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 {
// 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> 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"),
}
Variants§
Implementations§
source§impl<B> Cow<'_, B>
impl<B> Cow<'_, B>
sourcepub const fn is_borrowed(&self) -> bool
🔬This is a nightly-only experimental API. (cow_is_borrowed
)
pub const fn is_borrowed(&self) -> bool
cow_is_borrowed
)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());
sourcepub const fn is_owned(&self) -> bool
🔬This is a nightly-only experimental API. (cow_is_borrowed
)
pub const fn is_owned(&self) -> bool
cow_is_borrowed
)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());
1.0.0 · sourcepub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned
pub fn to_mut(&mut self) -> &mut <B as ToOwned>::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>
);
1.0.0 · sourcepub fn into_owned(self) -> <B as ToOwned>::Owned
pub fn into_owned(self) -> <B as ToOwned>::Owned
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)
);
Trait Implementations§
1.14.0 · source§impl<'a> AddAssign<&'a str> for Cow<'a, str>
Available on non-no_global_oom_handling
only.
impl<'a> AddAssign<&'a str> for Cow<'a, str>
no_global_oom_handling
only.source§fn add_assign(&mut self, rhs: &'a str)
fn add_assign(&mut self, rhs: &'a str)
+=
operation. Read more§impl<'a> Extend<CompactString> for Cow<'a, str>
impl<'a> Extend<CompactString> for Cow<'a, str>
§fn extend<T>(&mut self, iter: T)where
T: IntoIterator<Item = CompactString>,
fn extend<T>(&mut self, iter: T)where
T: IntoIterator<Item = CompactString>,
source§fn extend_one(&mut self, item: A)
fn extend_one(&mut self, item: A)
extend_one
)source§fn extend_reserve(&mut self, additional: usize)
fn extend_reserve(&mut self, additional: usize)
extend_one
)1.52.0 · source§impl<'a> Extend<Cow<'a, OsStr>> for OsString
impl<'a> Extend<Cow<'a, OsStr>> for OsString
source§fn extend<T>(&mut self, iter: T)
fn extend<T>(&mut self, iter: T)
source§fn extend_one(&mut self, item: A)
fn extend_one(&mut self, item: A)
extend_one
)source§fn extend_reserve(&mut self, additional: usize)
fn extend_reserve(&mut self, additional: usize)
extend_one
)§impl<'a> Extend<Cow<'a, str>> for CompactString
impl<'a> Extend<Cow<'a, str>> for CompactString
source§fn extend_one(&mut self, item: A)
fn extend_one(&mut self, item: A)
extend_one
)source§fn extend_reserve(&mut self, additional: usize)
fn extend_reserve(&mut self, additional: usize)
extend_one
)1.19.0 · source§impl<'a> Extend<Cow<'a, str>> for String
Available on non-no_global_oom_handling
only.
impl<'a> Extend<Cow<'a, str>> for String
no_global_oom_handling
only.source§fn extend<I>(&mut self, iter: I)
fn extend<I>(&mut self, iter: I)
source§fn extend_one(&mut self, s: Cow<'a, str>)
fn extend_one(&mut self, s: Cow<'a, str>)
extend_one
)source§fn extend_reserve(&mut self, additional: usize)
fn extend_reserve(&mut self, additional: usize)
extend_one
)§impl<'a> From<&'a CompactString> for Cow<'a, str>
impl<'a> From<&'a CompactString> for Cow<'a, str>
§fn from(s: &'a CompactString) -> Cow<'a, str>
fn from(s: &'a CompactString) -> Cow<'a, str>
1.28.0 · source§impl<'a> From<&'a String> for Cow<'a, str>
Available on non-no_global_oom_handling
only.
impl<'a> From<&'a String> for Cow<'a, str>
no_global_oom_handling
only.§impl From<CompactString> for Cow<'_, str>
impl From<CompactString> for Cow<'_, str>
§fn from(s: CompactString) -> Cow<'_, str>
fn from(s: CompactString) -> Cow<'_, str>
1.45.0 · source§impl<T> From<Cow<'_, [T]>> for Box<[T]>where
T: Clone,
Available on non-no_global_oom_handling
only.
impl<T> From<Cow<'_, [T]>> for Box<[T]>where
T: Clone,
no_global_oom_handling
only.1.45.0 · source§impl From<Cow<'_, str>> for Box<str>
Available on non-no_global_oom_handling
only.
impl From<Cow<'_, str>> for Box<str>
no_global_oom_handling
only.source§fn from(cow: Cow<'_, str>) -> Box<str>
fn from(cow: Cow<'_, str>) -> Box<str>
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}");
1.14.0 · source§impl<'a, T> From<Cow<'a, [T]>> for Vec<T>
impl<'a, T> From<Cow<'a, [T]>> for Vec<T>
source§fn from(s: Cow<'a, [T]>) -> Vec<T>
fn from(s: Cow<'a, [T]>) -> Vec<T>
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));
§impl<'a> From<Cow<'a, str>> for CompactString
impl<'a> From<Cow<'a, str>> for CompactString
§fn from(cow: Cow<'a, str>) -> CompactString
fn from(cow: Cow<'a, str>) -> CompactString
1.14.0 · source§impl<'a> From<Cow<'a, str>> for String
Available on non-no_global_oom_handling
only.
