Struct RefMut

pub struct RefMut<'a, T>
where
    T: ?Sized + 'a,
{ /* private fields */ }

A mutable reference to data in a Cell.

Access the value via std::ops::DerefMut (e.g. *val)

Implementations

impl<'a, T> RefMut<'a, T>

where T: ?Sized,

pub fn map<U, F>(self, f: F) -> RefMut<'a, U>

where F: FnOnce(&mut T) -> &mut U, U: ?Sized,

Makes a new RefMut for a component of the borrowed data which preserves the existing borrow.

The Cell is already mutably borrowed, so this cannot fail.

This is an associated function that needs to be used as RefMut::map(...). A method would interfere with methods of the same name on the contents of a RefMut used through DerefMut. Further this preserves the borrow of the value and hence does the proper cleanup when it’s dropped.

Examples

This can also be used to avoid pointer indirection when a boxed item is stored in the Cell.

use async_ecs::resource::cell::*;

let cb = Cell::new(Box::new(5));

// Borrowing the cell causes the `RefMut` to store a reference to the `Box`, which is a
// pointer to the value on the heap, and not a reference directly to the value.
let boxed_ref: RefMut<'_, Box<usize>> = cb.borrow_mut();
assert_eq!(**boxed_ref, 5); // Notice the double deref to get the actual value.

// By using `map` we can let `RefMut` store a reference directly to the value on the heap.
let pure_ref: RefMut<'_, usize> = RefMut::map(boxed_ref, Box::as_mut);

assert_eq!(*pure_ref, 5);

We can also use map to get a reference to a sub-part of the borrowed value.

let c = Cell::new((5, 'b'));

let b1: RefMut<'_, (u32, char)> = c.borrow_mut();
let b2: RefMut<'_, u32> = RefMut::map(b1, |t| &mut t.0);
assert_eq!(*b2, 5);

Trait Implementations

impl<'a, T> Debug for RefMut<'a, T>

where T: ?Sized + 'a + Debug,

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

Formats the value using the given formatter.

impl<'a, T> Deref for RefMut<'a, T>

where T: ?Sized,

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

impl<'a, T> DerefMut for RefMut<'a, T>

where T: ?Sized,

fn deref_mut(&mut self) -> &mut T

Mutably dereferences the value.

impl<'a, T> Drop for RefMut<'a, T>

where T: ?Sized,

fn drop(&mut self)

Executes the destructor for this type.