X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/f41a988984bb5487de50168403c5e9a666512595..8592458fd3494ebce80727341663c7b0036e3ede:/src/part11.rs diff --git a/src/part11.rs b/src/part11.rs index 9217be8..a088e77 100644 --- a/src/part11.rs +++ b/src/part11.rs @@ -2,34 +2,43 @@ // ====================================================== //@ We will play around with closures a bit more. Let us implement some kind of generic "callback" -//@ mechanism, providing two functions: Registering a new callback, and calling all registered callbacks. +//@ mechanism, providing two functions: Registering a new callback, and calling all registered +//@ callbacks. -//@ First of all, we need to find a way to store the callbacks. Clearly, there will be a `Vec` involved, so that we can -//@ always grow the number of registered callbacks. A callback will be a closure, i.e., something implementing -//@ `FnMut(i32)` (we want to call this multiple times, so clearly `FnOnce` would be no good). So our first attempt may be the following. +//@ First of all, we need to find a way to store the callbacks. Clearly, there will be a `Vec` +//@ involved, so that we can always grow the number of registered callbacks. A callback will be a +//@ closure, i.e., something implementing `FnMut(i32)` (we want to call this multiple times, so +//@ clearly `FnOnce` would be no good). So our first attempt may be the following. // For now, we just decide that the callbacks have an argument of type `i32`. struct CallbacksV1 { callbacks: Vec, } -//@ However, this will not work. Remember how the "type" of a closure is specific to the environment of captured variables. Different closures -//@ all implementing `FnMut(i32)` may have different types. However, a `Vec` is a *uniformly typed* vector. +//@ However, this will not work. Remember how the "type" of a closure is specific to the +//@ environment of captured variables. Different closures all implementing `FnMut(i32)` may have +//@ different types. However, a `Vec` is a *uniformly typed* vector. -//@ We will thus need a way to store things of *different* types in the same vector. We know all these types implement `FnMut(i32)`. For this scenario, -//@ Rust provides *trait objects*: The truth is, `FnMut(i32)` is not just a trait. It is also a type, that can be given to anything implementing -//@ this trait. So, we may write the following. +//@ We will thus need a way to store things of *different* types in the same vector. We know all +//@ these types implement `FnMut(i32)`. For this scenario, Rust provides *trait objects*: The truth +//@ is, `FnMut(i32)` is not just a trait. It is also a type, that can be given to anything +//@ implementing this trait. So, we may write the following. /* struct CallbacksV2 { callbacks: Vec, } */ -//@ 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 -//@ 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. +//@ 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 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. -//@ So, what can we do, if we can't store the callbacks in a vector? We can put them in a box. Semantically, `Box` is a lot like `T`: You fully own -//@ the data stored there. On the machine, however, `Box` is a *pointer* to a heap-allocated `T`. It is a lot like `std::unique_ptr` in C++. In our current example, -//@ the important bit is that since it's a pointer, `T` can be unsized, but `Box` itself will always be sized. So we can put it in a `Vec`. +//@ So, what can we do, if we can't store the callbacks in a vector? We can put them in a box. +//@ Semantically, `Box` is a lot like `T`: You fully own the data stored there. On the machine, +//@ however, `Box` is a *pointer* to a heap-allocated `T`. It is a lot like `std::unique_ptr` in +//@ C++. In our current example, the important bit is that since it's a pointer, `T` can be +//@ unsized, but `Box` itself will always be sized. So we can put it in a `Vec`. pub struct Callbacks { callbacks: Vec>, } @@ -45,16 +54,19 @@ impl Callbacks { self.callbacks.push(callback); } - // 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. + // 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 - //@ 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`. + //@ 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`. //@ Here, we use the special lifetime `'static`, which is the lifetime of the entire program. - //@ The same bound has been implicitly added in the version of `register` above, and in the definition of - //@ `Callbacks`. + //@ The same bound has been implicitly added in the version of `register` above, and in the + //@ definition of `Callbacks`. pub fn register_generic(&mut self, callback: F) { self.callbacks.push(Box::new(callback)); /*@*/ } @@ -63,15 +75,18 @@ impl Callbacks { pub fn call(&mut self, val: i32) { // 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`. + //@ 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 + //@ 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 - //@ dropped), the content it points to on the heap will be deleted. + //@ 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. } } } @@ -83,11 +98,14 @@ pub fn main() { c.call(0); { - //@ We can even register callbacks that modify their environment. Per default, Rust will attempt to borrow `count`. 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. - //@ 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. + //@ 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 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; c.register_generic(move |val| { count = count+1; @@ -98,23 +116,31 @@ 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 -//@ 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). -//@ In case of the `FnMut` trait, there's only a single action to be performed: Calling the closure. You can thus think of a pointer to `FnMut` as -//@ a pointer to the code, and a pointer to the environment. This is how Rust recovers the typical encoding of closures as a special case of a more -//@ general concept. +//@ 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 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). +//@ In case of the `FnMut` trait, there's only a single action to be performed: Calling the +//@ closure. You can thus think of a pointer to `FnMut` as a pointer to the code, and a pointer to +//@ the environment. This is how Rust recovers the typical encoding of closures as a special case +//@ of a more general concept. //@ -//@ Whenever you write a generic function, you have a choice: You can make it generic, like `register_generic`. Or you -//@ can use trait objects, like `register`. The latter will result in only a single compiled version (rather -//@ than one version per type it is instantiated with). This makes for smaller code, but you pay the overhead of the virtual function calls. -//@ (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)? +//@ Whenever you write a generic function, you have a choice: You can make it generic, like +//@ `register_generic`. Or you can use trait objects, like `register`. The latter will result in +//@ only a single compiled version (rather than one version per type it is instantiated with). This +//@ makes for smaller code, but you pay the overhead of the virtual function calls. (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 -// 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. +// **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 `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)