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,Borrowhas a blanket impl for anyT, and can be used to accept either a reference or a value. (See also note onAsRef’s reflexibility below.) Borrowalso requires thatHash,EqandOrdfor 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 implementAsRef, but notBorrow.
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<U> 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);