Enum std::borrow::Cow

1.0.0 · source ·

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's 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.

Enum std::borrow::Cow

Variants§

Borrowed(&'a B)

Owned(<B as ToOwned>::Owned)

Implementations§

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

pub fn is_borrowed(&self) -> bool

pub fn is_owned(&self) -> bool

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

pub fn into_owned(self) -> <B as ToOwned>::Owned

Trait Implementations§

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

type Output = Cow<'a, str>

fn add(self, rhs: &'a str) -> <Cow<'a, str> as Add<&'a str>>::Output

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

type Output = Cow<'a, str>

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

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

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

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

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

impl AsRef<Path> for Cow<'_, OsStr>

fn as_ref(&self) -> &Path

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

fn as_ref(&self) -> &T

impl<'a, B> Borrow<B> for Cow<'a, B>where B: ToOwned + ?Sized,

fn borrow(&self) -> &B

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

fn clone(&self) -> Cow<'_, B>

fn clone_from(&mut self, source: &Cow<'_, B>)

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

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

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

fn default() -> Cow<'_, B>

impl<B> Deref for Cow<'_, B>where B: ToOwned + ?Sized, <B as ToOwned>::Owned: Borrow<B>,

type Target = B

fn deref(&self) -> &B

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

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

impl<'a> Extend<Cow<'a, OsStr>> for OsString

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

fn extend_one(&mut self, item: A)

fn extend_reserve(&mut self, additional: usize)

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

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

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

fn extend_reserve(&mut self, additional: usize)

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

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

impl<'a> From<&'a CStr> for Cow<'a, CStr>

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

impl<'a> From<&'a CString> for Cow<'a, CStr>

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

impl<'a> From<&'a OsStr> for Cow<'a, OsStr>

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

impl<'a> From<&'a OsString> for Cow<'a, OsStr>

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

impl<'a> From<&'a Path> for Cow<'a, Path>

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

impl<'a> From<&'a PathBuf> for Cow<'a, Path>

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

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

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

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

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

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

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

impl<'a> From<CString> for Cow<'a, CStr>

fn from(s: CString) -> Cow<'a, CStr>

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

fn from(cow: Cow<'_, [T]>) -> Box<[T], Global>

impl From<Cow<'_, CStr>> for Box<CStr, Global>

fn from(cow: Cow<'_, CStr>) -> Box<CStr, Global>

impl From<Cow<'_, OsStr>> for Box<OsStr>

fn from(cow: Cow<'_, OsStr>) -> Box<OsStr>

impl From<Cow<'_, Path>> for Box<Path>

fn from(cow: Cow<'_, Path>) -> Box<Path>

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

fn from(cow: Cow<'_, str>) -> Box<str, Global>

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