}
// 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()`.
fn vec_min(v: &Vec<BigInt>) -> Option<BigInt> {
let mut min: Option<BigInt> = 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(); /*@*/
+ 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
+//@ 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 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.
+//@ 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`. <br/>
+//@ 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?
//@ ## 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/>
+//@ 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
//@ `Clone`. This makes the cost explicit.
// ## Lifetimes
-//@ To fix the performance problems of `vec_min`, we need to avoid using `clone()`. We'd like
+//@ 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.
-//@ The function `head` demonstrates how that could work: It borrows the first element of a vector if it is non-empty.
+//@ 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 borrow the first element of `v` and use it to construct the return value.
+//@ We can then obtain a reference to 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]) /*@*/
*first.unwrap()
}
-//@ To give the answer to this question, we have to talk about the *lifetime* of a borrow. The point is, saying that
+//@ 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<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
+//@ 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<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*.
+//@ 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. 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.
+//@ 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 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
+//@ 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 *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).
+//@ 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) | [next](part07.html)
+//@ [index](main.html) | [previous](part05.html) | [raw source](workspace/src/part06.rs) | [next](part07.html)
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