X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/17ab30e2988868e5f59b36bb0364cadb0a1c42f8..refs/remotes/github/master:/src/part06.rs diff --git a/src/part06.rs b/src/part06.rs index 21046c6..4c7ee24 100644 --- a/src/part06.rs +++ b/src/part06.rs @@ -1,56 +1,168 @@ -// 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 we use `unwrap` to explicitly assert 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)