use official plaintext version of CC license

[rust-101.git] / src / part11.rs

diff --git a/src/part11.rs b/src/part11.rs
index 1256e19b50219c98c1b4450aa7d5b2412690ba89..513440143eb453873fe2be46a74176b9350c4f6d 100644 (file)
--- a/src/part11.rs
+++ b/src/part11.rs
@@ -1,141 +1,120 @@
-// 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 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. <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() {
 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
 // 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](workspace/src/part11.rs) | [next](part12.html)