Trait AsRef

1.0.0 (const: unstable) · Source

pub trait AsRef<T>where
    T: ?Sized,{
    // Required method
    fn as_ref(&self) -> &T;
}

Expand description

Used to do a cheap reference-to-reference conversion.

This trait is similar to AsMut which is used for converting between mutable references. If you need to do a costly conversion it is better to implement From with type &T or write a custom function.

§Relation to `Borrow`

AsRef has the same signature as Borrow, but Borrow is different in a few aspects:

Unlike AsRef, Borrow has a blanket impl for any T, and can be used to accept either a reference or a value. (See also note on AsRef’s reflexibility below.)
Borrow also requires that Hash, Eq and Ord for a borrowed value are equivalent to those of the owned value. For this reason, if you want to borrow only a single field of a struct you can implement AsRef, but not Borrow.

Note: This trait must not fail. If the conversion can fail, use a dedicated method which returns an Option<T> or a Result<T, E>.

§Generic Implementations

AsRef auto-dereferences if the inner type is a reference or a mutable reference (e.g.: foo.as_ref() will work the same if foo has type &mut Foo or &&mut Foo).

Note that due to historic reasons, the above currently does not hold generally for all dereferenceable types, e.g. foo.as_ref() will not work the same as Box::new(foo).as_ref(). Instead, many smart pointers provide an as_ref implementation which simply returns a reference to the pointed-to value (but do not perform a cheap reference-to-reference conversion for that value). However, AsRef::as_ref should not be used for the sole purpose of dereferencing; instead ‘Deref coercion’ can be used:

let x = Box::new(5i32);
// Avoid this:
// let y: &i32 = x.as_ref();
// Better just write:
let y: &i32 = &x;

Types which implement Deref should consider implementing AsRef<T> as follows:

impl<T> AsRef<T> for SomeType
where
    T: ?Sized,
    <SomeType as Deref>::Target: AsRef<T>,
{
    fn as_ref(&self) -> &T {
        self.deref().as_ref()
    }
}

§Reflexivity

Ideally, AsRef would be reflexive, i.e. there would be an impl<T: ?Sized> AsRef<T> for T with as_ref simply returning its argument unchanged. Such a blanket implementation is currently not provided due to technical restrictions of Rust’s type system (it would be overlapping with another existing blanket implementation for &T where T: AsRef which allows AsRef to auto-dereference, see “Generic Implementations” above).

A trivial implementation of AsRef<T> for T must be added explicitly for a particular type T where needed or desired. Note, however, that not all types from std contain such an implementation, and those cannot be added by external code due to orphan rules.

§Examples

By using trait bounds we can accept arguments of different types as long as they can be converted to the specified type T.

For example: By creating a generic function that takes an AsRef<str> we express that we want to accept all references that can be converted to &str as an argument. Since both String and &str implement AsRef<str> we can accept both as input argument.

fn is_hello<T: AsRef<str>>(s: T) {
   assert_eq!("hello", s.as_ref());
}

let s = "hello";
is_hello(s);

let s = "hello".to_string();
is_hello(s);

Required Methods§

1.0.0 · Source

fn as_ref(&self) -> &T

Converts this type into a shared reference of the (usually inferred) input type.

AsRef

Trait AsRef Copy item path

§Relation to Borrow

§Generic Implementations

§Reflexivity

§Examples

Required Methods§

fn as_ref(&self) -> &T

Implementors§

impl AsRef<str> for InvalidTransaction

impl AsRef<str> for UnknownTransaction

impl AsRef<str> for Method

impl AsRef<str> for str

impl AsRef<str> for String

impl AsRef<str> for Ia5String

impl AsRef<str> for PrintableString

impl AsRef<str> for TeletexString

impl AsRef<str> for Buffer

impl AsRef<str> for Field

impl AsRef<ByteStr> for str

impl AsRef<ByteStr> for ByteStr

impl AsRef<ByteStr> for ByteString

impl AsRef<CryptoBytes<subsoil::::core::ecdsa::GenericPublic::{constant#0}, (PublicTag, EcdsaTag)>> for topsoil_core::runtime::app_crypto::ecdsa::AppPublic

impl AsRef<CryptoBytes<subsoil::::core::ecdsa::GenericPublic::{constant#0}, (PublicTag, EcdsaTag)>> for subsoil::consensus::beefy::ecdsa_crypto::Public

impl AsRef<CryptoBytes<subsoil::::core::ecdsa::GenericSignature::{constant#0}, (SignatureTag, CryptoBytes<subsoil::::core::ecdsa::GenericPublic::{constant#0}, (PublicTag, EcdsaTag)>)>> for topsoil_core::runtime::app_crypto::ecdsa::AppProofOfPossession

