collections::borrow::Cow - Rust
Description: API documentation for the Rust `Cow` enum in crate `collections`.
Keywords: rust, rustlang, rust-lang, Cow

# Enum collections::borrow::Cow 1.0.0 [−] [src]

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

## Variants

`Borrowed(&'a B)`

Borrowed data.

`Owned(<B as ToOwned>::Owned)`

Owned data.

## Methods

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

#### `fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned`

Acquires a mutable reference to the owned form of the data.

Clones the data if it is not already owned.

# Examples

    use std::ascii::AsciiExt;
    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>
    );

#### `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` 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(),
      Cow::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(),
      Cow::Owned(String::from(s))
    );

## Trait Implementations

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

#### `type Target = B`

#### `fn deref(&self) -> &B`

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

#### `fn add_assign(&mut self, rhs: &'a str)`

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

#### `fn add_assign(&mut self, rhs: Cow<'a, str>)`

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

#### `type Output = Cow<'a, str>`

#### `fn add(self, rhs: Cow<'a, str>) -> <Cow<'a, str> as Add<Cow<'a, str>>>::Output`

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

#### `type Output = Cow<'a, str>`

#### `fn add(self, rhs: &'a str) -> <Cow<'a, str> as Add<&'a str>>::Output`

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

#### `fn fmt(&self, f: &mut Formatter) -> Result<(), Error>`

Formats the value using the given formatter.

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

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

#### `fn from(v: Vec<T>) -> Cow<'a, [T]>`

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

#### `fn from(s: &'a [T]) -> Cow<'a, [T]>`

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

#### `fn from(s: &'a str) -> Cow<'a, str>`

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

#### `fn from(s: String) -> Cow<'a, str>`

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

#### `fn fmt(&self, f: &mut Formatter) -> Result<(), Error>`

Formats the value using the given formatter. Read more

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

#### `fn as_ref(&self) -> &T`

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

#### `fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering>`

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

#### `fn borrow(&self) -> &B`

Immutably borrows from an owned value. Read more

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

#### `fn to_string(&self) -> String`

Converts the given value to a `String`. Read more

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

#### `fn clone(&self) -> Cow<'a, B>`

#### `fn clone_from(&mut self, source: &Cow<'a, B>)`

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

#### `fn hash<H>(&self, state: &mut H) where
H: Hasher,`

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

#### `fn from_iter<I>(it: I) -> Cow<'a, str> where
I: IntoIterator<Item = &'b str>,`

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

#### `fn from_iter<I>(it: I) -> Cow<'a, [T]> where
I: IntoIterator<Item = T>,`

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

#### `fn from_iter<I>(it: I) -> Cow<'a, str> where
I: IntoIterator<Item = String>,`

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

#### `fn from_iter<I>(it: I) -> Cow<'a, str> where
I: IntoIterator<Item = char>,`

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

#### `fn cmp(&self, other: &Cow<'a, B>) -> Ordering`

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

#### `fn eq(&self, other: &&'b [B]) -> bool`

#### `fn ne(&self, other: &&'b [B]) -> bool`

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

#### `fn eq(&self, other: &Vec<B>) -> bool`

#### `fn ne(&self, other: &Vec<B>) -> bool`

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

#### `fn eq(&self, other: &&'b mut [B]) -> bool`

#### `fn ne(&self, other: &&'b mut [B]) -> bool`

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

#### `fn eq(&self, other: &String) -> bool`

#### `fn ne(&self, other: &String) -> bool`

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

#### `fn eq(&self, other: &&'b str) -> bool`

#### `fn ne(&self, other: &&'b str) -> bool`

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

#### `fn eq(&self, other: &str) -> bool`

#### `fn ne(&self, other: &str) -> bool`

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

#### `fn eq(&self, other: &Cow<'b, C>) -> bool`

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

#### `fn default() -> Cow<'a, B>`

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

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