-// Rust-101, Part 11: Trait Objects, Box (WIP)
-// ===========================================
-
-//@ Now that we know about closures, let's have some fun with them. We will try to implement some kind of generic "callback"
-//@ mechanism, providing two functions: Registering a new callback, and calling all registered callbacks. There will be two
-//@ versions, so to avoid clashes of names, we put them into modules.
+// Rust-101, Part 11: Trait Objects, Box, Lifetime bounds
+// ======================================================
+
+//@ 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.
+
+//@ 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<F: FnMut(i32)> {
+ callbacks: Vec<F>,
+}
+//@ 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<F>` 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.
+/* struct CallbacksV2 {
+ callbacks: Vec<FnMut(i32)>,
+} */
+//@ 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. <br/>
+//@ 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<T>` is a lot like `T`: You fully own
+//@ the data stored there. On the machine, however, `Box<T>` 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<T>` itself will always be sized. So we can put it in a `Vec`.
+pub struct Callbacks {
+ callbacks: Vec<Box<FnMut(i32)>>,
+}
-mod callbacks {
- //@ First of all, we need to find a way to store the callbacks. Clearly, there will be a `Vec` involves, 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 callbakcs have an argument of type `i32`.
- struct CallbacksV1<F: FnMut(i32)> {
- callbacks: Vec<F>,
+impl Callbacks {
+ // Now we can provide some functions. The constructor should be straight-forward.
+ pub fn new() -> Self {
+ Callbacks { callbacks: Vec::new() } /*@*/
}
- //@ However, this will not work. Remember how the "type" of a closure is specific to the environment of captures variables. Different closures
- //@ all implementing `FnMut(i32)` may be different types. However, a `Vec<F>` is a *uniformly typed* vector.
-
- //@ We will this 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, that `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:
- /* struct CallbacksV2 {
- callbacks: Vec<FnMut(i32)>,
- } */
- //@ 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 will this type 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 trouble 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, btw, 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<T>` is a lot like `T`: You fully own
- //@ the data stored there. On the machine, however, `Box<T>` is a *pointer* to `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<T>` itself will always be sized. So we can put it in a `Vec`.
- struct Callbacks {
- callbacks: Vec<Box<FnMut(i32)>>,
+ // Registration simply stores the callback.
+ pub fn register(&mut self, callback: Box<FnMut(i32)>) {
+ self.callbacks.push(callback);
}
- impl Callbacks {
- // Now we can provide some functions. The constructor should be straight-forward.
- fn new() -> Self {
- Callbacks { callbacks: Vec::new() } /*@*/
- }
-
- // Registration simply stores the callback.
- fn register(&mut self, callback: Box<FnMut(i32)>) {
- self.callbacks.push(callback); /*@*/
- }
-
- // And here we call all the stored callbacks.
- fn call(&mut self, val: i32) {
- // Since they are of type `FnMut`, we need to mutably iterate. Notice that boxes dereference implicitly.
- for callback in self.callbacks.iter_mut() {
- callback(val); /*@*/
- }
- }
+ // 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 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`.
+ pub fn register_generic<F: FnMut(i32)+'static>(&mut self, callback: F) {
+ self.callbacks.push(Box::new(callback)); /*@*/
}
- // Now we are read for the demo.
- pub fn demo() {
- let mut c = Callbacks::new();
- c.register(Box::new(|val| println!("Callback 1: {}", val)));
-
- c.call(0);
-
- //@ We can even register callbacks that modify their environment. Rust will again attempt to borrow `count`. However,
- //@ that doesn't work out this time: Since we want to put this thing in a `Box`, it could live longer than the function
- //@ we are in. Then the borrow of `count` would become invalid. However, we can tell rust to `move` ownership of the
- //@ variable into the closure. Its environment will then contain an `usize` rather than a `&mut uszie`, and have
- //@ no effect on this local variable anymore.
- let mut count: usize = 0;
- c.register(Box::new(move |val| { count = count+1; println!("Callback 2, {}. time: {}", count, val); } ));
- c.call(1);
- c.call(2);
+ // And here we call all the stored 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<FnMut(i32)>`. 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 references, this typically happens implicitly with smart pointers, so we can also directly call the function.
+ //@ Try removing the `&mut *`.
+ //@
+ //@ 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.
+ }
}
-
}
-// Remember to edit `main.rs` to run the demo.
+// Now we are ready for the demo. Remember to edit `main.rs` to run it.
pub fn main() {
- callbacks::demo();
-}
-
-mod callbacks_clone {
- //@ So, this worked great, didn't it! There's one point though that I'd like to emphasize: One cannot `clone` a closure.
- //@ Hence it becomes impossibly to implement `Clone` for our `Callbacks` type. What could we do about this?
-
- //@ You already learned about `Box` above. `Box` is historically a very special type in Rust (though it lost most of its
- //@ particularities by now, and people are working on making it just a normal library type). Effectively, however, it is
- //@ just an example of a *smart pointer*: It's like a pointer (i.e., a borrow), but with some additional smarts to it. For
- //@ `Box`, that's the part about ownership. Once you drop the box, the content it points to will also be deleted.
- //@
- //@ Another example of a smart pointer in Rust is `Rc<T>`. This is short for *reference-counter*, so you can already guess how
- //@ this pointer is smart: It has a reference count. You can `clone` an `Rc` as often as you want, that doesn't affect the
- //@ data it contains at all. It only creates more references to the same data. Once all the references are gone, the data is
- //@ deleted.
- //@
- //@ Wait a moment, you may here. Multiple references to the same data? That's aliasing! Indeed, we have to be careful here.
- //@ Once data is stored in an `Rc`, is is read-only: By dereferencing the smart `Rc`, you can only get a shared borrow of the data.
- use std::rc;
-
- //@ 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.
- #[derive(Clone)]
- struct Callbacks {
- callbacks: Vec<rc::Rc<Fn(i32)>>,
- }
-
- // The methods on these clonable callbacks are just like the ones above.
- impl Callbacks {
- fn new() -> Self {
- Callbacks { callbacks: Vec::new() } /*@*/
- }
-
- fn register(&mut self, callback: rc::Rc<Fn(i32)>) {
- self.callbacks.push(callback); /*@*/
- }
-
- fn call(&mut self, val: i32) {
- // We only need a shared iterator here. `Rc` also implicitly dereferences, so we can just call the callback.
- for callback in self.callbacks.iter() {
- callback(val); /*@*/
- }
- }
- }
-
- // The demo works just as above. Our counting callback doesn't work anymore though, because we are using `Fn` now.
- fn demo() {
- let mut c = Callbacks::new();
- c.register(rc::Rc::new(|val| println!("Callback 1: {}", val)));
-
- c.call(0);
- c.call(1);
+ let mut c = Callbacks::new();
+ c.register(Box::new(|val| println!("Callback 1: {}", val)));
+ c.call(0);
+
+ {
+ //@ 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;
+ println!("Callback 2: {} ({}. time)", val, count);
+ } );
}
+ c.call(1); c.call(2);
}
-// **Exercise 11.1**: We made the arbitrary choice of using `i32` for the arguments. Generalize the data-structures above
+//@ ## 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 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)?
+
+// **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 `t: T`, you will either have to restrict `T` to `Copy` types, or pass a reference.
-//@ [index](main.html) | [previous](part10.html) | [next](main.html)
+//@ [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)
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