reference - Rust (original) (raw)

Primitive Type reference

1.0.0 ·

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References, &T and &mut T.

A reference represents a borrow of some owned value. You can get one by using the & or &mutoperators on a value, or by using a ref or[ref](../std/keyword.ref.html) [mut](../std/keyword.mut.html) 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](primitive.bool.html "primitive bool") can only point to an allocation containing the integer values 1(true) or 0 (false), but creating a &[bool](primitive.bool.html "primitive bool") that points to an allocation containing the value 3 causes undefined behaviour. In fact, [Option](option/enum.Option.html "enum std::option::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, whilePartialEq 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));

Run

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:

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](marker/trait.Sync.html "trait std:📑:Sync"))
Sync

&mut T references get all of the above except ToSocketAddrs, plus the following, if Timplements 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.