1.0.0[][src]Enum alloc::borrow::Cow

pub enum Cow<'a, B: ?Sized + 'a> where
    B: ToOwned
{ 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.

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, ToOwned};

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 check it out.
match clone_on_write {
    Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
    _ => panic!("expect owned data"),
}

Variants

Borrowed data.

Owned data.

Methods

impl<'a, B: ?Sized> Cow<'a, B> where
    B: ToOwned
[src]

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>
);

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

impl<'a> ToString for Cow<'a, str>
1.17.0
[src]

Converts the given value to a String. Read more

impl<'a> From<Cow<'a, str>> for String
1.14.0
[src]

Performs the conversion.

impl<'a> From<&'a str> for Cow<'a, str>
[src]

Performs the conversion.

impl<'a> From<String> for Cow<'a, str>
[src]

Performs the conversion.

impl<'a> From<&'a String> for Cow<'a, str>
1.28.0
[src]

Performs the conversion.

impl<'a, T> From<Cow<'a, [T]>> for Vec<T> where
    [T]: ToOwned<Owned = Vec<T>>, 
1.14.0
[src]

Performs the conversion.

impl<'a, T: Clone> From<&'a [T]> for Cow<'a, [T]>
1.8.0
[src]

Performs the conversion.

impl<'a, T: Clone> From<Vec<T>> for Cow<'a, [T]>
1.8.0
[src]

Performs the conversion.

impl<'a, T: Clone> From<&'a Vec<T>> for Cow<'a, [T]>
1.28.0
[src]

Performs the conversion.

impl<'a, B: ?Sized> Debug for Cow<'a, B> where
    B: Debug + ToOwned,
    <B as ToOwned>::Owned: Debug
[src]

Formats the value using the given formatter. Read more

impl<'a, B: ?Sized> Display for Cow<'a, B> where
    B: Display + ToOwned,
    <B as ToOwned>::Owned: Display
[src]

Formats the value using the given formatter. Read more

impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B> where
    B: PartialEq<C> + ToOwned,
    C: ToOwned
[src]

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

This method tests for !=.

impl<'a, 'b> PartialEq<str> for Cow<'a, str>
[src]

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

This method tests for !=.

impl<'a, 'b> PartialEq<Cow<'a, str>> for str
[src]

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

This method tests for !=.

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

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

This method tests for !=.

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

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

This method tests for !=.

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

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

This method tests for !=.

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

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

This method tests for !=.

impl<'a, 'b, A: Clone, B> PartialEq<&'b [B]> for Cow<'a, [A]> where
    A: PartialEq<B>, 
[src]

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

This method tests for !=.

impl<'a, 'b, A: Clone, B> PartialEq<&'b mut [B]> for Cow<'a, [A]> where
    A: PartialEq<B>, 
[src]

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

This method tests for !=.

impl<'a, 'b, A: Clone, B> PartialEq<Vec<B>> for Cow<'a, [A]> where
    A: PartialEq<B>, 
[src]

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

This method tests for !=.

impl<'a, B: ?Sized> Eq for Cow<'a, B> where
    B: Eq + ToOwned
[src]

impl<'a, B: ?Sized> Ord for Cow<'a, B> where
    B: Ord + ToOwned
[src]

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

impl<'a, B: ?Sized> PartialOrd<Cow<'a, B>> for Cow<'a, B> where
    B: PartialOrd + ToOwned
[src]

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

impl<'a> Add<&'a str> for Cow<'a, str>
1.14.0
[src]

The resulting type after applying the + operator.

Performs the + operation.

impl<'a> Add<Cow<'a, str>> for Cow<'a, str>
1.14.0
[src]

The resulting type after applying the + operator.

Performs the + operation.

impl<'a> AddAssign<&'a str> for Cow<'a, str>
1.14.0
[src]

Performs the += operation.

impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str>
1.14.0
[src]

Performs the += operation.

impl<'a, B: ?Sized> Deref for Cow<'a, B> where
    B: ToOwned
[src]

The resulting type after dereferencing.

Dereferences the value.

impl<'a, B: ?Sized> Hash for Cow<'a, B> where
    B: Hash + ToOwned
[src]

Feeds this value into the given [Hasher]. Read more

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

impl<'a> Extend<Cow<'a, str>> for String
1.19.0
[src]

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

impl<'a> FromIterator<Cow<'a, str>> for String
1.19.0
[src]

Creates a value from an iterator. Read more

impl<'a> FromIterator<char> for Cow<'a, str>
1.12.0
[src]

Creates a value from an iterator. Read more

impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str>
1.12.0
[src]

Creates a value from an iterator. Read more

impl<'a> FromIterator<String> for Cow<'a, str>
1.12.0
[src]

Creates a value from an iterator. Read more

impl<'a, T> FromIterator<T> for Cow<'a, [T]> where
    T: Clone
[src]

Creates a value from an iterator. Read more

impl<'a, B: ?Sized> Clone for Cow<'a, B> where
    B: ToOwned
[src]

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

impl<'a, T: ?Sized + ToOwned> AsRef<T> for Cow<'a, T>
[src]

Performs the conversion.

impl<'a, B: ?Sized> Default for Cow<'a, B> where
    B: ToOwned,
    <B as ToOwned>::Owned: Default
1.11.0
[src]

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

impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B> where
    B: ToOwned,
    <B as ToOwned>::Owned: 'a, 
[src]

Immutably borrows from an owned value. Read more

Auto Trait Implementations

impl<'a, B: ?Sized> Send for Cow<'a, B> where
    B: Sync,
    <B as ToOwned>::Owned: Send

impl<'a, B: ?Sized> Sync for Cow<'a, B> where
    B: Sync,
    <B as ToOwned>::Owned: Sync

Blanket Implementations

impl<T> ToOwned for T where
    T: Clone
[src]

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

🔬 This is a nightly-only experimental API. (toowned_clone_into #41263)

recently added

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

impl<T> ToString for T where
    T: Display + ?Sized
[src]

Converts the given value to a String. Read more

impl<T> From for T
[src]

Performs the conversion.

impl<T, U> TryFrom for T where
    T: From<U>, 
[src]

🔬 This is a nightly-only experimental API. (try_from #33417)

The type returned in the event of a conversion error.

🔬 This is a nightly-only experimental API. (try_from #33417)

Performs the conversion.

impl<T, U> TryInto for T where
    U: TryFrom<T>, 
[src]

🔬 This is a nightly-only experimental API. (try_from #33417)

The type returned in the event of a conversion error.

🔬 This is a nightly-only experimental API. (try_from #33417)

Performs the conversion.

impl<T, U> Into for T where
    U: From<T>, 
[src]

Performs the conversion.

impl<T> Borrow for T where
    T: ?Sized
[src]

Immutably borrows from an owned value. Read more

impl<T> BorrowMut for T where
    T: ?Sized
[src]

Mutably borrows from an owned value. Read more

impl<T> Any for T where
    T: 'static + ?Sized
[src]

🔬 This is a nightly-only experimental API. (get_type_id #27745)

this method will likely be replaced by an associated static

Gets the TypeId of self. Read more