X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/6a83fbe44cc324f35f99da3ad290f0c0ef71260c..refs/remotes/github/master:/src/part05.rs

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-// 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>, // 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 little-endian vector of digits (i.e., least-significant digit first) 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 unique 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.
+
+//@ [index](main.html) | [previous](part04.html) | [raw source](workspace/src/part05.rs) |
+//@ [next](part06.html)