Enum alloc::borrow::Cow

pub enum Cow<'a, B>
where
    B: ToOwned + ?Sized + 'a,
{
    Borrowed(&'a B),
    Owned(<B as ToOwned>::Owned),
}

A clone-on-write smart pointer.

The type Cow is a smart pointer providing clone-on-write functionality: it can enclose and provide immutable access to borrowed data, and clone the data lazily when mutation or ownership is required. The type is designed to work with general borrowed data via the Borrow trait.

Cow implements Deref, which means that you can call non-mutating methods directly on the data it encloses. If mutation is desired, to_mut will obtain a mutable reference to an owned value, cloning if necessary.

If you need reference-counting pointers, note that Rc::make_mut and Arc::make_mut can provide clone-on-write functionality as well.

Examples

use std::borrow::Cow;

fn abs_all(input: &mut Cow<'_, [i32]>) {
    for i in 0..input.len() {
        let v = input[i];
        if v < 0 {
            // Clones into a vector if not already owned.
            input.to_mut()[i] = -v;
        }
    }
}

// No clone occurs because `input` doesn't need to be mutated.
let slice = [0, 1, 2];
let mut input = Cow::from(&slice[..]);
abs_all(&mut input);

// Clone occurs because `input` needs to be mutated.
let slice = [-1, 0, 1];
let mut input = Cow::from(&slice[..]);
abs_all(&mut input);

// No clone occurs because `input` is already owned.
let mut input = Cow::from(vec![-1, 0, 1]);
abs_all(&mut input);

Another example showing how to keep Cow in a struct:

use std::borrow::Cow;

struct Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
    values: Cow<'a, [X]>,
}

impl<'a, X: Clone + 'a> Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
    fn new(v: Cow<'a, [X]>) -> Self {
        Items { values: v }
    }
}

// Creates a container from borrowed values of a slice
let readonly = [1, 2];
let borrowed = Items::new((&readonly[..]).into());
match borrowed {
    Items { values: Cow::Borrowed(b) } => println!("borrowed {b:?}"),
    _ => panic!("expect borrowed value"),
}

let mut clone_on_write = borrowed;
// Mutates the data from slice into owned vec and pushes a new value on top
clone_on_write.values.to_mut().push(3);
println!("clone_on_write = {:?}", clone_on_write.values);

// The data was mutated. Let's check it out.
match clone_on_write {
    Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
    _ => panic!("expect owned data"),
}

Variants

Borrowed(&'a B)

Borrowed data.

Owned(<B as ToOwned>::Owned)

Owned data.

Implementations

impl<B: ?Sized + ToOwned> Cow<'_, B>

pub fn is_borrowed(&self) -> bool

🔬This is a nightly-only experimental API. (cow_is_borrowed #65143)

Returns true if the data is borrowed, i.e. if to_mut would require additional work.

Examples

#![feature(cow_is_borrowed)]
use std::borrow::Cow;

let cow = Cow::Borrowed("moo");
assert!(cow.is_borrowed());

let bull: Cow<'_, str> = Cow::Owned("...moo?".to_string());
assert!(!bull.is_borrowed());

pub fn is_owned(&self) -> bool

🔬This is a nightly-only experimental API. (cow_is_borrowed #65143)

Returns true if the data is owned, i.e. if to_mut would be a no-op.

Examples

#![feature(cow_is_borrowed)]
use std::borrow::Cow;

let cow: Cow<'_, str> = Cow::Owned("moo".to_string());
assert!(cow.is_owned());

let bull = Cow::Borrowed("...moo?");
assert!(!bull.is_owned());

pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned

Acquires a mutable reference to the owned form of the data.

Clones the data if it is not already owned.

Examples

use std::borrow::Cow;

let mut cow = Cow::Borrowed("foo");
cow.to_mut().make_ascii_uppercase();

assert_eq!(
  cow,
  Cow::Owned(String::from("FOO")) as Cow<'_, str>
);

pub fn into_owned(self) -> <B as ToOwned>::Owned

Extracts the owned data.

Clones the data if it is not already owned.

