pub trait Archive {
type Archived: Portable;
type Resolver;
const COPY_OPTIMIZATION: CopyOptimization<Self> = _;
// Required method
fn resolve(&self, resolver: Self::Resolver, out: Place<Self::Archived>);
}Expand description
A type that can be used without deserializing.
Archive is one of three basic traits used to work with zero-copy data and
controls the layout of the data in its archived zero-copy representation.
The Serialize trait helps transform types into that representation, and
the Deserialize trait helps transform types back out.
Types that implement Archive must have a well-defined archived size.
Unsized types can be supported using the ArchiveUnsized trait, along
with SerializeUnsized and DeserializeUnsized.
Archiving is done depth-first, writing any data owned by a type before writing the data for the type itself. The type must be able to create the archived type from only its own data and its resolver.
Archived data is always treated as if it is tree-shaped, with the root
owning its direct descendents and so on. Data that is not tree-shaped can be
supported using special serializer and deserializer bounds (see
ArchivedRc for example). In a buffer of
serialized data, objects are laid out in reverse order. This means that
the root object is located near the end of the buffer and leaf objects are
located near the beginning.
§Examples
Most of the time, #[derive(Archive)] will create an acceptable
implementation. You can use the #[rkyv(...)] attribute to control how the
implementation is generated. See the Archive
derive macro for more details.
use rkyv::{deserialize, rancor::Error, Archive, Deserialize, Serialize};
#[derive(Archive, Deserialize, Serialize, Debug, PartialEq)]
#[rkyv(
// This will generate a PartialEq impl between our unarchived
// and archived types
compare(PartialEq),
// Derives can be passed through to the generated type:
derive(Debug),
)]
struct Test {
int: u8,
string: String,
option: Option<Vec<i32>>,
}
fn main() {
let value = Test {
int: 42,
string: "hello world".to_string(),
option: Some(vec![1, 2, 3, 4]),
};
// Serializing is as easy as a single function call
let _bytes = rkyv::to_bytes::<Error>(&value).unwrap();
// Or you can customize your serialization for better performance or control
// over resource usage
use rkyv::{api::high::to_bytes_with_alloc, ser::allocator::Arena};
let mut arena = Arena::new();
let bytes =
to_bytes_with_alloc::<_, Error>(&value, arena.acquire()).unwrap();
// You can use the safe API for fast zero-copy deserialization
let archived = rkyv::access::<ArchivedTest, Error>(&bytes[..]).unwrap();
assert_eq!(archived, &value);
// Or you can use the unsafe API for maximum performance
let archived =
unsafe { rkyv::access_unchecked::<ArchivedTest>(&bytes[..]) };
assert_eq!(archived, &value);
// And you can always deserialize back to the original type
let deserialized = deserialize::<Test, Error>(archived).unwrap();
assert_eq!(deserialized, value);
}Note: the safe API requires the bytecheck feature.
Many of the core and standard library types already have Archive
implementations available, but you may need to implement Archive for your
own types in some cases the derive macro cannot handle.
In this example, we add our own wrapper that serializes a &'static str as
if it’s owned. Normally you can lean on the archived version of String to
do most of the work, or use the Inline to do
exactly this. This example does everything to demonstrate how to implement
Archive for your own types.
use core::{slice, str};
use rkyv::{
access_unchecked,
rancor::{Error, Fallible},
ser::Writer,
to_bytes,
Archive, ArchiveUnsized, Archived, Portable, RelPtr, Serialize,
SerializeUnsized, munge::munge, Place,
};
struct OwnedStr {
inner: &'static str,
}
#[derive(Portable)]
#[repr(transparent)]
struct ArchivedOwnedStr {
// This will be a relative pointer to our string
ptr: RelPtr<str>,
}
impl ArchivedOwnedStr {
// This will help us get the bytes of our type as a str again.
fn as_str(&self) -> &str {
unsafe {
// The as_ptr() function of RelPtr will get a pointer the str
&*self.ptr.as_ptr()
}
}
}
struct OwnedStrResolver {
// This will be the position that the bytes of our string are stored at.
// We'll use this to resolve the relative pointer of our
// ArchivedOwnedStr.
pos: usize,
}
// The Archive implementation defines the archived version of our type and
// determines how to turn the resolver into the archived form. The Serialize
// implementations determine how to make a resolver from the original value.
impl Archive for OwnedStr {
type Archived = ArchivedOwnedStr;
// This is the resolver we can create our Archived version from.
type Resolver = OwnedStrResolver;
// The resolve function consumes the resolver and produces the archived
// value at the given position.
fn resolve(
&self,
resolver: Self::Resolver,
out: Place<Self::Archived>,
) {
munge!(let ArchivedOwnedStr { ptr } = out);
RelPtr::emplace_unsized(
resolver.pos,
self.inner.archived_metadata(),
ptr,
);
}
}
// We restrict our serializer types with Writer because we need its
// capabilities to serialize the inner string. For other types, we might
// need more or less restrictive bounds on the type of S.
impl<S: Fallible + Writer + ?Sized> Serialize<S> for OwnedStr {
fn serialize(
&self,
serializer: &mut S,
) -> Result<Self::Resolver, S::Error> {
// This is where we want to write the bytes of our string and return
// a resolver that knows where those bytes were written.
// We also need to serialize the metadata for our str.
Ok(OwnedStrResolver {
pos: self.inner.serialize_unsized(serializer)?,
})
}
}
const STR_VAL: &'static str = "I'm in an OwnedStr!";
let value = OwnedStr { inner: STR_VAL };
// It works!
let buf = to_bytes::<Error>(&value).expect("failed to serialize");
let archived =
unsafe { access_unchecked::<ArchivedOwnedStr>(buf.as_ref()) };
// Let's make sure our data got written correctly
assert_eq!(archived.as_str(), STR_VAL);Provided Associated Constants§
Sourceconst COPY_OPTIMIZATION: CopyOptimization<Self> = _
const COPY_OPTIMIZATION: CopyOptimization<Self> = _
An optimization flag that allows the bytes of this type to be copied
directly to a writer instead of calling serialize.
This optimization is disabled by default. To enable this optimization,
you must unsafely attest that Self is trivially copyable using
CopyOptimization::enable or CopyOptimization::enable_if.
Required Associated Types§
Required Methods§
Sourcefn resolve(&self, resolver: Self::Resolver, out: Place<Self::Archived>)
fn resolve(&self, resolver: Self::Resolver, out: Place<Self::Archived>)
Creates the archived version of this value at the given position and writes it to the given output.
The output should be initialized field-by-field rather than by writing a whole struct. Performing a typed copy will mark all of the padding bytes as uninitialized, but they must remain set to the value they currently have. This prevents leaking uninitialized memory to the final archive.
Dyn Compatibility§
This trait is not dyn compatible.
In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.