-// Rust-101, Part 05: Copy, Clone
-// ==============================
+// 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>,
+}
+
+// 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`.
+ //
+ // **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 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 to its vector. 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`, the borrow will be 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); */
+ *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.
+
+// [index](main.html) | [previous](part04.html) | [next](part06.html)
projects / rust-101.git / blobdiff