1.0.0[][src]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);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(<B as ToOwned>::Owned)

Owned data.

Methods

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());Run

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());Run

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>
);Run

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

impl<'a> Extend<Cow<'a, str>> for String1.19.0[src]

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

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

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

type Target = B

The resulting type after dereferencing.

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

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

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

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

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

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

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

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

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

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

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

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

impl<'a> From<Cow<'a, str>> for String1.14.0[src]

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

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

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

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

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

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

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.

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.

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

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

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

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

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

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

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

impl<'a> FromIterator<Cow<'a, str>> for String1.19.0[src]

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

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

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

Important traits for Box<F>
fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a>[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<Cow<'a, str>> for Box<dyn Error>1.22.0[src]

Important traits for Box<F>
fn from(err: Cow<'a, str>) -> Box<dyn Error>[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> From<Cow<'a, CStr>> for CString1.28.0[src]

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

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

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

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

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

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

impl<'a> From<Cow<'a, OsStr>> for OsString1.28.0[src]

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

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

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

impl<'a> From<Cow<'a, Path>> for PathBuf1.28.0[src]

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

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for OsStr1.8.0[src]

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

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for &'b OsStr1.8.0[src]

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

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for OsString1.8.0[src]

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

impl<'a, 'b> PartialEq<Cow<'a, Path>> for Path1.6.0[src]

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

impl<'a, 'b> PartialEq<Cow<'a, Path>> for &'b Path1.6.0[src]

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

impl<'a, 'b> PartialEq<Cow<'a, Path>> for PathBuf1.6.0[src]

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for PathBuf1.8.0[src]

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

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for Path1.8.0[src]

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

impl<'a, 'b> PartialEq<Cow<'b, OsStr>> for &'a Path1.8.0[src]

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

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

impl<'a, 'b> PartialEq<Cow<'a, Path>> for OsStr1.8.0[src]

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

impl<'a, 'b> PartialEq<Cow<'a, Path>> for &'b OsStr1.8.0[src]

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

impl<'a, 'b> PartialEq<Cow<'a, Path>> for OsString1.8.0[src]

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

impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for OsStr1.8.0[src]

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

impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for &'b OsStr1.8.0[src]

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

impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for OsString1.8.0[src]

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

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for Path1.8.0[src]

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

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for &'b Path1.8.0[src]

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

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for PathBuf1.8.0[src]

impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for PathBuf1.8.0[src]

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

impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for Path1.8.0[src]

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

impl<'a, 'b> PartialOrd<Cow<'b, OsStr>> for &'a Path1.8.0[src]

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

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

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for OsStr1.8.0[src]

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

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for &'b OsStr1.8.0[src]

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

impl<'a, 'b> PartialOrd<Cow<'a, Path>> for OsString1.8.0[src]

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

Auto Trait Implementations

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

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

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

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

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.

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

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

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