X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/229b86d07e94cd3ec175051a44b3f3cb45b40b65..2d40516a8393db1f27bb822ff95c71a1a9c82537:/workspace/src/part06.rs diff --git a/workspace/src/part06.rs b/workspace/src/part06.rs index dea1d1d..3bc7d08 100644 --- a/workspace/src/part06.rs +++ b/workspace/src/part06.rs @@ -1,5 +1,3 @@ -// ***Remember to enable/add this part in `main.rs`!*** - // Rust-101, Part 06: Copy, Lifetimes // ================================== @@ -10,9 +8,6 @@ use part05::BigInt; // 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()); // 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. @@ -27,82 +22,28 @@ impl BigInt { } } -// Now we can write `vec_min`. However, in order to make it type-check, we have to make a full (deep) copy of e -// by calling `clone()`. +// 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) - }); + unimplemented!() } 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 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 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`.
-// 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. // ## `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. -// 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*. -// 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(v: &Vec) -> Option<&T> { if v.len() > 0 { - Some(&v[0]) + unimplemented!() } else { None } @@ -116,37 +57,10 @@ fn head(v: &Vec) -> Option<&T> { 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 borrow. 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 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) -> 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)