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/Rust

Enum std::borrow::Cow

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);

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"),
}

Variants

Borrowed(&'a B)

Borrowed data.

Owned(<B as ToOwned>::Owned)

Owned data.

Implementations

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

pub 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());

pub 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());

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

type Output = Cow<'a, str>

The resulting type after applying the + operator.

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

type Output = Cow<'a, str>

The resulting type after applying the + operator.

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

fn from(err: Cow<'a, str>) -> Box<dyn Error>ⓘ

Notable traits for Box<F>

impl<F> Future for Box<F> where
    F: Unpin + Future + ?Sized, 
    type Output = <F as Future>::Output;
impl<I> Iterator for Box<I> where
    I: Iterator + ?Sized, 
    type Item = <I as Iterator>::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))

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

fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a>ⓘ

Notable traits for Box<F>

impl<F> Future for Box<F> where
    F: Unpin + Future + ?Sized, 
    type Output = <F as Future>::Output;
impl<I> Iterator for Box<I> where
    I: Iterator + ?Sized, 
    type Item = <I as Iterator>::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))

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

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

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

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

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

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

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

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

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

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

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

impl<'_, '_, A, B> PartialEq<&'_ [B]> for Cow<'_, [A]> where
    A: PartialEq<B> + Clone, [src]

impl<'_, '_, A, B> PartialEq<&'_ mut [B]> for Cow<'_, [A]> where
    A: PartialEq<B> + Clone, [src]

impl<'a, 'b> PartialEq<&'a Path> for Cow<'b, OsStr>[src]1.8.0

impl<'a, 'b> PartialEq<&'b OsStr> for Cow<'a, OsStr>[src]1.8.0

impl<'a, 'b> PartialEq<&'b OsStr> for Cow<'a, Path>[src]1.8.0

impl<'a, 'b> PartialEq<&'b Path> for Cow<'a, Path>[src]1.6.0

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

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for OsStr[src]1.8.0

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for &'b OsStr[src]1.8.0

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for OsString[src]1.8.0

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for PathBuf[src]1.8.0

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for Path[src]1.8.0

impl<'a, 'b> PartialEq<Cow<'a, Path>> for Path[src]1.6.0

impl<'a, 'b> PartialEq<Cow<'a, Path>> for &'b Path[src]1.6.0

impl<'a, 'b> PartialEq<Cow<'a, Path>> for PathBuf[src]1.6.0

impl<'a, 'b> PartialEq<Cow<'a, Path>> for OsStr[src]1.8.0

impl<'a, 'b> PartialEq<Cow<'a, Path>> for &'b OsStr[src]1.8.0

impl<'a, 'b> PartialEq<Cow<'a, Path>> for OsString[src]1.8.0

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

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

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

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

impl<'a, 'b> PartialEq<Cow<'b, OsStr>> for &'a Path[src]1.8.0

impl<'a, 'b> PartialEq<OsStr> for Cow<'a, OsStr>[src]1.8.0

impl<'a, 'b> PartialEq<OsStr> for Cow<'a, Path>[src]1.8.0

impl<'a, 'b> PartialEq<OsString> for Cow<'a, OsStr>[src]1.8.0

impl<'a, 'b> PartialEq<OsString> for Cow<'a, Path>[src]1.8.0

impl<'a, 'b> PartialEq<Path> for Cow<'a, Path>[src]1.6.0

impl<'a, 'b> PartialEq<Path> for Cow<'a, OsStr>[src]1.8.0

impl<'a, 'b> PartialEq<PathBuf> for Cow<'a, Path>[src]1.6.0

impl<'a, 'b> PartialEq<PathBuf> for Cow<'a, OsStr>[src]1.8.0

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

impl<'_, A, B> PartialEq<Vec<B>> for Cow<'_, [A]> where
    A: PartialEq<B> + Clone, [src]

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

impl<'a, 'b> PartialOrd<&'a Path> for Cow<'b, OsStr>[src]1.8.0

impl<'a, 'b> PartialOrd<&'b OsStr> for Cow<'a, OsStr>[src]1.8.0

impl<'a, 'b> PartialOrd<&'b OsStr> for Cow<'a, Path>[src]1.8.0

impl<'a, 'b> PartialOrd<&'b Path> for Cow<'a, Path>[src]1.8.0

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

impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for OsStr[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for &'b OsStr[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for OsString[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for PathBuf[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for Path[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for Path[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for &'b Path[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for PathBuf[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for OsStr[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for &'b OsStr[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for OsString[src]1.8.0

impl<'a, 'b> PartialOrd<Cow<'b, OsStr>> for &'a Path[src]1.8.0

impl<'a, 'b> PartialOrd<OsStr> for Cow<'a, OsStr>[src]1.8.0

impl<'a, 'b> PartialOrd<OsStr> for Cow<'a, Path>[src]1.8.0

impl<'a, 'b> PartialOrd<OsString> for Cow<'a, OsStr>[src]1.8.0

impl<'a, 'b> PartialOrd<OsString> for Cow<'a, Path>[src]1.8.0

impl<'a, 'b> PartialOrd<Path> for Cow<'a, Path>[src]1.8.0

impl<'a, 'b> PartialOrd<Path> for Cow<'a, OsStr>[src]1.8.0

impl<'a, 'b> PartialOrd<PathBuf> for Cow<'a, Path>[src]1.8.0

impl<'a, 'b> PartialOrd<PathBuf> for Cow<'a, OsStr>[src]1.8.0

impl<'_> ToString for Cow<'_, str>[src]1.17.0

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

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

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

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

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

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

impl<T> ToOwned for T where
    T: Clone, [src]

type Owned = T

The resulting type after obtaining ownership.

impl<T> ToString for T where
    T: Display + ?Sized, [src]

impl<T, U> TryFrom<U> for T where
    U: Into<T>, [src]

type Error = Infallible

The type returned in the event of a conversion error.

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

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

The type returned in the event of a conversion error.

© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/borrow/enum.Cow.html