impl AsRef<CryptoBytes<subsoil::::core::ecdsa::GenericSignature::{constant#0}, (SignatureTag, CryptoBytes<subsoil::::core::ecdsa::GenericPublic::{constant#0}, (PublicTag, EcdsaTag)>)>> for topsoil_core::runtime::app_crypto::ecdsa::AppSignature

impl AsRef<CryptoBytes<subsoil::::core::ecdsa::GenericSignature::{constant#0}, (SignatureTag, CryptoBytes<subsoil::::core::ecdsa::GenericPublic::{constant#0}, (PublicTag, EcdsaTag)>)>> for subsoil::consensus::beefy::ecdsa_crypto::ProofOfPossession

impl AsRef<CryptoBytes<subsoil::::core::ecdsa::GenericSignature::{constant#0}, (SignatureTag, CryptoBytes<subsoil::::core::ecdsa::GenericPublic::{constant#0}, (PublicTag, EcdsaTag)>)>> for subsoil::consensus::beefy::ecdsa_crypto::Signature

impl AsRef<CryptoBytes<subsoil::::core::ed25519::Public::{constant#0}, (PublicTag, Ed25519Tag)>> for topsoil_core::runtime::app_crypto::ed25519::AppPublic

impl AsRef<CryptoBytes<subsoil::::core::ed25519::Signature::{constant#0}, (SignatureTag, Ed25519Tag)>> for topsoil_core::runtime::app_crypto::ed25519::AppProofOfPossession

impl AsRef<CryptoBytes<subsoil::::core::ed25519::Signature::{constant#0}, (SignatureTag, Ed25519Tag)>> for topsoil_core::runtime::app_crypto::ed25519::AppSignature

impl AsRef<CryptoBytes<subsoil::::core::sr25519::Signature::{constant#0}, (SignatureTag, Sr25519Tag)>> for topsoil_core::runtime::app_crypto::sr25519::AppProofOfPossession

impl AsRef<CryptoBytes<subsoil::::core::sr25519::Signature::{constant#0}, (SignatureTag, Sr25519Tag)>> for topsoil_core::runtime::app_crypto::sr25519::AppSignature

impl AsRef<Pair> for topsoil_core::runtime::app_crypto::ed25519::AppPair

impl AsRef<Pair> for topsoil_core::runtime::app_crypto::sr25519::AppPair

impl AsRef<VrfTranscript> for VrfSignData

impl AsRef<CStr> for CString

impl AsRef<CStr> for CStr

impl AsRef<LocalWaker> for Waker

impl AsRef<OsStr> for Component<'_>

impl AsRef<OsStr> for str

impl AsRef<OsStr> for String

impl AsRef<OsStr> for OsStr

impl AsRef<OsStr> for OsString

impl AsRef<OsStr> for Components<'_>

impl AsRef<OsStr> for std::path::Iter<'_>

impl AsRef<OsStr> for Path

impl AsRef<OsStr> for PathBuf

impl AsRef<Path> for Cow<'_, OsStr>

impl AsRef<Path> for Component<'_>

impl AsRef<Path> for str

impl AsRef<Path> for String

impl AsRef<Path> for OsStr

impl AsRef<Path> for OsString

impl AsRef<Path> for Components<'_>

impl AsRef<Path> for std::path::Iter<'_>

impl AsRef<Path> for Path

impl AsRef<Path> for PathBuf

impl AsRef<VerifyingKey> for ed25519_dalek::signing::SigningKey

impl AsRef<Scalar> for Scalar

impl AsRef<Keypair> for topsoil_core::runtime::testing::sr25519::Pair

impl AsRef<[u8; 8]> for bitcoin_hashes::siphash24::Hash

impl AsRef<[u8; 20]> for bitcoin_hashes::hash160::Hash

impl AsRef<[u8; 20]> for bitcoin_hashes::ripemd160::Hash

impl AsRef<[u8; 20]> for bitcoin_hashes::sha1::Hash

impl AsRef<[u8; 32]> for AccountId32

impl AsRef<[u8; 32]> for bitcoin_hashes::sha256::Hash

impl AsRef<[u8; 32]> for bitcoin_hashes::sha256d::Hash

impl AsRef<[u8; 32]> for bitcoin_hashes::sha512_256::Hash

impl AsRef<[u8; 32]> for SecretKey

impl AsRef<[u8; 32]> for Message

impl AsRef<[u8; 48]> for bitcoin_hashes::sha384::Hash

impl AsRef<[u8; 64]> for bitcoin_hashes::sha512::Hash

impl AsRef<[u8; 64]> for ElligatorSwift

impl AsRef<[u8; 64]> for secp256k1_sys::PublicKey

impl AsRef<[u8; 64]> for secp256k1_sys::Signature

impl AsRef<[u8; 64]> for XOnlyPublicKey

impl AsRef<[u8; 64]> for secp256k1::schnorr::Signature

impl AsRef<[u8; 65]> for RecoverableSignature

impl AsRef<[u8; 96]> for Keypair

Trait AsRef

§Relation to `Borrow`