القابلية للتغيير الداخلي (Interior Mutability)
In some situations, it’s necessary to modify data behind a shared (read-only) reference. For example, a shared data structure might have an internal cache, and wish to update that cache from read-only methods.
The “interior mutability” pattern allows exclusive (mutable) access behind a shared reference. The standard library provides several ways to do this, all while still ensuring safety, typically by performing a runtime check.
RefCell
Cell
Cell
wraps a value and allows getting or setting the value, even with a shared reference to the Cell
. However, it does not allow any references to the value. Since there are no references, borrowing rules cannot be broken.
Speaker Notes
This slide should take about 10 minutes.
The main thing to take away from this slide is that Rust provides safe ways to modify data behind a shared reference. There are a variety of ways to ensure that safety, and RefCell
and Cell
are two of them.
-
RefCell
enforces Rust’s usual borrowing rules (either multiple shared references or a single exclusive reference) with a runtime check. In this case, all borrows are very short and never overlap, so the checks always succeed.- The extra block in the
RefCell
example is to end the borrow created by the call toborrow_mut
before we print the cell. Trying to print a borrowedRefCell
just shows the message"{borrowed}"
.
- The extra block in the
-
Cell
is a simpler means to ensure safety: it has aset
method that takes&self
. This needs no runtime check, but requires moving values, which can have its own cost. -
Both
RefCell
andCell
are!Sync
, which means&RefCell
and&Cell
can’t be passed between threads. This prevents two threads trying to access the cell at once.