Examples

Calling into_owned on a Cow::Borrowed returns a clone of the borrowed data:

use std::borrow::Cow;

let s = "Hello world!";
let cow = Cow::Borrowed(s);

assert_eq!(
  cow.into_owned(),
  String::from(s)
);

Calling into_owned on a Cow::Owned returns the owned data. The data is moved out of the Cow without being cloned.

use std::borrow::Cow;

let s = "Hello world!";
let cow: Cow<'_, str> = Cow::Owned(String::from(s));

assert_eq!(
  cow.into_owned(),
  String::from(s)
);

Trait Implementations

impl<'a> Add<&'a str> for Cow<'a, str>

type Output = Cow<'a, str>

The resulting type after applying the + operator.

fn add(self, rhs: &'a str) -> Self::Output

Performs the + operation.

impl<'a> Add for Cow<'a, str>

type Output = Cow<'a, str>

The resulting type after applying the + operator.

fn add(self, rhs: Cow<'a, str>) -> Self::Output

Performs the + operation.

impl<'a> AddAssign<&'a str> for Cow<'a, str>

fn add_assign(&mut self, rhs: &'a str)

Performs the += operation.

impl<'a> AddAssign for Cow<'a, str>

fn add_assign(&mut self, rhs: Cow<'a, str>)

Performs the += operation.

impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T>

fn as_ref(&self) -> &T

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

impl<'a, B> Borrow<B> for Cow<'a, B>

where B: ToOwned + ?Sized,

fn borrow(&self) -> &B

Immutably borrows from an owned value.

impl<B: ?Sized + ToOwned> Clone for Cow<'_, B>

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<B> Debug for Cow<'_, B>

where B: Debug + ToOwned<Owned: Debug> + ?Sized,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<B> Default for Cow<'_, B>

where B: ToOwned<Owned: Default> + ?Sized,

fn default() -> Self

Creates an owned Cow<’a, B> with the default value for the contained owned value.

impl<B: ?Sized + ToOwned> Deref for Cow<'_, B>

where B::Owned: Borrow<B>,

type Target = B

The resulting type after dereferencing.

fn deref(&self) -> &B

Dereferences the value.

impl<B> Display for Cow<'_, B>

where B: Display + ToOwned<Owned: Display> + ?Sized,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'a> Extend<Cow<'a, str>> for String

fn extend<I: IntoIterator<Item = Cow<'a, str>>>(&mut self, iter: I)

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, s: Cow<'a, str>)

🔬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: Clone> From<&'a [T]> for Cow<'a, [T]>

fn from(s: &'a [T]) -> Cow<'a, [T]>

Creates a Borrowed variant of Cow from a slice.

This conversion does not allocate or clone the data.

impl<'a, T: Clone, const N: usize> From<&'a [T; N]> for Cow<'a, [T]>

fn from(s: &'a [T; N]) -> Cow<'a, [T]>

Creates a Borrowed variant of Cow from a reference to an array.

This conversion does not allocate or clone the data.

impl<'a> From<&'a CStr> for Cow<'a, CStr>

fn from(s: &'a CStr) -> Cow<'a, CStr>

Converts a CStr into a borrowed Cow without copying or allocating.

impl<'a> From<&'a CString> for Cow<'a, CStr>

fn from(s: &'a CString) -> Cow<'a, CStr>

Converts a &CString into a borrowed Cow without copying or allocating.

impl<'a> From<&'a String> for Cow<'a, str>

fn from(s: &'a String) -> Cow<'a, str>

Converts a String reference into a Borrowed variant. No heap allocation is performed, and the string is not copied.

Example

let s = "eggplant".to_string();
assert_eq!(Cow::from(&s), Cow::Borrowed("eggplant"));

impl<'a, T: Clone> From<&'a Vec<T>> for Cow<'a, [T]>

fn from(v: &'a Vec<T>) -> Cow<'a, [T]>

Creates a Borrowed variant of Cow from a reference to Vec.

This conversion does not allocate or clone the data.

impl<'a> From<&'a str> for Cow<'a, str>

fn from(s: &'a str) -> Cow<'a, str>

Converts a string slice into a Borrowed variant. No heap allocation is performed, and the string is not copied.

