Expand description
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][crate::rc::Rc::make_mut] and
[Arc::make_mut][crate::sync::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: 'a> 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
sourceimpl<B: ?Sized + ToOwned> Cow<'_, B>
impl<B: ?Sized + ToOwned> Cow<'_, B>
const: unstable · sourcepub fn is_borrowed(&self) -> bool
🔬This is a nightly-only experimental API. (cow_is_borrowed #65143)
pub fn is_borrowed(&self) -> bool
cow_is_borrowed #65143)const: unstable · sourcepub fn is_owned(&self) -> bool
🔬This is a nightly-only experimental API. (cow_is_borrowed #65143)
pub fn is_owned(&self) -> bool
cow_is_borrowed #65143)sourcepub fn into_owned(self) -> <B as ToOwned>::Owned
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
1.14.0 · sourceimpl<'a, T> From<Cow<'a, [T]>> for Vec<T>where
[T]: ToOwned<Owned = Vec<T>>,
impl<'a, T> From<Cow<'a, [T]>> for Vec<T>where
[T]: ToOwned<Owned = Vec<T>>,
sourcefn from(s: Cow<'a, [T]>) -> 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));