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 DecimalTypeVariant

impl AsRef<str> for IntegerTypeVariant

impl AsRef<str> for Signal

impl AsRef<str> for str

impl AsRef<str> for String

impl AsRef<str> for Graphemes

impl AsRef<str> for der::asn1::ia5_string::allocation::Ia5String

impl AsRef<str> for der::asn1::printable_string::allocation::PrintableString

impl AsRef<str> for der::asn1::teletex_string::allocation::TeletexString

impl AsRef<str> for HeaderName

impl AsRef<str> for Method

impl AsRef<str> for Authority

impl AsRef<str> for Scheme

impl AsRef<str> for Mime

impl AsRef<str> for OpensslString

impl AsRef<str> for OpensslStringRef

impl AsRef<str> for rcgen::string_types::Ia5String

impl AsRef<str> for rcgen::string_types::PrintableString

impl AsRef<str> for rcgen::string_types::TeletexString

impl AsRef<str> for DnsName<'_>

impl AsRef<str> for SmolStr

impl AsRef<str> for Field

impl AsRef<str> for Url

impl AsRef<CStr> for CStr

impl AsRef<CStr> for CString

impl AsRef<LocalWaker> for Waker

impl AsRef<ByteStr> for str

impl AsRef<ByteStr> for ByteString

impl AsRef<ByteStr> for ByteStr

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<OsStr> for Intern<str>

impl AsRef<OsStr> for Intern<Path>

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<Path> for Intern<str>

impl AsRef<Path> for Intern<OsStr>

impl AsRef<Graphemes> for Graphemes

impl AsRef<sigset_t> for SigSet

impl AsRef<aiocb> for AioFsync

impl AsRef<timespec> for TimeSpec

impl AsRef<sockaddr_alg> for AlgAddr

impl AsRef<sockaddr_nl> for NetlinkAddr

impl AsRef<sockaddr_vm> for VsockAddr

impl AsRef<sockaddr_in6> for SockaddrIn6

impl AsRef<sockaddr_in> for SockaddrIn

impl AsRef<sockaddr_ll> for LinkAddr

impl AsRef<sockaddr_un> for UnixAddr

impl AsRef<rusage> for Usage

impl AsRef<timeval> for TimeVal

impl AsRef<Asn1BitStringRef> for Asn1BitString

impl AsRef<Asn1EnumeratedRef> for Asn1Enumerated

impl AsRef<Asn1GeneralizedTimeRef> for Asn1GeneralizedTime

impl AsRef<Asn1IntegerRef> for Asn1Integer

impl AsRef<Asn1ObjectRef> for Asn1Object

impl AsRef<Asn1OctetStringRef> for Asn1OctetString

Trait AsRef

§Relation to `Borrow`