up the structure with serialized dependencies.
        let head_resolver = self.head.serialize(serializer)?;
        let mut resolvers = Vec::new();
        for tail in self.tail.iter() {
            resolvers.push(tail.serialize(serializer)?);
        }
        // Now we align our serializer for our archived type and resolve it.
        // We can't align for unsized types so we treat the trailing slice
        // like an array of 0 length for now.
        let result = serializer
            .align_for::<Block<Archived<H>, [Archived<T>; 0]>>()?;
        unsafe {
            serializer.resolve_aligned(&self.head, head_resolver)?;
        }
        serializer.align_for::<Archived<T>>()?;
        for (item, resolver) in self.tail.iter().zip(resolvers.drain(..)) {
            unsafe {
                serializer.resolve_aligned(item, resolver)?;
            }
        }
        Ok(result)
    }
}

let value = Box::new(Block {
    head: "Numbers 1-4".to_string(),
    tail: [1, 2, 3, 4],
});

// We have a Box<Block<String, [i32; 4]>> but we want to it to be a
// Box<Block<String, [i32]>>, so we need manually "unsize" the pointer.
let ptr = Box::into_raw(value);
let unsized_ptr = ptr_meta::from_raw_parts_mut::<Block<String, [i32]>>(
    ptr.cast::<()>(),
    4,
);
let unsized_value = unsafe { Box::from_raw(unsized_ptr) };

let bytes = to_bytes::<Error>(&unsized_value).unwrap();

let archived = unsafe {
    access_unchecked::<Archived<Box<Block<String, [i32]>>>>(&bytes)
};
assert_eq!(archived.head, "Numbers 1-4");
assert_eq!(archived.tail.len(), 4);
assert_eq!(archived.tail, [1, 2, 3, 4]);