pub enum Cow<'a, B: ?Sized + 'a> where
B: ToOwned, {
Borrowed(&'a B),
Owned(<B as ToOwned>::Owned),
}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 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: '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
Extracts the owned data.
Clones the data if it is not already owned.
Examples
Calling into_owned on a Cow::Borrowed clones the underlying data
and becomes a Cow::Owned:
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 is a no-op:
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
Performs the += operation. Read more
Performs the += operation. Read more
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.
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);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));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");This method returns an ordering between self and other values if one exists. Read more
This method tests less than (for self and other) and is used by the < operator. Read more
This method tests less than or equal to (for self and other) and is used by the <=
operator. Read more
This method tests greater than (for self and other) and is used by the > operator. Read more