X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/63b7a79069a399c32d575bb71832fcad134117c9..ab7f9b241429bd675b437d2437799de75d2f409b:/src/part12.rs diff --git a/src/part12.rs b/src/part12.rs index 8b04fb6..c2331ea 100644 --- a/src/part12.rs +++ b/src/part12.rs @@ -71,7 +71,7 @@ pub fn main() { 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 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 @@ -89,10 +89,10 @@ fn demo_cell(c: &mut Callbacks) { //@ 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 *exclusive* references, +//@ 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 sahred reference - as we saw with `Cell`. +//@ 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` @@ -135,8 +135,8 @@ impl CallbacksMut { //@ 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 an - //@ exclusive, mutable reference to its environment - so it may end up dereferencing a dangling pointer. Ouch! Lucky enough, + //@ 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(); @@ -166,4 +166,4 @@ fn demo_mut(c: &mut CallbacksMut) { // **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) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part12.rs) | [next](part13.html) +//@ [index](main.html) | [previous](part11.html) | [raw source](workspace/src/part12.rs) | [next](part13.html)