impl<'a> From<Cow<'a, str>> for String
no_global_oom_handling
only.source§fn from(s: Cow<'a, str>) -> String
fn from(s: Cow<'a, str>) -> String
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
// If the string is not owned...
let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
// It will allocate on the heap and copy the string.
let owned: String = String::from(cow);
assert_eq!(&owned[..], "eggplant");
1.22.0 · source§impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + 'a>
Available on non-no_global_oom_handling
only.
impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + 'a>
no_global_oom_handling
only.1.22.0 · source§impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a>
Available on non-no_global_oom_handling
only.
impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a>
no_global_oom_handling
only.source§fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a>
fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a>
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))
1.12.0 · source§impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str>
Available on non-no_global_oom_handling
only.
impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str>
no_global_oom_handling
only.§impl FromIterator<CompactString> for Cow<'_, str>
impl FromIterator<CompactString> for Cow<'_, str>
§fn from_iter<T>(iter: T) -> Cow<'_, str>where
T: IntoIterator<Item = CompactString>,
fn from_iter<T>(iter: T) -> Cow<'_, str>where
T: IntoIterator<Item = CompactString>,
1.80.0 · source§impl<'a> FromIterator<Cow<'a, str>> for Box<str>
Available on non-no_global_oom_handling
only.
impl<'a> FromIterator<Cow<'a, str>> for Box<str>
no_global_oom_handling
only.§impl<'a> FromIterator<Cow<'a, str>> for CompactString
impl<'a> FromIterator<Cow<'a, str>> for CompactString
§fn from_iter<T>(iter: T) -> CompactString
fn from_iter<T>(iter: T) -> CompactString
1.19.0 · source§impl<'a> FromIterator<Cow<'a, str>> for String
Available on non-no_global_oom_handling
only.
impl<'a> FromIterator<Cow<'a, str>> for String
no_global_oom_handling
only.1.12.0 · source§impl<'a> FromIterator<String> for Cow<'a, str>
Available on non-no_global_oom_handling
only.
impl<'a> FromIterator<String> for Cow<'a, str>
no_global_oom_handling
only.1.12.0 · source§impl<'a> FromIterator<char> for Cow<'a, str>
Available on non-no_global_oom_handling
only.
impl<'a> FromIterator<char> for Cow<'a, str>
no_global_oom_handling
only.1.0.0 · source§impl<B> Ord for Cow<'_, B>
impl<B> Ord for Cow<'_, B>
1.21.0 · source§fn max(self, other: Self) -> Selfwhere
Self: Sized,
fn max(self, other: Self) -> Selfwhere
Self: Sized,
§impl<'a> PartialEq<CompactString> for &Cow<'a, str>
impl<'a> PartialEq<CompactString> for &Cow<'a, str>
§impl<'a> PartialEq<CompactString> for Cow<'a, str>
impl<'a> PartialEq<CompactString> for Cow<'a, str>
§impl<'a> PartialEq<Cow<'a, str>> for &CompactString
impl<'a> PartialEq<Cow<'a, str>> for &CompactString
1.8.0 · source§impl<'a, 'b> PartialOrd<&'b OsStr> for Cow<'a, OsStr>
impl<'a, 'b> PartialOrd<&'b OsStr> for Cow<'a, OsStr>
1.8.0 · source§impl<'a, 'b> PartialOrd<&'b OsStr> for Cow<'a, Path>
impl<'a, 'b> PartialOrd<&'b OsStr> for Cow<'a, Path>
1.8.0 · source§impl<'a, 'b> PartialOrd<&'b Path> for Cow<'a, Path>
impl<'a, 'b> PartialOrd<&'b Path> for Cow<'a, Path>
1.8.0 · source§impl<'a, 'b> PartialOrd<&'a Path> for Cow<'b, OsStr>
impl<'a, 'b> PartialOrd<&'a Path> for Cow<'b, OsStr>
1.8.0 · source§impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for &'b OsStr
impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for &'b OsStr
1.8.0 · source§impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for OsStr
impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for OsStr
1.8.0 · source§impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for OsString
