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

// 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)>>,
}

impl Callbacks {
    // Now we can provide some functions. The constructor should be straight-forward.
    pub fn new() -> Self {
        Callbacks { callbacks: Vec::new() }                         /*@*/
    }

    // Registration simply stores the callback.
    pub fn register(&mut self, callback: Box<FnMut(i32)>) {
        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.
    
    //@ 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));                    /*@*/
    }

    // 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.
        }
    }
}

// Now we are ready for the demo. Remember to edit `main.rs` to run it.
pub fn main() {
    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);
}

//@ ## 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 reference.

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