pub enum Cow<'a, B>{
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 {
input.to_mut()[i] = -v;
}
}
}
let slice = [0, 1, 2];
let mut input = Cow::from(&slice[..]);
abs_all(&mut input);
let slice = [-1, 0, 1];
let mut input = Cow::from(&slice[..]);
abs_all(&mut input);
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> 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 }
}
}
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;
clone_on_write.values.to_mut().push(3);
println!("clone_on_write = {:?}", clone_on_write.values);
match clone_on_write {
Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
_ => panic!("expect owned data"),
}
🔬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());
🔬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());
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>
);
Extracts the owned data.
Clones the data if it is not already owned.
§Examples
Calling into_owned on a Cow::Borrowed returns a clone of the borrowed data:
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 returns the owned data. The data is moved out of the
Cow without being cloned.
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)
);
The resulting type after applying the + operator.
The resulting type after applying the + operator.
Converts this type into a shared reference of the (usually inferred) input type.
Immutably borrows from an owned value.
Read more
Performs copy-assignment from
source.
Read more
Formats the value using the given formatter.
Read more
Creates an owned Cow<’a, B> with the default value for the contained owned value.
The resulting type after dereferencing.
Dereferences the value.
Formats the value using the given formatter.
Read more
Extends a collection with the contents of an iterator.
Read more
🔬This is a nightly-only experimental API. (extend_one #72631) Extends a collection with exactly one element.
🔬This is a nightly-only experimental API. (extend_one #72631) Reserves capacity in a collection for the given number of additional elements.
Read more
Creates a Borrowed variant of Cow
from a slice.
This conversion does not allocate or clone the data.
Creates a Borrowed variant of Cow
from a reference to an array.
This conversion does not allocate or clone the data.
Converts a CStr into a borrowed Cow without copying or allocating.
Converts a &CString into a borrowed Cow without copying or allocating.
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"));
Creates a Borrowed variant of Cow
from a reference to Vec.
This conversion does not allocate or clone the data.
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"));
Converts a CString into an owned Cow without copying or allocating.
Converts a Cow<'_, [T]> into a Box<[T]>
When cow is the Cow::Borrowed variant, this
conversion allocates on the heap and copies the
underlying slice. Otherwise, it will try to reuse the owned
Vec’s allocation.
Converts a Cow<'a, CStr> into a Box<CStr>,
by copying the contents if they are borrowed.
Converts a Cow<'_, str> into a Box<str>
When cow is the Cow::Borrowed variant, this
conversion allocates on the heap and copies the
underlying str. Otherwise, it will try to reuse the owned
String’s allocation.
§Examples
use std::borrow::Cow;
let unboxed = Cow::Borrowed("hello");
let boxed: Box<str> = Box::from(unboxed);
println!("{boxed}");
let unboxed = Cow::Owned("hello".to_string());
let boxed: Box<str> = Box::from(unboxed);
println!("{boxed}");
Converts 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));
Creates 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[..]);
Creates 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[..]);
Converts a Cow<'a, CStr> into a CString, by copying the contents if they are
borrowed.
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
let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
let owned: String = String::from(cow);
assert_eq!(&owned[..], "eggplant");
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))
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))
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));
Creates an Owned variant of Cow
from an owned instance of Vec.
This conversion does not allocate or clone the data.
Compares and returns the maximum of two values.
Read more
Compares and returns the minimum of two values.
Read more
Restrict a value to a certain interval.
Read more
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient,
and should not be overridden without very good reason.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient,
and should not be overridden without very good reason.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient,
and should not be overridden without very good reason.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient,
and should not be overridden without very good reason.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient,
and should not be overridden without very good reason.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient,
and should not be overridden without very good reason.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient,
and should not be overridden without very good reason.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient,
and should not be overridden without very good reason.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient,
and should not be overridden without very good reason.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient,
and should not be overridden without very good reason.
This method returns an ordering between
self and
other values if one exists.
Read more
Tests less than (for
self and
other) and is used by the
< operator.
Read more
Tests less than or equal to (for
self and
other) and is used by the
<= operator.
Read more
Tests greater than (for
self and
other) and is used by the
>
operator.
Read more
Tests greater than or equal to (for
self and
other) and is used by
the
>= operator.
Read more
Immutably borrows from an owned value.
Read more
Mutably borrows from an owned value.
Read more
🔬This is a nightly-only experimental API. (clone_to_uninit #126799) Performs copy-assignment from
self to
dst.
Read more
Returns the argument unchanged.
Calls U::from(self).
That is, this conversion is whatever the implementation of
From<T> for U chooses to do.
The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning.
Read more
Uses borrowed data to replace owned data, usually by cloning.
Read more
Converts the given value to a
String.
Read more
The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.