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 &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));
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:

&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:

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

In addition, &T references implement Send if and only if T implements Sync.

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.

§Safety

For all types, T: ?Sized, and for all t: &T or t: &mut T, when such values cross an API boundary, the following invariants must generally be upheld:

  • t is aligned to align_of_val(t)
  • t is dereferenceable for size_of_val(t) many bytes

If t points at address a, being “dereferenceable” for N bytes means that the memory range [a, a + N) is all contained within a single allocated object.

For instance, this means that unsafe code in a safe function may assume these invariants are ensured of arguments passed by the caller, and it may assume that these invariants are ensured of return values from any safe functions it calls. In most cases, the inverse is also true: unsafe code must not violate these invariants when passing arguments to safe functions or returning values from safe functions; such violations may result in undefined behavior. Where exceptions to this latter requirement exist, they will be called out explicitly in documentation.

It is not decided yet whether unsafe code may violate these invariants temporarily on internal data. As a consequence, unsafe code which violates these invariants temporarily on internal data may become unsound in future versions of Rust depending on how this question is decided.

Trait Implementations§

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

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

This method tests for self and other values to be equal, and is used by ==.
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fn ne(&self, other: &&B) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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impl<A, B: ?Sized> PartialEq<&B> for &mut A
where A: PartialEq<B> + ?Sized,

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

This method tests for self and other values to be equal, and is used by ==.
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fn ne(&self, other: &&B) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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impl<A, B: ?Sized> PartialEq<&mut B> for &A
where A: PartialEq<B> + ?Sized,

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

This method tests for self and other values to be equal, and is used by ==.
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fn ne(&self, other: &&mut B) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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impl<A, B: ?Sized> PartialEq<&mut B> for &mut A
where A: PartialEq<B> + ?Sized,

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

This method tests for self and other values to be equal, and is used by ==.
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fn ne(&self, other: &&mut B) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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impl<A, B: ?Sized> PartialOrd<&B> for &A
where A: PartialOrd<B> + ?Sized,

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

This method returns an ordering between self and other values if one exists. Read more
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fn lt(&self, other: &&B) -> bool

This method tests less than (for self and other) and is used by the < operator. Read more
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fn le(&self, other: &&B) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator. Read more
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fn gt(&self, other: &&B) -> bool

This method tests greater than (for self and other) and is used by the > operator. Read more
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fn ge(&self, other: &&B) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator. Read more
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impl<A, B: ?Sized> PartialOrd<&mut B> for &mut A
where A: PartialOrd<B> + ?Sized,

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

This method returns an ordering between self and other values if one exists. Read more
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fn lt(&self, other: &&mut B) -> bool

This method tests less than (for self and other) and is used by the < operator. Read more
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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. Read more
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fn gt(&self, other: &&mut B) -> bool

This method tests greater than (for self and other) and is used by the > operator. Read more
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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. Read more
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impl<'a, 'b: 'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b T

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

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

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

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