Enum 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 const fn is_borrowed(c: &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.

Note: this is an associated function, which means that you have to call it as Cow::is_borrowed(&c) instead of c.is_borrowed(). This is so that there is no conflict with a method on the inner type.

Examples

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

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

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

pub const fn is_owned(c: &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.

Note: this is an associated function, which means that you have to call it as Cow::is_owned(&c) instead of c.is_owned(). This is so that there is no conflict with a method on the inner type.

Examples

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

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

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

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 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> 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> AddAssign for Cow<'a, str>

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

Performs the += operation.

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

fn add_assign(&mut self, rhs: &'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: ?Sized + ToOwned> Borrow<B> for Cow<'a, B>

fn borrow(&self) -> &B

Immutably borrows from an owned value.

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

fn clone(&self) -> Self

Returns a duplicate 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>

type Target = B

The resulting type after dereferencing.

fn deref(&self) -> &B

Dereferences the value.

impl<T: Clone> DerefPure for Cow<'_, T>

impl DerefPure for Cow<'_, str>

impl<T: Clone>