Enum alloc::borrow::Cow

1.0.0 · source · [−]
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);
Run

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"),
}
Run

Variants

Borrowed(&'a B)

Borrowed data.

Owned(<B as ToOwned>::Owned)

Owned data.

Implementations

🔬 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());
Run
🔬 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());
Run

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>
);
Run

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)
);
Run

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)
);
Run

Trait Implementations

The resulting type after applying the + operator.

Performs the + operation. Read more

The resulting type after applying the + operator.

Performs the + operation. Read more

Performs the += operation. Read more

Performs the += operation. Read more

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

Immutably borrows from an owned value. Read more

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

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

The resulting type after dereferencing.

Dereferences the value.

Formats the value using the given formatter. Read more

Extends a collection with the contents of an iterator. Read more

🔬 This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

🔬 This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements. Read more

Creates a Borrowed variant of Cow from a slice.

This conversion does not allocate or clone the data.

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

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

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"));
Run

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

This conversion does not allocate or clone the data.

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"));
Run

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

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

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

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));
Run

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[..]);
Run

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[..]);
Run

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

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");
Run

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));
Run

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

This conversion does not allocate or clone the data.

Creates a value from an iterator. Read more

Creates a value from an iterator. Read more

Creates a value from an iterator. Read more

Creates a value from an iterator. Read more

Creates a value from an iterator. Read more

Feeds this value into the given Hasher. Read more

Feeds a slice of this type into the given Hasher. Read more

This method returns an Ordering between self and other. Read more

Compares and returns the maximum of two values. Read more

Compares and returns the minimum of two values. Read more

Restrict a value to a certain interval. Read more

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

This method tests for !=.

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

This method tests for !=.

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

This method tests for !=.

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

This method tests for !=.

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

This method tests for !=.

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

This method tests for !=.

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

This method tests for !=.

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

This method tests for !=.

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

This method tests for !=.

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

This method tests for !=.

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

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

Converts the given value to a String. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

Uses borrowed data to replace owned data, usually by cloning. Read more

Converts the given value to a String. Read more

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.