//@ (There's not even an automatic derivation happening for the cases where it would be possible.)
//@ This restriction propagates up to `Callbacks` itself. What could we do about this?
+//@ ## `Rc`
//@ The solution is to find some way of cloning `Callbacks` without cloning the environments. This can be achieved with
//@ `Rc<T>`, a *reference-counted* pointer. This is is another example of a smart pointer. You can `clone` an `Rc` as often
//@ as you want, that doesn't affect the data it contains. It only creates more references to the same data. Once all the
impl Callbacks {
pub fn new() -> Self {
- Callbacks { callbacks: Vec::new() } /*@*/
+ Callbacks { callbacks: Vec::new() }
}
// Registration works just like last time, except that we are creating an `Rc` now.
pub fn call(&self, val: i32) {
// We only need a shared iterator here. Since `Rc` is a smart pointer, we can directly call the callback.
for callback in self.callbacks.iter() {
- callback(val); /*@*/
+ callback(val); /*@*/
}
}
}
//@ `set`, which overrides the content, only needs a *shared borrow* of the cell. The phenomenon of a type that permits mutation through
//@ shared borrows (i.e., mutation despite the possibility of aliasing) is called *interior mutability*. You can think
//@ of `set` changing only the *contents* of the cell, not its *identity*. In contrast, the kind of mutation we saw so far was
-//@ about replacing one piece of data by something else of the same type. This is called *exterior mutability*. <br/>
+//@ about replacing one piece of data by something else of the same type. This is called *inherited mutability*. <br/>
//@ Notice that it is impossible to *borrow* the contents of the cell, and that is actually the key to why this is safe.
// So, let us put our counter in a `Cell`, and replicate the example from the previous part.
impl CallbacksMut {
pub fn new() -> Self {
- CallbacksMut { callbacks: Vec::new() } /*@*/
+ CallbacksMut { callbacks: Vec::new() }
}
pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {
- let cell = Rc::new(RefCell::new(callback));
+ let cell = Rc::new(RefCell::new(callback)); /*@*/
self.callbacks.push(cell); /*@*/
}
//@ appropriately updates the number of active borrows.
//@
//@ Since `call` is the only place that borrows the environments of the closures, we should expect that
- //@ the check will always succeed. However, this function would still typecheck with an immutable borrow of `self` (since we are
- //@ relying on the interior mutability of `RefCell`). Under this condition, it could happen that a callback
- //@ will in turn trigger another round of callbacks, so that `call` would indirectly call itself.
- //@ This is called reentrancy. It would imply that we borrow the closure a second time, and
- //@ panic at run-time. I hope this also makes it clear that there's absolutely no hope of Rust
- //@ performing these checks statically, at compile-time: It would have to detect reentrancy!
+ //@ the check will always succeed. However, this is not actually true. Several different `CallbacksMut` could share
+ //@ a callback (as they were created with `clone`), and calling one callback here could trigger calling
+ //@ all callbacks of the other `CallbacksMut`, which would end up calling the initial callback again. This issue is called *reentrancy*,
+ //@ and it can lead to subtle bugs. Here, it would mean that the closure runs twice, each time thinking it has the only
+ //@ mutable borrow of its environment - so it may end up dereferencing a dangling pointer. Ouch! Lucky enough,
+ //@ Rust detects this at run-time and panics once we try to borrow the same environment again. I hope this also makes it
+ //@ clear that there's absolutely no hope of Rust performing these checks statically, at compile-time: It would have to detect reentrancy!
let mut closure = callback.borrow_mut();
// Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We thus have to explicitly
// dereference the smart pointer and obtain a mutable borrow of the content.
c.call(1); c.clone().call(2);
}
-// **Exercise 12.1**: Change the type of `call` to ask only for a shared borrow. Then write some piece of code using only the available, public
-// interface of `CallbacksMut` such that a reentrant call to `call` is happening, and the program aborts because the `RefCell` refuses to hand
-// out a second mutable borrow to its content.
+// **Exercise 12.1**: Write some piece of code using only the available, public interface of `CallbacksMut` such that a reentrant call to a closure
+// is happening, and the program aborts because the `RefCell` refuses to hand out a second mutable borrow of the closure's environment.
-//@ [index](main.html) | [previous](part11.html) | [next](part13.html)
+//@ [index](main.html) | [previous](part11.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part12.rs) | [next](part13.html)
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