Merge pull request #41 from bzindovic/bzindovic-part13-clarity-fix

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

diff --git a/src/part11.rs b/src/part11.rs
index a5cafa1ca6794419e1d29184b862ab0b1b0b98d7..a088e777ada93b66ec0a84bc3ab259e8c2973ca5 100644 (file)
--- 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"
 // ======================================================
 
 //@ 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<F: FnMut(i32)> {
     callbacks: Vec<F>,
 }
 // 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.
+//@ 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.
+//@ 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)>,
 } */
 /* 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.
+//@ 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`.
+//@ 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)>>,
 }
 pub struct Callbacks {
     callbacks: Vec<Box<FnMut(i32)>>,
 }


@@ -45,16 +54,19 @@ impl Callbacks {
         self.callbacks.push(callback);
     }
 
         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 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`.
+    //@ 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.
     //@ 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<F: FnMut(i32)+'static>(&mut self, callback: F) {
         self.callbacks.push(Box::new(callback));                    /*@*/
     }
     pub fn register_generic<F: FnMut(i32)+'static>(&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() {
     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`.
+            //@ 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);                                  /*@*/
             (&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.
+            //@ 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 *`.
             //@ 
             //@ 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.
+            //@ 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);
 
     {
     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.
+        //@ 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;
         let mut count: usize = 0;
         c.register_generic(move |val| {
             count = count+1;


@@ -98,23 +116,31 @@ pub fn main() {
 }
 
 //@ ## Run-time behavior
 }
 
 //@ ## 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.
+//@ 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 reference.
+// **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)