X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/6a83fbe44cc324f35f99da3ad290f0c0ef71260c..09a36e34a7b4f163c25fb971771bc4c7edd63e2b:/src/part05.rs

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+++ b/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 *bug* 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 of "small" numbers, which we will then consider the "digits"
+// of the big number. This is like "1337" being a vector of 4 small numbers (1, 3, 3, 7),
+// except that we will use `u64` as type of our base numbers. 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 collection of a bunch of named fields. Every field can be private to the current module
+// (which is the default), or public (which would be indicated by a `pub` in front of the name).
+// For the sake of the tutorial, we make `dat` 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 }
+    }
+}
+
+// ## 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!
+
+// 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 
+
+// [index](main.html) | [previous](part04.html) | [next](part06.html)