Example

assert_eq!(Cow::from("eggplant"), Cow::Borrowed("eggplant"));

impl<'a> From<CString> for Cow<'a, CStr>

fn from(s: CString) -> Cow<'a, CStr>

Converts a CString into an owned Cow without copying or allocating.

impl<T: Clone> From<Cow<'_, [T]>> for Box<[T]>

fn from(cow: Cow<'_, [T]>) -> Box<[T]>

Converts a Cow<'_, [T]> into a Box<[T]>

When cow is the Cow::Borrowed variant, this conversion allocates on the heap and copies the underlying slice. Otherwise, it will try to reuse the owned Vec’s allocation.

impl From<Cow<'_, CStr>> for Box<CStr>

fn from(cow: Cow<'_, CStr>) -> Box<CStr>

Converts a Cow<'a, CStr> into a Box<CStr>, by copying the contents if they are borrowed.

impl From<Cow<'_, str>> for Box<str>

fn from(cow: Cow<'_, str>) -> Box<str>

Converts a Cow<'_, str> into a Box<str>

When cow is the Cow::Borrowed variant, this conversion allocates on the heap and copies the underlying str. Otherwise, it will try to reuse the owned String’s allocation.

Examples

use std::borrow::Cow;

let unboxed = Cow::Borrowed("hello");
let boxed: Box<str> = Box::from(unboxed);
println!("{boxed}");

let unboxed = Cow::Owned("hello".to_string());
let boxed: Box<str> = Box::from(unboxed);
println!("{boxed}");

impl<'a, T> From<Cow<'a, [T]>> for Vec<T>

where [T]: ToOwned<Owned = Vec<T>>,

fn from(s: Cow<'a, [T]>) -> Vec<T>

Convert a clone-on-write slice into a vector.

If s already owns a Vec<T>, it will be returned directly. If s is borrowing a slice, a new Vec<T> will be allocated and filled by cloning s’s items into it.

Examples

let o: Cow<'_, [i32]> = Cow::Owned(vec![1, 2, 3]);
let b: Cow<'_, [i32]> = Cow::Borrowed(&[1, 2, 3]);
assert_eq!(Vec::from(o), Vec::from(b));

impl<'a, B> From<Cow<'a, B>> for Arc<B>

where B: ToOwned + ?Sized, Arc<B>: From<&'a B> + From<B::Owned>,

fn from(cow: Cow<'a, B>) -> Arc<B>

Create an atomically reference-counted pointer from a clone-on-write pointer by copying its content.

Example

let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
let shared: Arc<str> = Arc::from(cow);
assert_eq!("eggplant", &shared[..]);

impl<'a, B> From<Cow<'a, B>> for Rc<B>

where B: ToOwned + ?Sized, Rc<B>: From<&'a B> + From<B::Owned>,

fn from(cow: Cow<'a, B>) -> Rc<B>

Create a reference-counted pointer from a clone-on-write pointer by copying its content.

Example

let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
let shared: Rc<str> = Rc::from(cow);
assert_eq!("eggplant", &shared[..]);

impl<'a> From<Cow<'a, CStr>> for CString

fn from(s: Cow<'a, CStr>) -> Self

Converts a Cow<'a, CStr> into a CString, by copying the contents if they are borrowed.

impl<'a> From<Cow<'a, str>> for String

fn from(s: Cow<'a, str>) -> String

Converts a clone-on-write string to an owned instance of String.

This extracts the owned string, clones the string if it is not already owned.

Example

// If the string is not owned...
let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
// It will allocate on the heap and copy the string.
let owned: String = String::from(cow);
assert_eq!(&owned[..], "eggplant");

impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + 'a>

fn from(err: Cow<'b, str>) -> Box<dyn Error + 'a>

Converts a Cow into a box of dyn Error.

Examples

use std::error::Error;
use std::mem;
use std::borrow::Cow;

let a_cow_str_error = Cow::from("a str error");
let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))

impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a>

fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a>

Converts a Cow into a box of dyn Error + Send + Sync.

