use std::rc::Rc;
use std::cell::{Cell, RefCell};
-//@ Our generic callback mechanism is already working quite nicely. However, there's one point we may want to fix:
-//@ `Callbacks` does not implement `Clone`. The problem is that closures (or rather, their environment) can never be cloned.
+//@ Our generic callback mechanism is already working quite nicely. However, there's one point we
+//@ may want to fix: `Callbacks` does not implement `Clone`. The problem is that closures (or
+//@ rather, their environment) can never be cloned.
//@ (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?
-//@ 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
-//@ references are gone, the data is deleted.
+//@ ## `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 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 references are gone,
+//@ the data is deleted.
//@
//@ Wait a moment, you may say here. Multiple references to the same data? That's aliasing! Indeed:
-//@ Once data is stored in an `Rc`, it is read-only and you can only ever get a shared borrow of the data again.
+//@ Once data is stored in an `Rc`, it is read-only and you can only ever get a shared reference to the data again.
-//@ Because of this read-only restriction, we cannot use `FnMut` here: We'd be unable to call the function with a mutable borrow
-//@ of it's environment! So we have to go with `Fn`. We wrap that in an `Rc`, and then Rust happily derives `Clone` for us.
+//@ Because of this read-only restriction, we cannot use `FnMut` here: We'd be unable to call the
+//@ function with a mutable reference to it's environment! So we have to go with `Fn`. We wrap that
+//@ in an `Rc`, and then Rust happily derives `Clone` for us.
#[derive(Clone)]
struct Callbacks {
callbacks: Vec<Rc<Fn(i32)>>,
}
// ## Interior Mutability
-//@ Of course, the counting example from last time does not work anymore: It needs to mutate the environment, which a `Fn`
-//@ cannot do. The strict borrowing Rules of Rust are getting into our way. However, when it comes to mutating a mere number
-//@ (`usize`), there's not really any chance of problems coming up. Everybody can read and write that variable just as they want.
-//@ So it would be rather sad if we were not able to write this program. Lucky enough, Rust's standard library provides a
-//@ solution in the form of `Cell<T>`. This type represents a memory cell of some type `T`, providing the two basic operations
-//@ `get` and `set`. `get` returns a *copy* of the content of the cell, so all this works only if `T` is `Copy`.
-//@ `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/>
-//@ Notice that it is impossible to *borrow* the contents of the cell, and that is actually the key to why this is safe.
+//@ Of course, the counting example from last time does not work anymore: It needs to mutate the
+//@ environment, which a `Fn` cannot do. The strict borrowing Rules of Rust are getting into our
+//@ way. However, when it comes to mutating a mere number (`usize`), there's not really any chance
+//@ of problems coming up. Everybody can read and write that variable just as they want.
+//@ So it would be rather sad if we were not able to write this program. Lucky enough, Rust's
+//@ standard library provides a solution in the form of `Cell<T>`. This type represents a memory
+//@ cell of some type `T`, providing the two basic operations `get` and `set`. `get` returns a
+//@ *copy* of the content of the cell, so all this works only if `T` is `Copy`.
+//@ `set`, which overrides the content, only needs a *shared reference* to the cell. The phenomenon
+//@ of a type that permits mutation through shared references (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
+//@ *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.
fn demo_cell(c: &mut Callbacks) {
{
let count = Cell::new(0);
- // Again, we have to move ownership if the `count` into the environment closure.
+ // Again, we have to move ownership of the `count` into the environment closure.
c.register(move |val| {
- // In here, all we have is a shared borrow of our environment. But that's good enough for the `get` and `set` of the cell!
- //@ At run-time, the `Cell` will be almost entirely compiled away, so this becomes pretty much equivalent to the version
- //@ we wrote in the previous part.
+ // In here, all we have is a shared reference of our environment. But that's good enough
+ // for the `get` and `set` of the cell!
+ //@ At run-time, the `Cell` will be almost entirely compiled away, so this becomes
+ //@ pretty much equivalent to the version we wrote in the previous part.
let new_count = count.get()+1;
count.set(new_count);
println!("Callback 2: {} ({}. time)", val, new_count);
c.call(2); c.clone().call(3);
}
-//@ It is worth mentioning that `Rc` itself also has to make use of interior mutability: When you `clone` an `Rc`, all it has available
-//@ is a shared borrow. However, it has to increment the reference count! Internally, `Rc` uses `Cell` for the count, such that it
-//@ can be updated during `clone`.