impl<'a, 'b> PartialOrd<Cow<'a, OsStr>> for OsString
1.8.0 · source§impl<'a> PartialOrd<Cow<'a, OsStr>> for Path
impl<'a> PartialOrd<Cow<'a, OsStr>> for Path
1.8.0 · source§impl<'a> PartialOrd<Cow<'a, OsStr>> for PathBuf
impl<'a> PartialOrd<Cow<'a, OsStr>> for PathBuf
1.8.0 · source§impl<'a, 'b> PartialOrd<Cow<'a, Path>> for &'b OsStr
impl<'a, 'b> PartialOrd<Cow<'a, Path>> for &'b OsStr
1.8.0 · source§impl<'a, 'b> PartialOrd<Cow<'a, Path>> for &'b Path
impl<'a, 'b> PartialOrd<Cow<'a, Path>> for &'b Path
1.8.0 · source§impl<'a> PartialOrd<Cow<'a, Path>> for OsStr
impl<'a> PartialOrd<Cow<'a, Path>> for OsStr
1.8.0 · source§impl<'a> PartialOrd<Cow<'a, Path>> for OsString
impl<'a> PartialOrd<Cow<'a, Path>> for OsString
1.8.0 · source§impl<'a> PartialOrd<Cow<'a, Path>> for Path
impl<'a> PartialOrd<Cow<'a, Path>> for Path
1.8.0 · source§impl<'a> PartialOrd<Cow<'a, Path>> for PathBuf
impl<'a> PartialOrd<Cow<'a, Path>> for PathBuf
1.8.0 · source§impl<'a, 'b> PartialOrd<Cow<'b, OsStr>> for &'a Path
impl<'a, 'b> PartialOrd<Cow<'b, OsStr>> for &'a Path
1.8.0 · source§impl<'a, 'b> PartialOrd<OsStr> for Cow<'a, OsStr>
impl<'a, 'b> PartialOrd<OsStr> for Cow<'a, OsStr>
1.8.0 · source§impl<'a> PartialOrd<OsStr> for Cow<'a, Path>
impl<'a> PartialOrd<OsStr> for Cow<'a, Path>
1.8.0 · source§impl<'a, 'b> PartialOrd<OsString> for Cow<'a, OsStr>
impl<'a, 'b> PartialOrd<OsString> for Cow<'a, OsStr>
1.8.0 · source§impl<'a> PartialOrd<OsString> for Cow<'a, Path>
impl<'a> PartialOrd<OsString> for Cow<'a, Path>
1.8.0 · source§impl<'a> PartialOrd<Path> for Cow<'a, OsStr>
impl<'a> PartialOrd<Path> for Cow<'a, OsStr>
1.8.0 · source§impl<'a> PartialOrd<Path> for Cow<'a, Path>
impl<'a> PartialOrd<Path> for Cow<'a, Path>
1.8.0 · source§impl<'a> PartialOrd<PathBuf> for Cow<'a, OsStr>
impl<'a> PartialOrd<PathBuf> for Cow<'a, OsStr>
1.8.0 · source§impl<'a> PartialOrd<PathBuf> for Cow<'a, Path>
impl<'a> PartialOrd<PathBuf> for Cow<'a, Path>
1.0.0 · source§impl<'a, B> PartialOrd for Cow<'a, B>
impl<'a, B> PartialOrd for Cow<'a, B>
impl<B> DerefPure for Cow<'_, B>
impl<B> Eq for Cow<'_, B>
Auto Trait Implementations§
impl<'a, B> Freeze for Cow<'a, B>
impl<'a, B> RefUnwindSafe for Cow<'a, B>
impl<'a, B> Send for Cow<'a, B>
impl<'a, B> Sync for Cow<'a, B>
impl<'a, B> Unpin for Cow<'a, B>
impl<'a, B> UnwindSafe for Cow<'a, B>
Blanket Implementations§
source§impl<T> AsCleanDebug for Twhere
T: Display,
impl<T> AsCleanDebug for Twhere
T: Display,
source§fn as_clean(&self) -> CleanDebug<'_, Self>where
Self: Display,
fn as_clean(&self) -> CleanDebug<'_, Self>where
Self: Display,
CleanDebug
for Self. Read moresource§impl<T> BorrowMut<T> for Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere
T: ?Sized,
source§fn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
source§impl<T> CloneToUninit for Twhere
T: Clone,
impl<T> CloneToUninit for Twhere
T: Clone,
source§unsafe fn clone_to_uninit(&self, dst: *mut T)
unsafe fn clone_to_uninit(&self, dst: *mut T)
clone_to_uninit
)§impl<Q, K> Comparable<K> for Q
impl<Q, K> Comparable<K> for Q
§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
key
and return true
if they are equal.§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
§impl<T> Instrument for T
impl<T> Instrument for T
§fn instrument(self, span: Span) -> Instrumented<Self> ⓘ
fn instrument(self, span: Span) -> Instrumented<Self> ⓘ
§impl<T> Pointable for T
impl<T> Pointable for T
§impl<T> ToCompactString for Twhere
T: Display,
impl<T> ToCompactString for Twhere
T: Display,
§fn try_to_compact_string(&self) -> Result<CompactString, ToCompactStringError>
fn try_to_compact_string(&self) -> Result<CompactString, ToCompactStringError>
ToCompactString::to_compact_string()
Read more§fn to_compact_string(&self) -> CompactString
fn to_compact_string(&self) -> CompactString
CompactString
. Read more