Primitive Type reference

References, both shared and mutable.

A reference represents a borrow of some owned value. You can get one by using the & or &mut operators on a value, or by using a ref or ref mut pattern.

For those familiar with pointers, a reference is just a pointer that is assumed to be aligned, not null, and pointing to memory containing a valid value of T - for example, &bool can only point to an allocation containing the integer values 1 (true) or 0 (false), but creating a &bool that points to an allocation containing the value 3 causes undefined behaviour. In fact, Option<&T> has the same memory representation as a nullable but aligned pointer, and can be passed across FFI boundaries as such.

In most cases, references can be used much like the original value. Field access, method calling, and indexing work the same (save for mutability rules, of course). In addition, the comparison operators transparently defer to the referent’s implementation, allowing references to be compared the same as owned values.

References have a lifetime attached to them, which represents the scope for which the borrow is valid. A lifetime is said to “outlive” another one if its representative scope is as long or longer than the other. The 'static lifetime is the longest lifetime, which represents the total life of the program. For example, string literals have a 'static lifetime because the text data is embedded into the binary of the program, rather than in an allocation that needs to be dynamically managed.

&mut T references can be freely coerced into &T references with the same referent type, and references with longer lifetimes can be freely coerced into references with shorter ones.

Reference equality by address, instead of comparing the values pointed to, is accomplished via implicit reference-pointer coercion and raw pointer equality via ptr::eq, while PartialEq compares values.

use std::ptr;

let five = 5;
let other_five = 5;
let five_ref = &five;
let same_five_ref = &five;
let other_five_ref = &other_five;

assert!(five_ref == same_five_ref);
assert!(five_ref == other_five_ref);

assert!(ptr::eq(five_ref, same_five_ref));
assert!(!ptr::eq(five_ref, other_five_ref));

For more information on how to use references, see the book’s section on “References and Borrowing”.

Trait implementations

The following traits are implemented for all &T, regardless of the type of its referent:

• Copy
• Clone (Note that this will not defer to T’s Clone implementation if it exists!)
• Deref
• Borrow
• fmt::Pointer

&mut T references get all of the above except Copy and Clone (to prevent creating multiple simultaneous mutable borrows), plus the following, regardless of the type of its referent:

• DerefMut
• BorrowMut

The following traits are implemented on &T references if the underlying T also implements that trait:

• All the traits in std::fmt except fmt::Pointer (which is implemented regardless of the type of its referent) and fmt::Write
• PartialOrd
• Ord
• PartialEq
• Eq
• AsRef
• Fn (in addition, &T references get FnMut and FnOnce if T: Fn)
• Hash
• ToSocketAddrs
• Send (&T references also require T: Sync)

&mut T references get all of the above except ToSocketAddrs, plus the following, if T implements that trait:

• AsMut
• FnMut (in addition, &mut T references get FnOnce if T: FnMut)
• fmt::Write
• Iterator
• DoubleEndedIterator
• ExactSizeIterator
• FusedIterator
• TrustedLen
• io::Write
• Read
• Seek
• BufRead

Note that due to method call deref coercion, simply calling a trait method will act like they work on references as well as they do on owned values! The implementations described here are meant for generic contexts, where the final type T is a type parameter or otherwise not locally known.

Trait Implementations

impl<'a, T> Extend<&'a T> for BinaryHeap<T> where
    T: 'a + Ord + Copy, 

fn extend<I>(&mut self, iter: I) where
    I: IntoIterator<Item = &'a T>, 

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, &'a T)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<'a, T> Extend<&'a T> for LinkedList<T> where
    T: 'a + Copy, 

fn extend<I>(&mut self, iter: I) where
    I: IntoIterator<Item = &'a T>, 

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, &'a T)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<'a, T, A> Extend<&'a T> for VecDeque<T, A> where
    T: 'a + Copy,
    A: Allocator, 

fn extend<I>(&mut self, iter: I) where
    I: IntoIterator<Item = &'a T>, 

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, &T)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<'a, T, A> Extend<&'a T> for Vec<T, A> where
    T: 'a + Copy,
    A: 'a + Allocator, 

Extend implementation that copies elements out of references before pushing them onto the Vec.

This implementation is specialized for slice iterators, where it uses copy_from_slice to append the entire slice at once.

fn extend<I>(&mut self, iter: I) where
    I: IntoIterator<Item = &'a T>, 

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, &'a T)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<'a, T> Extend<&'a T> for BTreeSet<T> where
    T: 'a + Ord + Copy, 

fn extend<I>(&mut self, iter: I) where
    I: IntoIterator<Item = &'a T>, 

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, &'a T)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<'a, T, S> Extend<&'a T> for HashSet<T, S> where
    T: 'a + Eq + Hash + Copy,
    S: BuildHasher, 

fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I)

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, item: &'a T)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<'_, T> From<&'_ T> for NonNull<T> where
    T: ?Sized, 

fn from(reference: &T) -> NonNull<T>

Converts a &T to a NonNull<T>.

