Trait AsRef

1.55.0 · 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.

Implementors§

1.0.0 · Source§

impl AsRef<str> for Field

1.0.0 · Source§

Trait AsRefCopy item path

§Relation to Borrow

§Generic Implementations

§Reflexivity

§Examples

Required Methods§

fn as_ref(&self) -> &T

Implementors§

impl AsRef<str> for str

impl AsRef<str> for PyBackedStr

impl AsRef<str> for ArchivedString

impl AsRef<str> for Field

impl AsRef<str> for devela::all::String

impl AsRef<ContentStyle> for ContentStyle

impl AsRef<Collator> for Collator

impl AsRef<DateTimeFormat> for DateTimeFormat

impl AsRef<NumberFormat> for NumberFormat

impl AsRef<PluralRules> for PluralRules

impl AsRef<RelativeTimeFormat> for RelativeTimeFormat

impl AsRef<CompileError> for CompileError

impl AsRef<Exception> for Exception

impl AsRef<Global> for Global

impl AsRef<Instance> for Instance

impl AsRef<LinkError> for LinkError

impl AsRef<Memory> for Memory

impl AsRef<Module> for Module

impl AsRef<RuntimeError> for RuntimeError

impl AsRef<Table> for Table

impl AsRef<Tag> for Tag

impl AsRef<Array> for devela::_dep::js_sys::Array

impl AsRef<ArrayBuffer> for ArrayBuffer

impl AsRef<AsyncIterator> for AsyncIterator

impl AsRef<BigInt64Array> for BigInt64Array

impl AsRef<BigInt> for BigInt

impl AsRef<BigUint64Array> for BigUint64Array

impl AsRef<Boolean> for Boolean

impl AsRef<DataView> for DataView

impl AsRef<Date> for Date

impl AsRef<Error> for CompileError

impl AsRef<Error> for LinkError

impl AsRef<Error> for RuntimeError

impl AsRef<Error> for Error

impl AsRef<Error> for EvalError

impl AsRef<Error> for RangeError

impl AsRef<Error> for ReferenceError

impl AsRef<Error> for SyntaxError

impl AsRef<Error> for TypeError

impl AsRef<Error> for UriError

impl AsRef<EvalError> for EvalError

impl AsRef<Float32Array> for Float32Array

impl AsRef<Float64Array> for Float64Array

impl AsRef<Function> for Function

impl AsRef<Generator> for Generator

impl AsRef<Int8Array> for Int8Array

impl AsRef<Int16Array> for Int16Array

impl AsRef<Int32Array> for Int32Array

impl AsRef<Iterator> for Iterator

impl AsRef<IteratorNext> for IteratorNext

impl AsRef<JsString> for JsString

impl AsRef<Map> for Map

impl AsRef<Number> for Number

impl AsRef<Object> for Collator

impl AsRef<Object> for DateTimeFormat

impl AsRef<Object> for NumberFormat

impl AsRef<Object> for PluralRules

impl AsRef<Object> for RelativeTimeFormat

impl AsRef<Object> for Exception

impl AsRef<Object> for Global

impl AsRef<Object> for Instance

impl AsRef<Object> for Memory

impl AsRef<Object> for Module

impl AsRef<Object> for Table

impl AsRef<Object> for Tag

impl AsRef<Object> for devela::_dep::js_sys::Array

impl AsRef<Object> for ArrayBuffer

impl AsRef<Object> for BigInt64Array

impl AsRef<Object> for BigInt

impl AsRef<Object> for BigUint64Array

impl AsRef<Object> for Boolean

impl AsRef<Object> for DataView

Trait AsRef

§Relation to `Borrow`