Cow in alloc::borrow - Rust
Description: A clone-on-write smart pointer.
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),
    }

Expand description

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.

If you need reference-counting pointers, note that `Rc::make_mut` and `Arc::make_mut` can provide clone-on-write functionality as well.

# 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);

Run

Another example showing how to keep `Cow` in a struct:

    use std::borrow::Cow;

    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"),
    }

Run

## Variants

`Borrowed(&'a B)`

Borrowed data.

### Tuple Fields of Borrowed

`0: &'a B`
`Owned(<B as ToOwned>::Owned)`

Owned data.

### Tuple Fields of Owned

`0: <B as ToOwned>::Owned`

## Implementations

[src]

### impl<B: ?Sized + ToOwned> Cow<'_, B>

[src]

#### pub fn is_borrowed(&self) -> bool

🔬 This is a nightly-only experimental API. (`cow_is_borrowed` #65143)

Returns true if the data is borrowed, i.e. if `to_mut` would require additional work.

# Examples

    #![feature(cow_is_borrowed)]
    use std::borrow::Cow;

    let cow = Cow::Borrowed("moo");
    assert!(cow.is_borrowed());

    let bull: Cow<'_, str> = Cow::Owned("...moo?".to_string());
    assert!(!bull.is_borrowed());

Run
[src]

#### pub fn is_owned(&self) -> bool

🔬 This is a nightly-only experimental API. (`cow_is_borrowed` #65143)

Returns true if the data is owned, i.e. if `to_mut` would be a no-op.

# Examples

    #![feature(cow_is_borrowed)]
    use std::borrow::Cow;

    let cow: Cow<'_, str> = Cow::Owned("moo".to_string());
    assert!(cow.is_owned());

    let bull = Cow::Borrowed("...moo?");
    assert!(!bull.is_owned());

Run
[src]

#### pub 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::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>
    );

Run
[src]

#### pub 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(),
      String::from(s)
    );

Run

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)
    );

Run

## Trait Implementations

1.14.0[src]

### impl<'a> Add<&'a str> for Cow<'a, str>

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

The resulting type after applying the `+` operator.

[src]

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

Performs the `+` operation. Read more

1.14.0[src]

### impl<'a> Add<Cow<'a, str>> for Cow<'a, str>

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

The resulting type after applying the `+` operator.

[src]

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

Performs the `+` operation. Read more

1.14.0[src]

### impl<'a> AddAssign<&'a str> for Cow<'a, str>

[src]

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

Performs the `+=` operation. Read more

1.14.0[src]

### impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str>

[src]

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

Performs the `+=` operation. Read more

[src]

### impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T>

[src]

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

Performs the conversion.

[src]

### 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

[src]

### impl<B: ?Sized + ToOwned> Clone for Cow<'_, B>

[src]

#### fn clone(&self) -> Self

Returns a copy of the value. Read more

[src]

#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more

[src]

### impl<B: ?Sized> Debug for Cow<'_, B> where
B: Debug + ToOwned<Owned: Debug>,

[src]

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

Formats the value using the given formatter. Read more

1.11.0[src]

### impl<B: ?Sized> Default for Cow<'_, B> where
B: ToOwned<Owned: Default>,

[src]

#### fn default() -> Self

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

[src]

### impl<B: ?Sized + ToOwned> Deref for Cow<'_, B>

#### type Target = B

The resulting type after dereferencing.

[src]

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

Dereferences the value.

[src]

### impl<B: ?Sized> Display for Cow<'_, B> where
B: Display + ToOwned<Owned: Display>,

[src]

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

Formats the value using the given formatter. Read more

1.19.0[src]

### impl<'a> Extend<Cow<'a, str>> for String

[src]

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

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

[src]

#### fn extend_one(&mut self, s: Cow<'a, str>)

🔬 This is a nightly-only experimental API. (`extend_one` #72631)

Extends a collection with exactly one element.

[src]

#### fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (`extend_one` #72631)

Reserves capacity in a collection for the given number of additional elements. Read more

1.8.0[src]

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

[src]

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

Creates a `Borrowed` variant of `Cow` from a slice.

This conversion does not allocate or clone the data.

1.28.0[src]

### impl<'a> From<&'a String> for Cow<'a, str>

[src]

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

Converts a `String` reference into a `Borrowed` variant. No heap allocation is performed, and the string is not copied.

