# Trait core::convert::AsRef

1.0.0 · source ·

    pub trait AsRef<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;

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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()
        }
    }

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## 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);

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## Required Methods§

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#### fn as_ref(&self) -> &T

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

## Implementors§

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### impl AsRef<str> for str

1.7.0 · source§

### impl AsRef<CStr> for CStr

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### impl AsRef<[u8]> for str

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### impl<T> AsRef<[T]> for [T]

1.13.0 · source§

### impl<T> AsRef<[T]> for Iter<'_, T>

1.53.0 · source§

### impl<T> AsRef<[T]> for IterMut<'_, T>

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### impl<T, U: ?Sized> AsRef<U> for &T
where T: AsRef<U> + ?Sized,

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### impl<T, U: ?Sized> AsRef<U> for &mut T
where T: AsRef<U> + ?Sized,

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### impl<T, const N: usize> AsRef<[T; N]> for Simd<T, N>
where LaneCount<N>: SupportedLaneCount, T: SimdElement,

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### impl<T, const N: usize> AsRef<[T]> for [T; N]

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### impl<T, const N: usize> AsRef<[T]> for Simd<T, N>
where LaneCount<N>: SupportedLaneCount, T: SimdElement,