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
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impl<'a, B: ?Sized> Display for Cow<'a, B> where
    B: Display + ToOwned,
    <B as ToOwned>::Owned: Display
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impl<'a, B: ?Sized> Debug for Cow<'a, B> where
    B: Debug + ToOwned,
    <B as ToOwned>::Owned: Debug
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impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B> where
    B: PartialEq<C> + ToOwned,
    C: ToOwned
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This method tests for !=.

impl<'a, 'b> PartialEq<str> for Cow<'a, str>
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impl<'a, 'b> PartialEq<Cow<'a, str>> for str
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impl<'a, 'b> PartialEq<&'b str> for Cow<'a, str>
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impl<'a, 'b> PartialEq<Cow<'a, str>> for &'b str
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impl<'a, 'b> PartialEq<String> for Cow<'a, str>
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impl<'a, 'b> PartialEq<Cow<'a, str>> for String
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impl<'a, 'b, A: Clone, B> PartialEq<&'b [B]> for Cow<'a, [A]> where
    A: PartialEq<B>, 
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impl<'a, 'b, A: Clone, B> PartialEq<&'b mut [B]> for Cow<'a, [A]> where
    A: PartialEq<B>, 
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impl<'a, 'b, A: Clone, B> PartialEq<Vec<B>> for Cow<'a, [A]> where
    A: PartialEq<B>, 
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impl<'a, B: ?Sized> Eq for Cow<'a, B> where
    B: Eq + ToOwned
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impl<'a, B: ?Sized> Ord for Cow<'a, B> where
    B: Ord + ToOwned
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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
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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, B: ?Sized> Hash for Cow<'a, B> where
    B: Hash + ToOwned
[src]

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

impl<'a> From<Cow<'a, str>> for String
1.14.0
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impl<'a> From<&'a str> for Cow<'a, str>
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impl<'a> From<String> for Cow<'a, str>
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impl<'a> From<&'a String> for Cow<'a, str>
1.28.0
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impl<'a, T> From<Cow<'a, [T]>> for Vec<T> where
    [T]: ToOwned<Owned = Vec<T>>, 
1.14.0
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impl<'a, T: Clone> From<&'a [T]> for Cow<'a, [T]>
1.8.0
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impl<'a, T: Clone> From<Vec<T>> for Cow<'a, [T]>
1.8.0
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impl<'a, T: Clone> From<&'a Vec<T>> for Cow<'a, [T]>
1.28.0
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impl<'a> Add<&'a str> for Cow<'a, str>
1.14.0
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The resulting type after applying the + operator.

impl<'a> Add<Cow<'a, str>> for Cow<'a, str>
1.14.0
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The resulting type after applying the + operator.

impl<'a> AddAssign<&'a str> for Cow<'a, str>
1.14.0
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impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str>
1.14.0
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impl<'a, B: ?Sized> Deref for Cow<'a, B> where
    B: ToOwned
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The resulting type after dereferencing.

impl<'a> FromIterator<Cow<'a, str>> for String
1.19.0
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impl<'a> FromIterator<char> for Cow<'a, str>
1.12.0
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impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str>
1.12.0
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impl<'a> FromIterator<String> for Cow<'a, str>
1.12.0
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impl<'a, T> FromIterator<T> for Cow<'a, [T]> where
    T: Clone
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impl<'a> Extend<Cow<'a, str>> for String
1.19.0
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impl<'a, B: ?Sized> Clone for Cow<'a, B> where
    B: ToOwned
[src]

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, T: ?Sized + ToOwned> AsRef<T> for Cow<'a, T>
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impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B> where
    B: ToOwned,
    <B as ToOwned>::Owned: 'a, 
[src]

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]

impl<T> ToString for T where
    T: Display + ?Sized
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impl<T> From for T
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impl<T, U> TryFrom for T where
    T: From<U>, 
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🔬 This is a nightly-only experimental API. (try_from #33417)

The type returned in the event of a conversion error.

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.

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

impl<T> Borrow for T where
    T: ?Sized
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impl<T> BorrowMut for T where
    T: ?Sized
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impl<T> Any for T where
    T: 'static + ?Sized
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