Trait AsRef

1.0.0 · 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.

Trait AsRefCopy item path

§Relation to Borrow

§Generic Implementations

§Reflexivity

§Examples

Required Methods§

fn as_ref(&self) -> &T

Implementors§

impl AsRef<str> for Signal

impl AsRef<str> for str

impl AsRef<str> for OpensslString

impl AsRef<str> for OpensslStringRef

impl AsRef<str> for DnsName<'_>

impl AsRef<str> for InternalString

impl AsRef<str> for Field

impl AsRef<str> for Str

impl AsRef<str> for Id

impl AsRef<str> for Authority

impl AsRef<str> for Scheme

impl AsRef<str> for Mime

impl AsRef<str> for HeaderName

impl AsRef<str> for Method

impl AsRef<str> for String

impl AsRef<str> for Url

impl AsRef<BStr> for str

impl AsRef<BStr> for bstr::bstr::BStr

impl AsRef<BStr> for BString

impl AsRef<BStr> for [u8]

impl AsRef<FormatSizeOptions> for FormatSizeOptions

impl AsRef<LanguageIdentifier> for LanguageIdentifier

impl AsRef<LanguageIdentifier> for Locale

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

impl AsRef<Asn1StringRef> for Asn1String

impl AsRef<Asn1TimeRef> for Asn1Time

impl AsRef<BigNumContextRef> for BigNumContext

impl AsRef<BigNumRef> for BigNum

impl AsRef<CipherCtxRef> for CipherCtx

impl AsRef<CmsContentInfoRef> for CmsContentInfo

impl AsRef<ConfRef> for Conf

impl AsRef<DsaSigRef> for DsaSig

impl AsRef<EcGroupRef> for EcGroup

impl AsRef<EcPointRef> for EcPoint

impl AsRef<EcdsaSigRef> for EcdsaSig

impl AsRef<LibCtxRef> for LibCtx

impl AsRef<MdCtxRef> for MdCtx

impl AsRef<OcspBasicResponseRef> for OcspBasicResponse

impl AsRef<OcspCertIdRef> for OcspCertId

impl AsRef<OcspOneReqRef> for OcspOneReq

impl AsRef<OcspRequestRef> for OcspRequest

impl AsRef<OcspResponseRef> for OcspResponse

impl AsRef<Pkcs7Ref> for Pkcs7

impl AsRef<Pkcs7SignedRef> for Pkcs7Signed

impl AsRef<Pkcs7SignerInfoRef> for Pkcs7SignerInfo

impl AsRef<Pkcs12Ref> for Pkcs12

impl AsRef<ProviderRef> for Provider

impl AsRef<SrtpProtectionProfileRef> for SrtpProtectionProfile

impl AsRef<SslContextRef> for SslContext

impl AsRef<SslRef> for Ssl

impl AsRef<SslSessionRef> for SslSession

impl AsRef<OpensslStringRef> for OpensslString

impl AsRef<X509StoreBuilderRef> for X509StoreBuilder

impl AsRef<X509StoreRef> for X509Store

impl AsRef<AccessDescriptionRef> for AccessDescription

impl AsRef<DistPointNameRef> for DistPointName

impl AsRef<DistPointRef> for DistPoint

impl AsRef<GeneralNameRef> for GeneralName

impl AsRef<X509AlgorithmRef> for X509Algorithm

impl AsRef<X509CrlRef> for X509Crl

impl AsRef<X509ExtensionRef> for X509Extension

impl AsRef<X509NameEntryRef> for X509NameEntry

impl AsRef<X509NameRef> for X509Name

impl AsRef<X509ObjectRef> for X509Object

impl AsRef<X509Ref> for X509

impl AsRef<X509Ref> for X509Ref

impl AsRef<X509ReqRef> for X509Req

Trait AsRef

§Relation to `Borrow`