+//@ It is worth mentioning that `Rc` itself also has to make use of interior mutability: When you
+//@ `clone` an `Rc`, all it has available is a shared reference. However, it has to increment the
+//@ reference count! Internally, `Rc` uses `Cell` for the count, such that it can be updated during
+//@ `clone`.
+//@
+//@ Putting it all together, the story around mutation and ownership through references looks as
+//@ follows: There are *unique* references, which - because of their exclusivity - are always safe
+//@ to mutate through. And there are *shared* references, where the compiler cannot generally
+//@ promise that mutation is safe. However, if extra circumstances guarantee that mutation *is*
+//@ safe, then it can happen even through a shared reference - as we saw with `Cell`.
// ## `RefCell`
-//@ As the next step in the evolution of `Callbacks`, we could try to solve this problem of mutability once and for all, by adding `Cell`
-//@ to `Callbacks` such that clients don't have to worry about this. However, that won't end up working: Remember that `Cell` only works
-//@ with types that are `Copy`, which the environment of a closure will never be. We need a variant of `Cell` that allows borrowing its
-//@ contents, such that we can provide a `FnMut` with its environment. But if `Cell` would allow that, we could write down all those
-//@ crashing C++ programs that we wanted to get rid of.
+//@ As the next step in the evolution of `Callbacks`, we could try to solve this problem of
+//@ mutability once and for all, by adding `Cell` to `Callbacks` such that clients don't have to
+//@ worry about this. However, that won't end up working: Remember that `Cell` only works with
+//@ types that are `Copy`, which the environment of a closure will never be. We need a variant of
+//@ `Cell` that allows borrowing its contents, such that we can provide a `FnMut` with its
+//@ environment. But if `Cell` would allow that, we could write down all those crashing C++
+//@ programs that we wanted to get rid of.
//@
-//@ This is the point where our program got too complex for Rust to guarantee at compile-time that nothing bad will happen. Since we don't
-//@ want to give up the safety guarantee, we are going to need some code that actually checks at run-time that the borrowing rules
-//@ are not violated. Such a check is provided by `RefCell<T>`: Unlike `Cell<T>`, this lets us borrow the contents, and it works for
-//@ non-`Copy` `T`. But, as we will see, it incurs some run-time overhead.
+//@ This is the point where our program got too complex for Rust to guarantee at compile-time that
+//@ nothing bad will happen. Since we don't want to give up the safety guarantee, we are going to
+//@ need some code that actually checks at run-time that the borrowing rules are not violated. Such
+//@ a check is provided by `RefCell<T>`: Unlike `Cell<T>`, this lets us borrow the contents, and it
+//@ works for non-`Copy` `T`. But, as we will see, it incurs some run-time overhead.
// Our final version of `Callbacks` puts the closure environment into a `RefCell`.
#[derive(Clone)]
pub fn call(&mut self, val: i32) {
for callback in self.callbacks.iter() {
- // We have to *explicitly* borrow the contents of a `RefCell` by calling `borrow` or `borrow_mut`.
- //@ At run-time, the cell will keep track of the number of outstanding shared and mutable borrows,
- //@ and panic if the rules are violated. <br />
- //@ For this check to be performed, `closure` is a *guard*: Rather than a normal borrow, `borrow_mut` returns
- //@ a smart pointer (`RefMut`, in this case) that waits until is goes out of scope, and then
- //@ appropriately updates the number of active borrows.
+ // We have to *explicitly* borrow the contents of a `RefCell` by calling `borrow` or
+ // `borrow_mut`.
+ //@ At run-time, the cell will keep track of the number of outstanding shared and
+ //@ mutable references, and panic if the rules are violated. <br />
+ //@ For this check to be performed, `closure` is a *guard*: Rather than a normal
+ //@ reference, `borrow_mut` returns a smart pointer ([`RefMut`](https://doc.rust-
+ //@ lang.org/stable/std/cell/struct.RefMut.html), in this case) that waits until is
+ //@ goes out of scope, and then appropriately updates the number of active references.
//@
- //@ 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!
+ //@ Since `call` is the only place that borrows the environments of the closures, we
+ //@ should expect that the check will always succeed, as is actually entirely useless.
+ //@ 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 of functions accidentally recursively calling themselves is called
+ //@ *reentrancy*, and it can lead to subtle bugs. Here, it would mean that the closure
+ //@ runs twice, each time thinking it has a unique, mutable reference to 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.
+ // 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 reference
+ // to the content.
(&mut *closure)(val);
}
}
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 panics
+// 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](workspace/src/part12.rs) |
+//@ [next](part13.html)
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