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

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

    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);

## Variants

`Borrowed(&'a B)`

Borrowed data.

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

Owned data.

## Methods

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

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

Immutably borrows from an owned value. Read more

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

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

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

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

#### `type Target = B`

The resulting type after dereferencing

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

The method called to dereference a value

### `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]

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

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

#### `fn max(self, other: Self) -> Self`

🔬 This is a nightly-only experimental API. (`ord_max_min` #25663)

Compares and returns the maximum of two values. Read more

#### `fn min(self, other: Self) -> Self`

🔬 This is a nightly-only experimental API. (`ord_max_min` #25663)

Compares and returns the minimum of two values. 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]

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

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

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

This method tests for `!=`.

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

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

This method returns an ordering between `self` and `other` values if one exists. Read more

#### `fn lt(&self, other: &Rhs) -> bool`

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more

#### `fn le(&self, other: &Rhs) -> bool`

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

#### `fn gt(&self, other: &Rhs) -> bool`

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

#### `fn ge(&self, other: &Rhs) -> bool`

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

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

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

Formats the value using the given formatter.

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

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

Formats the value using the given formatter. Read more

### `impl<'a, B: ?Sized> Default for Cow<'a, B> where
B: ToOwned,
<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.

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

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

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

#### `fn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher,`
1.3.0

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

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

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

Performs the conversion.

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

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

The resulting type after applying the `+` operator

#### `fn add(self, rhs: &'a str) -> Self::Output`

The method for the `+` operator

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

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

The resulting type after applying the `+` operator

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

The method for the `+` operator

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

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

The method for the `+=` operator

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

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

The method for the `+=` operator

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

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

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

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

This method tests for `!=`.

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

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

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

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

This method tests for `!=`.

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

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

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

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

This method tests for `!=`.

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

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

Performs the conversion.

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

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

Performs the conversion.

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

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

Creates a value from an iterator. Read more

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

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

Creates a value from an iterator. Read more

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

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

Creates a value from an iterator. Read more

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

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

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

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

This method tests for `!=`.

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

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

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

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

This method tests for `!=`.

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

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

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

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

This method tests for `!=`.

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

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

Performs the conversion.

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

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

Performs the conversion.

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

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

Creates a value from an iterator. Read more

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