This conversion is safe and infallible since references cannot be null.

impl<T: ?Sized + AsRef<OsStr>> From<&'_ T> for OsString

fn from(s: &T) -> OsString

Copies any value implementing AsRef<OsStr> into a newly allocated OsString.

impl<T: ?Sized + AsRef<OsStr>> From<&'_ T> for PathBuf

fn from(s: &T) -> PathBuf

Converts a borrowed OsStr to a PathBuf.

Allocates a PathBuf and copies the data into it.

impl<'_, T> From<&'_ mut T> for NonNull<T> where
    T: ?Sized, 

fn from(reference: &mut T) -> NonNull<T>

Converts a &mut T to a NonNull<T>.

This conversion is safe and infallible since references cannot be null.

impl<'_, K, Q, V> Index<&'_ Q> for BTreeMap<K, V> where
    K: Borrow<Q> + Ord,
    Q: Ord + ?Sized, 

fn index(&self, key: &Q) -> &V

Returns a reference to the value corresponding to the supplied key.

Panics

Panics if the key is not present in the BTreeMap.

type Output = V

The returned type after indexing.

impl<K, Q: ?Sized, V, S> Index<&'_ Q> for HashMap<K, V, S> where
    K: Eq + Hash + Borrow<Q>,
    Q: Eq + Hash,
    S: BuildHasher, 

fn index(&self, key: &Q) -> &V

Returns a reference to the value corresponding to the supplied key.

Panics

Panics if the key is not present in the HashMap.

type Output = V

The returned type after indexing.

impl<'_, T, V> Join<&'_ T> for [V] where
    T: Clone,
    V: Borrow<[T]>, 

type Output = Vec<T, Global>

🔬 This is a nightly-only experimental API. (slice_concat_trait #27747)

The resulting type after concatenation

fn join(slice: &[V], sep: &T) -> Vec<T, Global>

🔬 This is a nightly-only experimental API. (slice_concat_trait #27747)

Implementation of [T]::join

impl<'_, '_, A, B> PartialEq<&'_ B> for &'_ mut A where
    A: PartialEq<B> + ?Sized,
    B: ?Sized, 

fn eq(&self, other: &&B) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &&B) -> bool

This method tests for !=.

impl<'_, '_, A, B> PartialEq<&'_ B> for &'_ A where
    A: PartialEq<B> + ?Sized,
    B: ?Sized, 

fn eq(&self, other: &&B) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &&B) -> bool

This method tests for !=.

impl<'_, '_, A, B> PartialEq<&'_ mut B> for &'_ mut A where
    A: PartialEq<B> + ?Sized,
    B: ?Sized, 

fn eq(&self, other: &&mut B) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &&mut B) -> bool

This method tests for !=.

impl<'_, '_, A, B> PartialEq<&'_ mut B> for &'_ A where
    A: PartialEq<B> + ?Sized,
    B: ?Sized, 

fn eq(&self, other: &&mut B) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &&mut B) -> bool

This method tests for !=.

impl<'_, '_, A, B> PartialOrd<&'_ B> for &'_ A where
    A: PartialOrd<B> + ?Sized,
    B: ?Sized, 

fn partial_cmp(&self, other: &&B) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &&B) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &&B) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &&B) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &&B) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<'_, '_, A, B> PartialOrd<&'_ mut B> for &'_ mut A where
    A: PartialOrd<B> + ?Sized,
    B: ?Sized, 

fn partial_cmp(&self, other: &&mut B) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &&mut B) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &&mut B) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &&mut B) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &&mut B) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<'a, 'b, T, U> CoerceUnsized<&'a U> for &'b mut T where
    'b: 'a,
    T: Unsize<U> + ?Sized,
    U: ?Sized, 

impl<'a, 'b, T, U> CoerceUnsized<&'a U> for &'b T where
    'b: 'a,
    T: Unsize<U> + ?Sized,
    U: ?Sized, 

impl<'a, T, U> CoerceUnsized<&'a mut U> for &'a mut T where
    T: Unsize<U> + ?Sized,
    U: ?Sized, 

impl<'a, T, U> DispatchFromDyn<&'a U> for &'a T where
    T: Unsize<U> + ?Sized,
    U: ?Sized, 

impl<'a, T, U> DispatchFromDyn<&'a mut U> for &'a mut T where
    T: Unsize<U> + ?Sized,
    U: ?Sized,