# 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(&'a B)`

Borrowed data.

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

Owned data.

## Methods

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

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

#### `pub fn into_owned(self) -> <B as ToOwned>::Owned`[src]

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]

#### `fn to_string(&self) -> String`[src]

Converts the given value to a `String`. 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`[src]

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

#### `
#[must_use]
fn ne(&self, other: &Rhs) -> bool`[src]

This method tests for `!=`.

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

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

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

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

This method tests for `!=`.

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

#### `fn eq(&self, other: &Cow<'a, str>) -> bool`[src]

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

#### `fn ne(&self, other: &Cow<'a, str>) -> bool`[src]

This method tests for `!=`.

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

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

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

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

This method tests for `!=`.

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

#### `fn eq(&self, other: &Cow<'a, str>) -> bool`[src]

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

#### `fn ne(&self, other: &Cow<'a, str>) -> bool`[src]

This method tests for `!=`.

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

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

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

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

This method tests for `!=`.

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

#### `fn eq(&self, other: &Cow<'a, str>) -> bool`[src]

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

#### `fn ne(&self, other: &Cow<'a, str>) -> bool`[src]

This method tests for `!=`.

### `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`[src]

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

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

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`[src]

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`[src]

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`[src]

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

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

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]

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

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

#### `fn max(self, other: Self) -> Self`
1.21.0
[src]

Compares and returns the maximum of two values. Read more

#### `fn min(self, other: Self) -> Self`
1.21.0
[src]

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]

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

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

#### `
#[must_use]
fn lt(&self, other: &Rhs) -> bool`[src]

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

#### `
#[must_use]
fn le(&self, other: &Rhs) -> bool`[src]

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

#### `
#[must_use]
fn gt(&self, other: &Rhs) -> bool`[src]

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

#### `
#[must_use]
fn ge(&self, other: &Rhs) -> bool`[src]

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

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

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

Performs the conversion.

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

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

Performs the conversion.

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

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

Performs the conversion.

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

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

Performs the conversion.

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

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

Performs the conversion.

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

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

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]>`[src]

Performs the conversion.

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

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

Performs the conversion.

### `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`[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]

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

Formats the value using the given formatter. Read more

### `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`[src]

Performs the `+` operation.

### `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`[src]

Performs the `+` operation.

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

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

Performs the `+=` operation.

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

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

Performs the `+=` operation.

### `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`[src]

Dereferences the value.

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

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

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

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

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

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

#### `fn extend<I: IntoIterator<Item = Cow<'a, str>>>(&mut self, iter: I)`[src]

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

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

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

Creates a value from an iterator. Read more

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

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

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>`[src]

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>`[src]

Creates a value from an iterator. Read more

### `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]>`[src]

Creates a value from an iterator. Read more

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

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

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

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

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

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

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

#### `type Owned = T`

#### `fn to_owned(&Self) -> T`[src]

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

#### `fn clone_into(&Self, &mut T)`[src]

🔬 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]

#### `fn to_string(&Self) -> String`[src]

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

### `impl<T> From for T`[src]

#### `fn from(t: T) -> T`[src]

Performs the conversion.

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

#### `type Error = !`

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

The type returned in the event of a conversion error.

#### `fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

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

Performs the conversion.

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

#### `type Error = <U as TryFrom<T>>::Error`

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

The type returned in the event of a conversion error.

#### `fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

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

Performs the conversion.

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

#### `fn into(self) -> U`[src]

Performs the conversion.

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

#### `fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

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

#### `fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

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

#### `fn get_type_id(&self) -> TypeId`[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

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