-// Rust-101, Part 06: Abstract Datastructure, Testing
-// ==================================================
+// Rust-101, Part 06: Copy, Lifetimes
+// ==================================
-use std::cmp;
-use std::ops;
-
-pub struct BigInt {
- data: Vec<u64>, // least significant digits first. The last block will *not* be 0.
-}
+// We continue to work on our `BigInt`, so we start by importing what we already established.
+use part05::BigInt;
+// 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`. First, we have to write `min` for `BigInt`.
impl BigInt {
- pub fn new(x: u64) -> Self {
- if x == 0 {
- BigInt { data: vec![] }
+ fn min_try1(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`.
+ debug_assert!(self.test_invariant() && other.test_invariant());
+ // Now our assumption of having no trailing zeros comes in handy:
+ // 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 {
- BigInt { data: vec![x] }
+ // **Exercise 06.1**: Fill in this code.
+ unimplemented!()
}
}
}
-/// Add with carry, returning the sum and the carry
-fn overflowing_add(a: u64, b: u64, carry: bool) -> (u64, bool) {
- match u64::checked_add(a, b) {
- Some(sum) if !carry => (sum, false),
- Some(sum) => { // we have to increment the sum by 1, where it may overflow again
- match u64::checked_add(sum, 1) {
- Some(total_sum) => (total_sum, false),
- None => (0, true) // we overflowed incrementing by 1, so we are just "at the edge"
- }
- },
- None => {
- // Get the remainder, i.e., the wrapping sum. This cannot overflow again by adding just 1, so it is safe
- // to add the carry here.
- let rem = u64::wrapping_add(a, b) + if carry { 1 } else { 0 };
- (rem, true)
- }
+// Now we can write `vec_min`. In order to make it type-check, we have make a deep copy of e.
+fn vec_min(v: &Vec<BigInt>) -> Option<BigInt> {
+ let mut min: Option<BigInt> = None;
+ for e in v {
+ let e = e.clone();
+ min = Some(match min {
+ None => e,
+ Some(n) => e.min_try1(n)
+ });
}
+ min
}
+// Now, what's happening here? Why do we have to clone `e`, and why did we not
+// have to do that in our previous version?
+//
+// The answer is already hidden in the type of `vec_min`: `v` is just borrowed, but
+// the Option<BigInt> that it returns is *owned*. We can't just return one of the elements of `v`,
+// as that would mean that it is no longer in the vector! In our code, this comes up when we update
+// the intermediate variable `min`, which also has type `Option<BigInt>`. If you replace get rid of the
+// `e.clone()`, Rust will complain "Cannot move out of borrowed content". That's because
+// `e` is a `&BigInt`. Assigning `min = Some(*e)` works just like a function call: Ownership of the
+// underlying data is transferred from where `e` borrows from to `min`. But that's not allowed, since
+// we just borrowed `e`, so we cannot empty it! We can, however, call `clone()` on it. Then we own
+// the copy that was created, and hence we can store it in `min`.<br/>
+// Of course, making such a full copy is expensive, so we'd like to avoid it. We'll some to that in the next part.
-#[test]
-fn test_overflowing_add() {
- assert_eq!(overflowing_add(10, 100, false), (110, false));
- assert_eq!(overflowing_add(10, 100, true), (111, false));
- assert_eq!(overflowing_add(1 << 63, 1 << 63, false), (0, true));
- assert_eq!(overflowing_add(1 << 63, 1 << 63, true), (1, true));
- assert_eq!(overflowing_add(1 << 63, (1 << 63) -1 , true), (0, true));
-}
+// ## `Copy` types
+// But before we go there, I should answer the second question I brought up above: Why did our old `vec_min` work?
+// We stored the minimal `i32` locally without cloning, and Rust did not complain. That's because there isn't
+// really much of an "ownership" when it comes to types like `i32` or `bool`: If you move the value from one
+// place to another, then both instances are "complete". We also say the value has been *duplicated*. This is in
+// stark contrast to types like `Vec<i32>`, where moving the value results in both the old and the new vector to
+// point to the same underlying buffer. We don't have two vectors, there's no proper duplication.
+//
+// Rust calls types that can be easily duplicated `Copy` types. `Copy` is another trait, and it is implemented for
+// types like `i32` and `bool`. Remember how we defined the trait `Minimum` by writing `trait Minimum : Copy { ...`?
+// This tells Rust that every type that implements `Minimum` must also implement `Copy`, and that's why the compiler
+// accepted our generic `vec_min` in part 02. `Copy` is the first *marker trait* that we encounter: It does not provide
+// any methods, but makes a promise about the behavior of the type - in this case, being duplicable.
-impl ops::Add<BigInt> for BigInt {
- type Output = BigInt;
- fn add(self, rhs: BigInt) -> Self::Output {
- let mut result_vec:Vec<u64> = Vec::with_capacity(cmp::max(self.data.len(), rhs.data.len()));
- let mut carry:bool = false; // the carry bit
- for (i, val) in self.data.into_iter().enumerate() {
- // compute next digit and carry
- let rhs_val = if i < rhs.data.len() { rhs.data[i] } else { 0 };
- let (sum, new_carry) = overflowing_add(val, rhs_val, carry);
- // store them
- result_vec.push(sum);
- carry = new_carry;
- }
- BigInt { data: result_vec }
+// If you try to implement `Copy` for `BigInt`, you will notice that Rust
+// does not let you do that. A type can only be `Copy` if all its elements
+// are `Copy`, and that's not the case for `BigInt`. However, we can make
+// `SomethingOrNothing<T>` copy if `T` is `Copy`.