Examples

use std::error::Error;
use std::mem;
use std::borrow::Cow;

let a_cow_str_error = Cow::from("a str error");
let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
assert!(
    mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))

impl<'a> From<String> for Cow<'a, str>

fn from(s: String) -> Cow<'a, str>

Converts a String into an Owned variant. No heap allocation is performed, and the string is not copied.

Example

let s = "eggplant".to_string();
let s2 = "eggplant".to_string();
assert_eq!(Cow::from(s), Cow::<'static, str>::Owned(s2));

impl<'a, T: Clone> From<Vec<T>> for Cow<'a, [T]>

fn from(v: Vec<T>) -> Cow<'a, [T]>

Creates an Owned variant of Cow from an owned instance of Vec.

This conversion does not allocate or clone the data.

impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str>

fn from_iter<I: IntoIterator<Item = &'b str>>(it: I) -> Cow<'a, str>

Creates a value from an iterator.

impl<'a> FromIterator<Cow<'a, str>> for String

fn from_iter<I: IntoIterator<Item = Cow<'a, str>>>(iter: I) -> String

Creates a value from an iterator.

impl<'a> FromIterator<String> for Cow<'a, str>

fn from_iter<I: IntoIterator<Item = String>>(it: I) -> Cow<'a, str>

Creates a value from an iterator.

impl<'a, T> FromIterator<T> for Cow<'a, [T]>

where T: Clone,

fn from_iter<I: IntoIterator<Item = T>>(it: I) -> Cow<'a, [T]>

Creates a value from an iterator.

impl<'a> FromIterator<char> for Cow<'a, str>

fn from_iter<I: IntoIterator<Item = char>>(it: I) -> Cow<'a, str>

Creates a value from an iterator.

impl<B> Hash for Cow<'_, B>

where B: Hash + ToOwned + ?Sized,

fn hash<H: Hasher>(&self, state: &mut H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<B> Ord for Cow<'_, B>

where B: Ord + ToOwned + ?Sized,

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized + PartialOrd,

Restrict a value to a certain interval.

impl<T, U> PartialEq<&[U]> for Cow<'_, [T]>

where T: PartialEq<U> + Clone,

fn eq(&self, other: &&[U]) -> bool

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

fn ne(&self, other: &&[U]) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T, U> PartialEq<&mut [U]> for Cow<'_, [T]>

where T: PartialEq<U> + Clone,

fn eq(&self, other: &&mut [U]) -> bool

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

fn ne(&self, other: &&mut [U]) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'a, 'b> PartialEq<&'b str> for Cow<'a, str>

fn eq(&self, other: &&'b str) -> bool

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

fn ne(&self, other: &&'b str) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'a, 'b> PartialEq<Cow<'a, str>> for &'b str

fn eq(&self, other: &Cow<'a, str>) -> bool

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

fn ne(&self, other: &Cow<'a, str>) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'a, 'b> PartialEq<Cow<'a, str>> for String

fn eq(&self, other: &Cow<'a, str>) -> bool

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

fn ne(&self, other: &Cow<'a, str>) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'a, 'b> PartialEq<Cow<'a, str>> for str

fn eq(&self, other: &Cow<'a, str>) -> bool

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

fn ne(&self, other: &Cow<'a, str>) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'a, 'b, B, C> PartialEq<Cow<'b, C>> for Cow<'a, B>

where B: PartialEq<C> + ToOwned + ?Sized, C: ToOwned + ?Sized,

fn eq(&self, other: &Cow<'b, C>) -> bool

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

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

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'a, 'b> PartialEq<String> for Cow<'a, str>

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

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

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

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T, U, A: Allocator> PartialEq<Vec<U, A>> for Cow<'_, [T]>

where T: PartialEq<U> + Clone,

fn eq(&self, other: &Vec<U, A>) -> bool

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

fn ne(&self, other: &Vec<U, A>) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'a, 'b> PartialEq<str> for Cow<'a, str>

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

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

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

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'a, B> PartialOrd for Cow<'a, B>

where B: PartialOrd + ToOwned + ?Sized,

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

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

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

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

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

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

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

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

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

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

impl<B> Eq for Cow<'_, B>

where B: Eq + ToOwned + ?Sized,