# Example

    let s = "eggplant".to_string();
    assert_eq!(Cow::from(&s), Cow::Borrowed("eggplant"));

Run
1.28.0[src]

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

[src]

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

Creates a `Borrowed` variant of `Cow` from a reference to `Vec`.

This conversion does not allocate or clone the data.

[src]

### impl<'a> From<&'a str> for Cow<'a, str>

[src]

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

Converts a string slice into a `Borrowed` variant. No heap allocation is performed, and the string is not copied.

# Example

    assert_eq!(Cow::from("eggplant"), Cow::Borrowed("eggplant"));

Run
1.45.0[src]

### impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]>

[src]

#### fn from(cow: Cow<'_, [T]>) -> Box<[T]>ⓘ
Notable traits for Box<I, A>
`impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> type Item = I::Item;impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A> where
A: 'static, type Output = F::Output;`

Performs the conversion.

1.45.0[src]

### impl From<Cow<'_, str>> for Box<str>

[src]

#### fn from(cow: Cow<'_, str>) -> Box<str>ⓘ
Notable traits for Box<I, A>
`impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> type Item = I::Item;impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A> where
A: 'static, type Output = F::Output;`

Performs the conversion.

1.14.0[src]

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

[src]

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

Convert a clone-on-write slice into a vector.

If `s` already owns a `Vec<T>`, it will be returned directly. If `s` is borrowing a slice, a new `Vec<T>` will be allocated and filled by cloning `s`’s items into it.

# Examples

    let o: Cow<[i32]> = Cow::Owned(vec![1, 2, 3]);
    let b: Cow<[i32]> = Cow::Borrowed(&[1, 2, 3]);
    assert_eq!(Vec::from(o), Vec::from(b));

Run
1.45.0[src]

### impl<'a, B: ?Sized> From<Cow<'a, B>> for Rc<B> where
B: ToOwned + ?Sized,
Rc<B>: From<&'a B> + From<B::Owned>,

[src]

#### fn from(cow: Cow<'a, B>) -> Rc<B>

Create a reference-counted pointer from a clone-on-write pointer by copying its content.

# Example

    let cow: Cow<str> = Cow::Borrowed("eggplant");
    let shared: Rc<str> = Rc::from(cow);
    assert_eq!("eggplant", &shared[..]);

Run
1.45.0[src]

### impl<'a, B: ?Sized> From<Cow<'a, B>> for Arc<B> where
B: ToOwned + ?Sized,
Arc<B>: From<&'a B> + From<B::Owned>,

[src]

#### fn from(cow: Cow<'a, B>) -> Arc<B>

Create an atomically reference-counted pointer from a clone-on-write pointer by copying its content.

# Example

    let cow: Cow<str> = Cow::Borrowed("eggplant");
    let shared: Arc<str> = Arc::from(cow);
    assert_eq!("eggplant", &shared[..]);

Run
1.14.0[src]

### impl<'a> From<Cow<'a, str>> for String

[src]

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

Converts a clone-on-write string to an owned instance of `String`.

This extracts the owned string, clones the string if it is not already owned.

# Example

    // If the string is not owned...
    let cow: Cow<str> = Cow::Borrowed("eggplant");
    // It will allocate on the heap and copy the string.
    let owned: String = String::from(cow);
    assert_eq!(&owned[..], "eggplant");

Run
[src]

### impl<'a> From<String> for Cow<'a, str>

[src]

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

Converts a `String` into an `Owned` variant. No heap allocation is performed, and the string is not copied.

# Example

    let s = "eggplant".to_string();
    let s2 = "eggplant".to_string();
    assert_eq!(Cow::from(s), Cow::<'static, str>::Owned(s2));

Run
1.8.0[src]

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

[src]

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

Creates an `Owned` variant of `Cow` from an owned instance of `Vec`.

This conversion does not allocate or clone the data.

