-// Rust-101, Part 05: Copy, Clone
-// ==============================
-
-use std::cmp;
-use std::ops;
+// 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
}
}
+// ## 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,
// `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 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
}
}
// 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!
-//
-// To put this in perspective, `clone` in Rust corresponds to what people usually manually do in
-// the copy constructor of a C++ class: It creates new, independent instance containing the
-// same values. Contrary to that, if you pass something to a function normally (like the
-// second call to `from_vec` in `clone_demo`), only a *shallow* copy is created: The fields
-// are copied, but pointers are simply duplicated. This corresponds to the default copy
-// constructor in C++. Rust assumes that after such a copy, the old value is useless
-// (as the new one uses the same pointers), and hence considers the data semantically
-// moved to the copy. That's another explanation of why Rust does not let you access
-// a vector anymore after you passed ownership to some function.
+// 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!
-// With `BigInt` being about numbers, we should be able to write a version of `vec_min`
-// that computes the minimum of a list of `BigInt`. We start by writing `min` for
-// `BigInt`. Now our assumption of having no trailing zeros comes in handy!
-impl BigInt {
- fn min(self, other: Self) -> Self {
- // Just to be sure, we first check that both operands actually satisfy our invariant.
- // `debug_assert!` is a macro that checks that its argument (must be of type `bool`)
- // is `true`, and panics otherwise. It gets removed in release builds, which you do with
- // `cargo build --release`.
- //
- // If you carefully check the type of `BigInt::test_invariant`, you may be surprised that
- // we can call the function this way. Doesn't it take `self` in borrowed form? Indeed,
- // the explicit way to do that would be to call `(&other).test_invariant()`. However, the
- // `self` argument of a method is treated specially by Rust, and borrowing happens automatically here.
- debug_assert!(self.test_invariant() && other.test_invariant());
- // If the lengths of the two numbers differ, we already know which is larger.
- if self.data.len() < other.data.len() {
- self
- } else if self.data.len() > other.data.len() {
- other
- } else {
- // **Exercise**: Fill in this code.
- panic!("Not yet implemented.");
+// 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`.
-fn vec_min(v: &Vec<BigInt>) -> Option<BigInt> {
- let mut min: Option<BigInt> = None;
- for e in v {
- // In the loop, `e` now has type `&i32`, so we have to dereference it.
- min = Some(match min {
- None => e.clone(),
- Some(n) => e.clone().min(n)
- });
+// ## 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,
}
- min
+ /* 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)
projects / rust-101.git / blobdiff