Enum std::borrow::Cow1.0.0[−][src]

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

Owned(::Owned)

Owned data.

Implementations

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

`pub const fn is_borrowed(&self) -> bool`[src]

🔬 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

`pub const fn is_owned(&self) -> bool`[src]

🔬 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

`pub fn to_mut(&mut self) -> &mut ::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>
);

Run

`pub fn into_owned(self) -> ::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)
);

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

`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.

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

Performs the + operation. Read more

`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.

`pub fn add( self, rhs: Cow<'a, str> ) -> <Cow<'a, str> as Add<Cow<'a, str>>>::Output`[src]

Performs the + operation. Read more

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

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

Performs the += operation. Read more

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

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

Performs the += operation. Read more

`impl AsRef<Path> for Cow<'_, OsStr>`1.8.0[src]

`fn as_ref(&self) -> &Path`[src]

Performs the conversion.

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

`pub fn as_ref(&self) -> &T`[src]

Performs the conversion.

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

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

Immutably borrows from an owned value. Read more

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

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

Returns a copy of the value. Read more

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

Performs copy-assignment from source. Read more

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

`pub fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>`[src]

Formats the value using the given formatter. Read more

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

`pub fn default() -> Cow<'_, B>`[src]

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

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

`type Target = B`

The resulting type after dereferencing.

`pub fn deref(&self) -> &B`[src]

Dereferences the value.

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

`pub fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>`[src]

Formats the value using the given formatter. Read more

`impl<'a> Extend<Cow<'a, OsStr>> for OsString`1.52.0[src]

`fn extend<T: IntoIterator<Item = Cow<'a, OsStr>>>(&mut self, iter: T)`[src]

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

`fn extend_one(&mut self, item: A)`[src]

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

Extends a collection with exactly one element.

`fn extend_reserve(&mut self, additional: usize)`[src]

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

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

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

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

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

`pub fn extend_one(&mut self, s: Cow<'a, str>)`[src]

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

Extends a collection with exactly one element.

`fn extend_reserve(&mut self, additional: usize)`[src]

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

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

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

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

Performs the conversion.

`impl<'a> From<&'a CStr> for Cow<'a, CStr>`1.28.0[src]

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

Performs the conversion.

`impl<'a> From<&'a CString> for Cow<'a, CStr>`1.28.0[src]

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

Performs the conversion.

`impl<'a> From<&'a OsStr> for Cow<'a, OsStr>`1.28.0[src]

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

Performs the conversion.

`impl<'a> From<&'a OsString> for Cow<'a, OsStr>`1.28.0[src]

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

Performs the conversion.

`impl<'a> From<&'a Path> for Cow<'a, Path>`1.6.0[src]

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

Performs the conversion.

`impl<'a> From<&'a PathBuf> for Cow<'a, Path>`1.28.0[src]

`fn from(p: &'a PathBuf) -> Cow<'a, Path>`[src]

Performs the conversion.

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

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

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

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

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

Performs the conversion.

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

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

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

`impl<'a> From<CString> for Cow<'a, CStr>`1.28.0[src]

`fn from(s: CString) -> Cow<'a, CStr>`[src]

Performs the conversion.

`impl<'_, T> From<Cow<'_, [T]>> for Box<[T], Global> where T: Copy,` 1.45.0[src]

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

Performs the conversion.

`impl From<Cow<'_, CStr>> for Box<CStr>`1.45.0[src]

`fn from(cow: Cow<'_, CStr>) -> Box<CStr>ⓘNotable traits for Box<F, A> impl<F, A> Future for Box<F, A> where F: Future + Unpin + ?Sized, A: Allocator + 'static, type Output = <F as Future>::Output;impl<I, A> Iterator for Box<I, A> where I: Iterator + ?Sized, A: Allocator, type Item = ::Item;impl<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`[src]

Performs the conversion.

`impl From<Cow<'_, OsStr>> for Box<OsStr>`1.45.0[src]

`fn from(cow: Cow<'_, OsStr>) -> Box<OsStr>ⓘNotable traits for Box<F, A> impl<F, A> Future for Box<F, A> where F: Future + Unpin + ?Sized, A: Allocator + 'static, type Output = <F as Future>::Output;impl<I, A> Iterator for Box<I, A> where I: Iterator + ?Sized, A: Allocator, type Item = ::Item;impl<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`[src]

Performs the conversion.

