[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 big 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. 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>,
}

// 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 ...`, it says that we will locally
    // change the vector `v`. In this case, we need to make that annotation to be able to call `pop`
    // on `v`.
    pub fn from_vec(mut v: Vec<u64>) -> Self {
        while v.len() > 0 && v[v.len()-1] == 0 {
            v.pop();
        }
        BigInt { data: v }
    }
}

// **Exercise 05.1**: Write a function on `BigInt` that returns the number of digits. Write another one
// that increments the number by 1.
// 
// *Hint*: To take `self` as a mutable borrow, write `fn inc1(&mut self)`.

// ## Cloning
// If you have a close look at the type of `BigInt::from_vec`, you will notice that it
// consumes the vector `v`. The caller hence loses access. There is however 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, 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 its `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 would 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`.

// ## Mutation + aliasing considered harmful (part 2)
// Now that we know how to borrow a part of an `enum` (like `v` above), there's another example 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 borrow of a part of `var`,
// and since we wrote `ref mut`, they will be mutable borrows. 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); */
    *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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