the official term is mutable references, be more explicit about that

[rust-101.git] / src / part05.rs

// Rust-101, Part 05: Clone
// ========================

// ## Big Numbers
//@ In the course of the next few parts, we are going to build a data-structure for computations with
//@ *big* numbers. We would like to not have an upper bound to how large these numbers can get, with
//@ the memory of the machine being the only limit.
//@ 
//@ We start by deciding how to represent such big numbers. One possibility here is
//@ to use a vector "digits" of the number. This is like "1337" being a vector of four digits (1, 3, 3, 7),
//@ except that we will use `u64` as type of our digits, meaning we have 2^64 individual digits. Now we just
//@ have to decide the order in which we store numbers. I decided that we will store the least significant
//@ digit first. This means that "1337" would actually become (7, 3, 3, 1). <br/>
//@ Finally, we declare that there must not be any trailing zeros (corresponding to
//@ useless leading zeros in our usual way of writing numbers). This is to ensure that
//@ the same number can only be stored in one way.

//@ To write this down in Rust, we use a `struct`, which is a lot like structs in C:
//@ Just a bunch of named fields. Every field can be private to the current module (which is the default),
//@ or public (which is indicated by a `pub` in front of the name). For the sake of the tutorial, we make
//@ `data` public - otherwise, the next parts of this course could not work on `BigInt`s. Of course, in a
//@ real program, one would make the field private to ensure that the invariant (no trailing zeros) is maintained.
pub struct BigInt {
    pub data: Vec<u64>, // least significant digit first, no trailing zeros
}

// Now that we fixed the data representation, we can start implementing methods on it.
impl BigInt {
    //@ Let's start with a constructor, creating a `BigInt` from an ordinary integer.
    //@ To create an instance of a struct, we write its name followed by a list of
    //@ fields and initial values assigned to them.
    pub fn new(x: u64) -> Self {
        if x == 0 {
            BigInt { data: vec![] }                                 /*@*/
        } else {
            BigInt { data: vec![x] }                                /*@*/
        }
    }

    //@ It can often be useful to encode the invariant of a data-structure in code, so here
    //@ is a check that detects useless trailing zeros.
    pub fn test_invariant(&self) -> bool {
        if self.data.len() == 0 {
            true
        } else {
            self.data[self.data.len() - 1] != 0                     /*@*/
        }
    }

    // We can convert any vector of digits into a number, by removing trailing zeros. The `mut`
    // declaration for `v` here is just like the one in `let mut ...`: We completely own `v`, but Rust
    // still asks us to make our intention of modifying it explicit. This `mut` is *not* part of the
    // type of `from_vec` - the caller has to give up ownership of `v` anyway, so they don't care anymore
    // what you do to it.
    // 
    // **Exercise 05.1**: Implement this function.
    // 
    // *Hint*: You can use `pop` to remove the last element of a vector.
    pub fn from_vec(mut v: Vec<u64>) -> Self {
        unimplemented!()
    }
}

// ## Cloning
//@ If you take a close look at the type of `BigInt::from_vec`, you will notice that it
//@ consumes the vector `v`. The caller hence loses access to its vector. However, there is something
//@ we can do if we don't want that to happen: We can explicitly `clone` the vector,
//@ which means that a full (or *deep*) copy will be performed. Technically,
//@ `clone` takes a borrowed vector in the form of a shared reference, and returns a fully owned one.
fn clone_demo() {
    let v = vec![0,1 << 16];
    let b1 = BigInt::from_vec((&v).clone());
    let b2 = BigInt::from_vec(v);
}
//@ Rust has special treatment for methods that borrow their `self` argument (like `clone`, or
//@ like `test_invariant` above): It is not necessary to explicitly borrow the receiver of the
//@ method. Hence you could replace `(&v).clone()` by `v.clone()` above. Just try it!

//@ To be clonable is a property of a type, and as such, naturally expressed with a trait.
//@ In fact, Rust already comes with a trait `Clone` for exactly this purpose. We can hence
//@ make our `BigInt` clonable as well.
impl Clone for BigInt {
    fn clone(&self) -> Self {
        BigInt { data: self.data.clone() }                          /*@*/
    }
}
//@ Making a type clonable is such a common exercise that Rust can even help you doing it:
//@ If you add `#[derive(Clone)]` right in front of the definition of `BigInt`, Rust will
//@ generate an implementation of `Clone` that simply clones all the fields. Try it!
//@ These `#[...]` annotations at types (and functions, modules, crates) are called *attributes*.
//@ We will see some more examples of attributes later.

// We can also make the type `SomethingOrNothing<T>` implement `Clone`. 
//@ However, that can only work if `T` is `Clone`! So we have to add this bound to `T` when we introduce
//@ the type variable.
use part02::{SomethingOrNothing,Something,Nothing};
impl<T: Clone> Clone for SomethingOrNothing<T> {
    fn clone(&self) -> Self {
        match *self {                                               /*@*/
            Nothing => Nothing,                                     /*@*/
            //@ In the second arm of the match, we need to talk about the value `v`
            //@ that's stored in `self`. However, if we were to write the pattern as
            //@ `Something(v)`, that would indicate that we *own* `v` in the code
            //@ after the arrow. That can't work though, we have to leave `v` owned by
            //@ whoever called us - after all, we don't even own `self`, we just borrowed it.
            //@ By writing `Something(ref v)`, we borrow `v` for the duration of the match
            //@ arm. That's good enough for cloning it.
            Something(ref v) => Something(v.clone()),               /*@*/
        }                                                           /*@*/
    }
}
//@ Again, Rust will generate this implementation automatically if you add
//@ `#[derive(Clone)]` right before the definition of `SomethingOrNothing`.

// **Exercise 05.2**: Write some more functions on `BigInt`. What about a function that returns the number of
// digits? The number of non-zero digits? The smallest/largest digit? Of course, these should all take `self` as a shared reference (i.e., in borrowed form).

// ## Mutation + aliasing considered harmful (part 2)
//@ Now that we know how to create references to contents of an `enum` (like `v` above), there's another example we can look at for why we
//@ have to rule out mutation in the presence of aliasing. First, we define an `enum` that can hold either
//@ a number, or a string.
enum Variant {
    Number(i32),
    Text(String),
}
//@ Now consider the following piece of code. Like above, `n` will be a reference to a part of `var`,
//@ and since we wrote `ref mut`, the reference will be uniqie and mutable. In other words, right after the match, `ptr`
//@ points to the number that's stored in `var`, where `var` is a `Number`. Remember that `_` means
//@ "we don't care".
fn work_on_variant(mut var: Variant, text: String) {
    let mut ptr: &mut i32;
    match var {
        Variant::Number(ref mut n) => ptr = n,
        Variant::Text(_) => return,
    }
    /* var = Variant::Text(text); */                                /* BAD! */
    *ptr = 1337;
}
//@ Now, imagine what would happen if we were permitted to also mutate `var`. We could, for example,
//@ make it a `Text`. However, `ptr` still points to the old location! Hence `ptr` now points somewhere
//@ into the representation of a `String`. By changing `ptr`, we manipulate the string in completely
//@ unpredictable ways, and anything could happen if we were to use it again! (Technically, the first field
//@ of a `String` is a pointer to its character data, so by overwriting that pointer with an integer,
//@ we make it a completely invalid address. When the destructor of `var` runs, it would try to deallocate
//@ that address, and Rust would eat your laundry - or whatever.)
//@ 
//@ I hope this example clarifies why Rust has to rule out mutation in the presence of aliasing *in general*,
//@ not just for the specific case of a buffer being reallocated, and old pointers becoming hence invalid.

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