X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/f41a988984bb5487de50168403c5e9a666512595..683f369ba51d22cb8b89ab31f1b20c01c117361c:/src/part11.rs diff --git a/src/part11.rs b/src/part11.rs index 9217be8..757f205 100644 --- a/src/part11.rs +++ b/src/part11.rs @@ -23,7 +23,7 @@ struct CallbacksV1 { //@ But, Rust complains about this definition. It says something about "Sized". What's the trouble? See, for many things we want to do, it is crucial that //@ Rust knows the precise, fixed size of the type - that is, how large this type will be when represented in memory. For example, for a `Vec`, the //@ elements are stored one right after the other. How should that be possible, without a fixed size? The point is, `FnMut(i32)` could be of any size. -//@ We don't know how large that "type that implemenets `FnMut(i32)`" is. Rust calls this an *unsized* type. Whenever we introduce a type variable, Rust +//@ We don't know how large that "type that implements `FnMut(i32)`" is. Rust calls this an *unsized* type. Whenever we introduce a type variable, Rust //@ will implicitly add a bound to that variable, demanding that it is sized. That's why we did not have to worry about this so far.
//@ You can opt-out of this implicit bound by saying `T: ?Sized`. Then `T` may or may not be sized. @@ -48,7 +48,7 @@ impl Callbacks { // We can also write a generic version of `register`, such that it will be instantiated with some concrete closure type `F` // and do the creation of the `Box` and the conversion from `F` to `FnMut(i32)` itself. - //@ For this to work, we need to demand that the type `F` does not contain any short-lived borrows. After all, we will store it + //@ For this to work, we need to demand that the type `F` does not contain any short-lived references. After all, we will store it //@ in our list of callbacks indefinitely. If the closure contained a pointer to our caller's stackframe, that pointer //@ could be invalid by the time the closure is called. We can mitigate this by bounding `F` by a *lifetime*: `F: 'a` says //@ that all data of type `F` will *outlive* (i.e., will be valid for at least as long as) lifetime `'a`. @@ -64,13 +64,13 @@ impl Callbacks { // Since they are of type `FnMut`, we need to mutably iterate. for callback in self.callbacks.iter_mut() { //@ Here, `callback` has type `&mut Box`. We can make use of the fact that `Box` is a *smart pointer*: In - //@ particular, we can use it as if it were a normal pointer, and use `*` to get to its contents. Then we mutably borrow - //@ these contents, because we call a `FnMut`. + //@ particular, we can use it as if it were a normal reference, and use `*` to get to its contents. Then we obtain a + //@ mutable reference to these contents, because we call a `FnMut`. (&mut *callback)(val); /*@*/ - //@ Just like it is the case with normal borrows, this typically happens implicitly, so we can also directly call the function. + //@ Just like it is the case with normal references, this typically happens implicitly with smart pointers, so we can also directly call the function. //@ Try removing the `&mut *`. //@ - //@ The difference to a normal pointer is that `Box` implies ownership: Once you drop the box (i.e., when the entire `Callbacks` instance is + //@ The difference to a reference is that `Box` implies full ownership: Once you drop the box (i.e., when the entire `Callbacks` instance is //@ dropped), the content it points to on the heap will be deleted. } } @@ -83,9 +83,9 @@ pub fn main() { c.call(0); { - //@ We can even register callbacks that modify their environment. Per default, Rust will attempt to borrow `count`. However, + //@ We can even register callbacks that modify their environment. Per default, Rust will attempt to capture a reference to `count`, to borrow it. However, //@ that doesn't work out this time. Remember the `'static` bound above? Borrowing `count` in the environment would - //@ violate that bound, as the borrow is only valid for this block. If the callbacks are triggered later, we'd be in trouble. + //@ violate that bound, as the reference is only valid for this block. If the callbacks are triggered later, we'd be in trouble. //@ We have to explicitly tell Rust to `move` ownership of the variable into the closure. Its environment will then contain a //@ `usize` rather than a `&mut usize`, and the closure has no effect on this local variable anymore. let mut count: usize = 0; @@ -99,7 +99,7 @@ pub fn main() { //@ ## Run-time behavior //@ When you run the program above, how does Rust know what to do with the callbacks? Since an unsized type lacks some information, -//@ a *pointer* to such a type (be it a `Box` or a borrow) will need to complete this information. We say that pointers to +//@ a *pointer* to such a type (be it a `Box` or a reference) will need to complete this information. We say that pointers to //@ trait objects are *fat*. They store not only the address of the object, but (in the case of trait objects) also a *vtable*: A //@ table of function pointers, determining the code that's run when a trait method is called. There are some restrictions for traits to be usable //@ as trait objects. This is called *object safety* and described in [the documentation](https://doc.rust-lang.org/stable/book/trait-objects.html) and [the reference](https://doc.rust-lang.org/reference.html#trait-objects). @@ -113,8 +113,8 @@ pub fn main() { //@ (Of course, in the case of `register` above, there's no function called on the trait object.) //@ Isn't it beautiful how traits can nicely handle this tradeoff (and much more, as we saw, like closures and operator overloading)? -// **Exercise 11.1**: We made the arbitrary choice of using `i32` for the arguments. Generalize the data-structures above +// **Exercise 11.1**: We made the arbitrary choice of using `i32` for the arguments. Generalize the data structures above // to work with an arbitrary type `T` that's passed to the callbacks. Since you need to call multiple callbacks with the -// same `t: T`, you will either have to restrict `T` to `Copy` types, or pass a borrow. +// same `val: T` (in our `call` function), you will either have to restrict `T` to `Copy` types, or pass a reference. -//@ [index](main.html) | [previous](part10.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part11.rs) | [next](part12.html) +//@ [index](main.html) | [previous](part10.html) | [raw source](workspace/src/part11.rs) | [next](part12.html)