+use part02::{SomethingOrNothing,Something,Nothing};
+impl<T: Copy> Copy for SomethingOrNothing<T> {}
+// Again, Rust can generate implementations of `Copy` automatically. If
+// you add `#[derive(Copy,Clone)]` right before the definition of `SomethingOrNothing`,
+// both `Copy` and `Clone` will automatically be implemented.
+
+// ## An operational perspective
+// Instead of looking at what happens "at the surface" (i.e., visible in Rust), one can also explain
+// ownership passing and how `Copy` and `Clone` fit in by looking at what happens on the machine.<br/>
+// When Rust code is executed, passing a value (like `i32` or `Vec<i32>`) to a function will always
+// result in a shallow copy being performed: Rust just copies the bytes representing that value, and
+// considers itself done. That's just like the default copy constructor in C++. Rust, however, will
+// consider this a destructive operation: After copying the bytes elsewhere, the original value must
+// no longer be used. After all, the two could now share a pointer! If, however, you mark a type `Copy`,
+// then Rust will *not* consider a move destructive, and just like in C++, the old and new value
+// can happily coexist. Now, Rust does not allow you to overload the copy constructor. This means that
+// passing a value around will always be a fast operation, no allocation or any other kind of heap access
+// will happen. In the situations where you would write a copy constructor in C++ (and hence
+// incur a hidden cost on every copy of this type), you'd have the type *not* implement `Copy`, but only
+// `Clone`. This makes the cost explicit.
+
+// ## Lifetimes
+// To fix the performance problems of `vec_min`, we need to avoid using `clone()`. We'd like
+// the return value to not be owned (remember that this was the source of our need for cloning), but *borrowed*.
+
+// The function `head` demonstrates how that could work: It borrows the first element of a vector if it is non-empty.
+// The type of the function says that it will either return nothing, or it will return a borrowed `T`.
+// We can then borrow the first element of `v` and use it to construct the return value.
+fn head<T>(v: &Vec<T>) -> Option<&T> {
+ if v.len() > 0 {
+ Some(&v[0])
+ } else {
+ None
}
}
+// Technically, we are returning a pointer to the first element. But doesn't that mean that callers have to be
+// careful? Imagine `head` would be a C++ function, and we would write the following code.
+/*
+ int foo(std::vector<int> v) {
+ int *first = head(v);
+ v.push_back(42);
+ return *first;
+ }
+*/
+// This is very much like our very first motivating example for ownership, at the beginning of part 04:
+// `push_back` could reallocate the buffer, making `first` an invalid pointer. Again, we have aliasing (of `first`
+// and `v`) and mutation. But this time, the bug is hidden behind the call to `head`. How does Rust solve this? If we translate
+// the code above to Rust, it doesn't compile, so clearly we are good - but how and why?
+// (Notice that have to explicitly assert using `unwrap` that `first` is not `None`, whereas the C++ code
+// above would silently dereference a `NULL`-pointer. But that's another point.)
+fn rust_foo(mut v: Vec<i32>) -> i32 {
+ let first: Option<&i32> = head(&v);
+ /* v.push(42); */
+ *first.unwrap()
+}
+// To give the answer to this question, we have to talk about the *lifetime* of a borrow. The point is, saying that
+// you borrowed your friend a `Vec<i32>`, or a book, is not good enough, unless you also agree on *how long*
+// your friend can borrow it. After all, you need to know when you can rely on owning your data (or book) again.
+//
+// Every borrow in Rust has an associated lifetime, written `&'a T` for a borrow of type `T` with lifetime `'a`. The full
+// type of `head` reads as follows: `fn<'a, T>(&'a Vec<T>) -> Option<&'a T>`. Here, `'a` is a *lifetime variable*, which
+// represents how long the vector has been borrowed. The function type expresses that argument and return value have *the same lifetime*.
+//
+// When analyzing the code of `rust_foo`, Rust has to assign a lifetime to `first`. It will choose the scope
+// where `first` is valid, which is the entire rest of the function. Because `head` ties the lifetime of its
+// argument and return value together, this means that `&v` also has to borrow `v` for the entire duration of
+// the function. So when we try to borrow `v` mutable for `push`, Rust complains that the two borrows (the one
+// for `head`, and the one for `push`) overlap. Lucky us! Rust caught our mistake and made sure we don't crash the program.
+//
+// So, to sum this up: Lifetimes enable Rust to reason about *how long* a pointer has been borrowed. We can thus
+// safely write functions like `head`, that return pointers into data they got as argument, and make sure they
+// are used correctly, *while looking only at the function type*. At no point in our analysis of `rust_foo` did
+// we have to look *into* `head`. That's, of course, crucial if we want to separate library code from application code.
+// Most of the time, we don't have to explicitly add lifetimes to function types. This is thanks to *lifetimes elision*,
+// where Rust will automatically insert lifetimes we did not specify, following some [simple, well-documented rules](http://doc.rust-lang.org/stable/book/lifetimes.html#lifetime-elision).
-
+// [index](main.html) | [previous](part05.html) | [next](part07.html)
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