`impl From<Cow<'_, Path>> for Box<Path>`1.45.0[src]

`fn from(cow: Cow<'_, Path>) -> Box<Path>ⓘNotable traits for Box<F, A> impl<F, A> Future for Box<F, A> where F: Future + Unpin + ?Sized, A: Allocator + 'static, type Output = <F as Future>::Output;impl<I, A> Iterator for Box<I, A> where I: Iterator + ?Sized, A: Allocator, type Item = ::Item;impl<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`[src]

Performs the conversion.

`impl<'_> From<Cow<'_, str>> for Box<str, Global>`1.45.0[src]

`pub fn from(cow: Cow<'_, str>) -> Box<str, Global>ⓘNotable traits for Box<F, A> impl<F, A> Future for Box<F, A> where F: Future + Unpin + ?Sized, A: Allocator + 'static, type Output = <F as Future>::Output;impl<I, A> Iterator for Box<I, A> where I: Iterator + ?Sized, A: Allocator, type Item = ::Item;impl<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`[src]

Performs the conversion.

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

`pub fn from(s: Cow<'a, [T]>) -> Vec<T, Global>ⓘNotable traits for Vec<u8, A> impl<A: Allocator> Write for Vec<u8, A>`[src]

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

`impl<'a, B> From<Cow<'a, B>> for Rc where B: ToOwned + ?Sized, Rc: From<&'a B>, Rc: From<::Owned>,` 1.45.0[src]

`pub fn from(cow: Cow<'a, B>) -> Rc`[src]

Performs the conversion.

`impl<'a, B> From<Cow<'a, B>> for Arc where B: ToOwned + ?Sized, Arc: From<&'a B>, Arc: From<::Owned>,` 1.45.0[src]

`pub fn from(cow: Cow<'a, B>) -> Arc`[src]

Performs the conversion.

`impl<'a> From<Cow<'a, CStr>> for CString`1.28.0[src]

`fn from(s: Cow<'a, CStr>) -> Self`[src]

Performs the conversion.

`impl<'a> From<Cow<'a, OsStr>> for OsString`1.28.0[src]

`fn from(s: Cow<'a, OsStr>) -> Self`[src]

Performs the conversion.

`impl<'a> From<Cow<'a, Path>> for PathBuf`1.28.0[src]

`fn from(p: Cow<'a, Path>) -> Self`[src]

Performs the conversion.

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

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

Performs the conversion.

`impl<'a> From<Cow<'a, str>> for Box<dyn Error>`1.22.0[src]

`fn from(err: Cow<'a, str>) -> Box<dyn Error>ⓘNotable traits for Box<F, A> impl<F, A> Future for Box<F, A> where F: Future + Unpin + ?Sized, A: Allocator + 'static, type Output = <F as Future>::Output;impl<I, A> Iterator for Box<I, A> where I: Iterator + ?Sized, A: Allocator, type Item = ::Item;impl<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`[src]

Converts a Cow into a box of dyn Error.

Examples

use std::error::Error;
use std::mem;
use std::borrow::Cow;

let a_cow_str_error = Cow::from("a str error");
let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))

Run

`impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a>`1.22.0[src]

`fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a>ⓘNotable traits for Box<F, A> impl<F, A> Future for Box<F, A> where F: Future + Unpin + ?Sized, A: Allocator + 'static, type Output = <F as Future>::Output;impl<I, A> Iterator for Box<I, A> where I: Iterator + ?Sized, A: Allocator, type Item = ::Item;impl<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`[src]

Converts a Cow into a box of dyn Error + Send + Sync.

Examples

use std::error::Error;
use std::mem;
use std::borrow::Cow;

let a_cow_str_error = Cow::from("a str error");
let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
assert!(
    mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))

Run

`impl<'a> From<OsString> for Cow<'a, OsStr>`1.28.0[src]

`fn from(s: OsString) -> Cow<'a, OsStr>`[src]

Performs the conversion.

`impl<'a> From<PathBuf> for Cow<'a, Path>`1.6.0[src]

`fn from(s: PathBuf) -> Cow<'a, Path>`[src]

Performs the conversion.

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

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

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

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

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

Performs the conversion.

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

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

Creates a value from an iterator. Read more

`impl<'a> FromIterator<Cow<'a, OsStr>> for OsString`1.52.0[src]

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

Creates a value from an iterator. Read more

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

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

Creates a value from an iterator. Read more

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

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

Creates a value from an iterator. Read more

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

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

Creates a value from an iterator. Read more

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

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

Creates a value from an iterator. Read more

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

`pub fn hash<H>(&self, state: &mut H) where H: Hasher,` [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<'_, B> Ord for Cow<'_, B> where B: Ord + ToOwned + ?Sized,` [src]

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

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

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

Compares and returns the maximum of two values. Read more

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

Compares and returns the minimum of two values. Read more