X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/17ab30e2988868e5f59b36bb0364cadb0a1c42f8..a0ae4ec8a5da0e171cb2d2f68621fa98f5ea610b:/src/part06.rs diff --git a/src/part06.rs b/src/part06.rs index 21046c6..7113094 100644 --- a/src/part06.rs +++ b/src/part06.rs @@ -1,56 +1,151 @@ -// Rust-101, Part 06: Lifetimes, Testing -// ===================================== +// Rust-101, Part 06: Copy, Lifetimes +// ================================== -use std::cmp; -use std::ops; -use std::fmt; +// We continue to work on our `BigInt`, so we start by importing what we already established. use part05::BigInt; - -impl PartialEq for BigInt { - fn eq(&self, other: &BigInt) -> bool { +// 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 { + 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()); - self.data == other.data + // 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 { + // **Exercise 06.1**: Fill in this code. + unimplemented!() + } } } -fn call_eq() { - let b1 = BigInt::new(13); - let b2 = BigInt::new(37); - println!("b1 == b1: {} ; b1 == b2: {}; b1 != b2: {}", b1 == b1, b1 == b2, b1 != b2); +// Now we can write `vec_min`. +fn vec_min(v: &Vec) -> Option { + let mut min: Option = None; + // If `v` is a shared reference to a vector, then the default for iterating over it is to call `iter`, the iterator that borrows the elements. + 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 to make a full (deep) copy of `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 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`. If you 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 `e` 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`.
+//@ Of course, making such a full copy is expensive, so we'd like to avoid it. We'll come to that in the next part. +// ## `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`, 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 fmt::Debug for BigInt { - fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { - self.data.fmt(f) - } -} +//@ 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` copy if `T` is `Copy`. +use part02::{SomethingOrNothing,Something,Nothing}; +impl Copy for SomethingOrNothing {} +//@ 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.
+//@ When Rust code is executed, passing a value (like `i32` or `Vec`) 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*. +//@ In other words, we want to return a shared reference to the minimal element. -impl BigInt { - pub fn inc(&mut self, mut by: u64) { - panic!("Not yet implemented."); +//@ The function `head` demonstrates how that could work: It returns a reference to 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 obtain a reference to the first element of `v` and use it to construct the return value. +fn head(v: &Vec) -> Option<&T> { + if v.len() > 0 { + Some(&v[0]) /*@*/ + } else { + None } } - - -#[test] -fn test_inc() { - let mut b = BigInt::new(1337); - b.inc(1337); - assert!(b == BigInt::new(1337 + 1337)); - - b = BigInt::new(0); - assert_eq!(b, BigInt::from_vec(vec![0])); - b.inc(1 << 63); - assert_eq!(b, BigInt::from_vec(vec![1 << 63])); - b.inc(1 << 63); - assert_eq!(b, BigInt::from_vec(vec![0, 1])); - b.inc(1 << 63); - assert_eq!(b, BigInt::from_vec(vec![1 << 63, 1])); - b.inc(1 << 63); - assert_eq!(b, BigInt::from_vec(vec![0, 2])); +// 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 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 { + 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 reference. The point is, saying that +//@ you borrowed your friend a `Vec`, 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 reference in Rust has an associated lifetime, written `&'a T` for a reference with lifetime `'a` to something of type `T`. The full +//@ type of `head` reads as follows: `fn<'a, T>(&'a Vec) -> Option<&'a T>`. Here, `'a` is a *lifetime variable*, which +//@ represents for 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 `rust_foo`. So when we try to create a unique reference to `v` for `push`, Rust complains that the two references (the one +//@ for `head`, and the one for `push`) overlap, so neither of them can be unique. 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 reference is valid, how long ownership has been borrowed. We can thus +//@ safely write functions like `head`, that return references 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 *lifetime elision*, +//@ where Rust will automatically insert lifetimes we did not specify, following some [simple, well-documented rules](https://doc.rust-lang.org/stable/book/lifetimes.html#lifetime-elision). + +//@ [index](main.html) | [previous](part05.html) | [raw source](workspace/src/part06.rs) | [next](part07.html)