Enum kvarn_utils::prelude::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);
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"),
}
Variants§
Implementations§
source§impl<B> Cow<'_, B>where
B: ToOwned + ?Sized,
impl<B> Cow<'_, B>where B: ToOwned + ?Sized,
const: unstable · sourcepub fn is_borrowed(&self) -> bool
🔬This is a nightly-only experimental API. (cow_is_borrowed
)
pub 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());
const: unstable · sourcepub fn is_owned(&self) -> bool
🔬This is a nightly-only experimental API. (cow_is_borrowed
)
pub 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());
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>
);
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>
impl<'a> AddAssign<&'a str> for Cow<'a, str>
source§fn add_assign(&mut self, rhs: &'a str)
fn add_assign(&mut self, rhs: &'a str)
+=
operation. Read more1.11.0 · source§impl<B> Default for Cow<'_, B>where
B: ToOwned + ?Sized,
<B as ToOwned>::Owned: Default,
impl<B> Default for Cow<'_, B>where B: ToOwned + ?Sized, <B as ToOwned>::Owned: Default,
const: unstable · source§impl<B> Deref for Cow<'_, B>where
B: ToOwned + ?Sized,
<B as ToOwned>::Owned: Borrow<B>,
impl<B> Deref for Cow<'_, B>where B: ToOwned + ?Sized, <B as ToOwned>::Owned: Borrow<B>,
source§impl<B> Display for Cow<'_, B>where
B: Display + ToOwned + ?Sized,
<B as ToOwned>::Owned: Display,
impl<B> Display for Cow<'_, B>where B: Display + ToOwned + ?Sized, <B as ToOwned>::Owned: Display,
§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
)§impl<'a> Extend<Cow<'a, str>> for CompactString
impl<'a> Extend<Cow<'a, str>> for CompactString
§fn extend<T>(&mut self, iter: T)where
T: IntoIterator<Item = Cow<'a, str>>,
fn extend<T>(&mut self, iter: T)where T: IntoIterator<Item = Cow<'a, str>>,
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> 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>
§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<'a, B> From<Cow<'a, B>> for Arc<B>where
B: ToOwned + ?Sized,
Arc<B>: From<&'a B> + From<<B as ToOwned>::Owned>,
impl<'a, B> From<Cow<'a, B>> for Arc<B>where B: ToOwned + ?Sized, Arc<B>: From<&'a B> + From<<B as ToOwned>::Owned>,
§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
§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>,
§impl<'a> FromIterator<Cow<'a, str>> for CompactString
impl<'a> FromIterator<Cow<'a, str>> for CompactString
§fn from_iter<T>(iter: T) -> CompactStringwhere
T: IntoIterator<Item = Cow<'a, str>>,
fn from_iter<T>(iter: T) -> CompactStringwhere T: IntoIterator<Item = Cow<'a, str>>,
source§impl<B> Ord for Cow<'_, B>where
B: Ord + ToOwned + ?Sized,
impl<B> Ord for Cow<'_, B>where B: Ord + ToOwned + ?Sized,
1.21.0 · source§fn max(self, other: Self) -> Selfwhere
Self: Sized,
fn max(self, other: Self) -> Selfwhere Self: Sized,
source§impl<T, U> PartialEq<&mut [U]> for Cow<'_, [T]>where
T: PartialEq<U> + Clone,
impl<T, U> PartialEq<&mut [U]> for Cow<'_, [T]>where T: PartialEq<U> + Clone,
§impl<'a> PartialEq<CompactString> for Cow<'a, str>
impl<'a> PartialEq<CompactString> for Cow<'a, str>
1.8.0 · source§impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for &'b OsStr
impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for &'b OsStr
1.8.0 · source§impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for OsStr
impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for OsStr
1.8.0 · source§impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for OsString
impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for OsString
1.8.0 · source§impl<'a> PartialEq<Cow<'a, OsStr>> for Path
impl<'a> PartialEq<Cow<'a, OsStr>> for Path
1.8.0 · source§impl<'a> PartialEq<Cow<'a, OsStr>> for PathBuf
impl<'a> PartialEq<Cow<'a, OsStr>> for PathBuf
1.8.0 · source§impl<'a, 'b> PartialEq<Cow<'a, Path>> for &'b OsStr
impl<'a, 'b> PartialEq<Cow<'a, Path>> for &'b OsStr
1.6.0 · source§impl<'a, 'b> PartialEq<Cow<'a, Path>> for &'b Path
impl<'a, 'b> PartialEq<Cow<'a, Path>> for &'b Path
1.8.0 · source§impl<'a> PartialEq<Cow<'a, Path>> for OsStr
impl<'a> PartialEq<Cow<'a, Path>> for OsStr
1.8.0 · source§impl<'a> PartialEq<Cow<'a, Path>> for OsString
impl<'a> PartialEq<Cow<'a, Path>> for OsString
1.6.0 · source§impl<'a> PartialEq<Cow<'a, Path>> for Path
impl<'a> PartialEq<Cow<'a, Path>> for Path
1.6.0 · source§impl<'a> PartialEq<Cow<'a, Path>> for PathBuf
impl<'a> PartialEq<Cow<'a, Path>> for PathBuf
source§impl<'a, 'b> PartialEq<Cow<'a, str>> for &'b str
impl<'a, 'b> PartialEq<Cow<'a, str>> for &'b str
source§impl<'a, 'b> PartialEq<Cow<'a, str>> for String
impl<'a, 'b> PartialEq<Cow<'a, str>> for String
source§impl<'a, 'b> PartialEq<Cow<'a, str>> for str
impl<'a, 'b> PartialEq<Cow<'a, str>> for str
source§impl<'a, 'b, B, C> PartialEq<Cow<'b, C>> for Cow<'a, B>where
B: PartialEq<C> + ToOwned + ?Sized,
C: ToOwned + ?Sized,
impl<'a, 'b, B, C> PartialEq<Cow<'b, C>> for Cow<'a, B>where B: PartialEq<C> + ToOwned + ?Sized, C: ToOwned + ?Sized,
1.8.0 · source§impl<'a, 'b> PartialEq<Cow<'b, OsStr>> for &'a Path
impl<'a, 'b> PartialEq<Cow<'b, OsStr>> for &'a Path
source§impl<T, U, A> PartialEq<Vec<U, A>> for Cow<'_, [T]>where
A: Allocator,
T: PartialEq<U> + Clone,
impl<T, U, A> PartialEq<Vec<U, A>> for Cow<'_, [T]>where A: Allocator, T: PartialEq<U> + Clone,
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<&'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>
source§impl<'a, B> PartialOrd<Cow<'a, B>> for Cow<'a, B>where
B: PartialOrd<B> + ToOwned + ?Sized,
impl<'a, B> PartialOrd<Cow<'a, B>> for Cow<'a, B>where B: PartialOrd<B> + ToOwned + ?Sized,
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>
impl<B> Eq for Cow<'_, B>where B: Eq + ToOwned + ?Sized,
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§
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 more§impl<T> ToCompactString for Twhere
T: Display,
impl<T> ToCompactString for Twhere T: Display,
§fn to_compact_string(&self) -> CompactString
fn to_compact_string(&self) -> CompactString
CompactString
. Read more