Struct Ref

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

An immutable reference to data in a Cell.

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

Implementations

impl<'a, T> Ref<'a, T>

where T: ?Sized,

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

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

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

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

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

Examples

This can 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 `Ref` to store a reference to the `Box`, which is a
// pointer to the value on the heap, not the actual value.
let boxed_ref: Ref<'_, Box<usize>> = cb.borrow();
assert_eq!(**boxed_ref, 5); // Notice the double deref to get the actual value.

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

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

Trait Implementations

impl<'a, T> Clone for Ref<'a, T>

where T: ?Sized,

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

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

where T: ?Sized,

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

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

where T: ?Sized,

fn drop(&mut self)

Executes the destructor for this type.