1.12.0[src]

### impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str>

[src]

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

Creates a value from an iterator. Read more

1.19.0[src]

### impl<'a> FromIterator<Cow<'a, str>> for String

[src]

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

Creates a value from an iterator. Read more

1.12.0[src]

### impl<'a> FromIterator<String> for Cow<'a, str>

[src]

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

Creates a value from an iterator. Read more

[src]

### 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

1.12.0[src]

### impl<'a> FromIterator<char> for Cow<'a, str>

[src]

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

Creates a value from an iterator. Read more

[src]

### impl<B: ?Sized> Hash for Cow<'_, B> where
B: Hash + ToOwned,

[src]

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

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

1.3.0[src]

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

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

[src]

### impl<B: ?Sized> Ord for Cow<'_, B> where
B: Ord + ToOwned,

[src]

#### fn cmp(&self, other: &Self) -> Ordering

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

1.21.0[src]

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

Compares and returns the maximum of two values. Read more

1.21.0[src]

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

Compares and returns the minimum of two values. Read more

1.50.0[src]

#### fn clamp(self, min: Self, max: Self) -> Self

Restrict a value to a certain interval. Read more

[src]

### impl<T, U> PartialEq<&'_ [U]> for Cow<'_, [T]> where
T: PartialEq<U>,
T: Clone,

[src]

#### fn eq(&self, other: &&[U]) -> bool

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

[src]

#### fn ne(&self, other: &&[U]) -> bool

This method tests for `!=`.

[src]

### impl<T, U> PartialEq<&'_ mut [U]> for Cow<'_, [T]> where
T: PartialEq<U>,
T: Clone,

[src]

#### fn eq(&self, other: &&mut [U]) -> bool

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

[src]

#### fn ne(&self, other: &&mut [U]) -> bool

This method tests for `!=`.

[src]

### 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

[src]

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

This method tests for `!=`.

[src]

### impl<'a, 'b> PartialEq<Cow<'a, str>> for str

[src]

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

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

[src]

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

This method tests for `!=`.

[src]

### impl<'a, 'b> PartialEq<Cow<'a, str>> for &'b str

[src]

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

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

[src]

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

This method tests for `!=`.

[src]

### impl<'a, 'b> PartialEq<Cow<'a, str>> for String

[src]

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

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

[src]

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

This method tests for `!=`.

[src]

### 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

[src]

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

This method tests for `!=`.

[src]

### 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

[src]

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

This method tests for `!=`.

[src]

### impl<T, U, A: Allocator> PartialEq<Vec<U, A>> for Cow<'_, [T]> where
T: PartialEq<U>,
T: Clone,

[src]

#### fn eq(&self, other: &Vec<U, A>) -> bool

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

[src]

#### fn ne(&self, other: &Vec<U, A>) -> bool

This method tests for `!=`.

[src]

### 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

[src]

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

This method tests for `!=`.

[src]

### 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>

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

[src]

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

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

[src]

#### 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

[src]

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

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

[src]

#### 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

1.17.0[src]

### impl ToString for Cow<'_, str>

[src]

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

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

[src]

### impl<B: ?Sized> Eq for Cow<'_, B> where
B: Eq + ToOwned,

## Auto Trait Implementations

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

### 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,

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

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

## Blanket Implementations

[src]

### impl<T> Any for T where
T: 'static + ?Sized,

[src]

#### pub fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more

[src]

### impl<T> Borrow<T> for T where
T: ?Sized,

[src]

#### pub fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more

[src]

### impl<T> BorrowMut<T> for T where
T: ?Sized,

[src]

#### pub fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more

[src]

### impl<T> From<T> for T

[src]

#### pub fn from(t: T) -> T

Performs the conversion.

[src]

### impl<T, U> Into<U> for T where
U: From<T>,

[src]

#### pub fn into(self) -> U

Performs the conversion.

[src]

### impl<T> ToOwned for T where
T: Clone,

#### type Owned = T

The resulting type after obtaining ownership.

[src]

#### pub fn to_owned(&Self) -> T

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

[src]

#### pub fn clone_into(&Self, &mut T)

🔬 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

[src]

### impl<T> ToString for T where
T: Display + ?Sized,

[src]

#### pub default fn to_string(&Self) -> String

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

[src]

### impl<T, U> TryFrom<U> for T where
U: Into<T>,

#### type Error = Infallible

The type returned in the event of a conversion error.

[src]

#### pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.

[src]

### impl<T, U> TryInto<U> for T where
U: TryFrom<T>,

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

The type returned in the event of a conversion error.

[src]

#### pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.