//@
//@ If you give a book to your friend, you cannot just come to his place next day and get the book!
//@ It's no longer yours. Rust makes sure you don't break this rule. Try enabling the commented
-//@ line in `ownership_demo`. Rust will tell you that `v` has been *moved*, which is to say that ownership
-//@ has been transferred somewhere else. In this particular case, the buffer storing the data
-//@ does not even exist anymore, so we are lucky that Rust caught this problem!
+//@ line in `ownership_demo`. Rust will tell you that `v` has been *moved*, which is to say that
+//@ ownership has been transferred somewhere else. In this particular case, the buffer storing the
+//@ data does not even exist anymore, so we are lucky that Rust caught this problem!
//@ Essentially, ownership rules out aliasing, hence making the kind of problem discussed above
//@ impossible.
//@ vector. Hence, when `vec_min` finishes, the entire vector is deleted. That's of course not what
//@ we wanted! Can't we somehow give `vec_min` access to the vector, while retaining ownership of it?
//@
-//@ Rust calls this *a reference* to the vector, and it considers references as *borrowing* ownership. This
-//@ works a bit like borrowing does in the real world: If your friend borrows a book from you, your friend
-//@ can have it and work on it (and you can't!) as long as the book is still borrowed. Your friend could
-//@ even lend the book to someone else. Eventually however, your friend has to give the book back to you,
-//@ at which point you again have full control.
+//@ Rust calls this *a reference* to the vector, and it considers references as *borrowing*
+//@ ownership. This works a bit like borrowing does in the real world: If your friend borrows a
+//@ book from you, your friend can have it and work on it (and you can't!) as long as the book is
+//@ still borrowed. Your friend could even lend the book to someone else. Eventually however, your
+//@ friend has to give the book back to you, at which point you again have full control.
//@
//@ Rust distinguishes between two kinds of references. First of all, there's the *shared* reference.
//@ This is where the book metaphor kind of breaks down... you can give a shared reference to
//@ *the same data* to lots of different people, who can all access the data. This of course
-//@ introduces aliasing, so in order to live up to its promise of safety, Rust generally does not allow
-//@ mutation through a shared reference.
+//@ introduces aliasing, so in order to live up to its promise of safety, Rust generally does not
+//@ allow mutation through a shared reference.
//@ So, let's re-write `vec_min` to work on a shared reference to a vector, written `&Vec<i32>`.
//@ I also took the liberty to convert the function from `SomethingOrNothing` to the standard
use std::cmp;
let mut min = None;
- // This time, we explicitly request an iterator for the vector `v`. The method `iter` just borrows the vector
- // it works on, and provides shared references to the elements.
+ // This time, we explicitly request an iterator for the vector `v`. The method `iter` just
+ // borrows the vector it works on, and provides shared references to the elements.
for e in v.iter() {
// In the loop, `e` now has type `&i32`, so we have to dereference it to obtain an `i32`.
min = Some(match min {
vec_min(&v);
println!("The first element is: {}", *first);
}
-//@ What's going on here? First, `&` is how you lend ownership to someone - this operator creates a shared reference.
-//@ `shared_ref_demo` creates three shared references to `v`:
-//@ The reference `first` begins in the 2nd line of the function and lasts all the way to the end. The other two
+//@ What's going on here? First, `&` is how you lend ownership to someone - this operator creates a
+//@ shared reference. `shared_ref_demo` creates three shared references to `v`: The reference
+//@ `first` begins in the 2nd line of the function and lasts all the way to the end. The other two
//@ references, created for calling `vec_min`, only last for the duration of that respective call.
//@
//@ Technically, of course, references are pointers. Notice that since `vec_min` only gets a shared
//@ that was created before calling `vec_min` remains valid.
// ## Unique, mutable references
-//@ There is a second way to borrow something, a second kind of reference: The *mutable reference*. This is a reference that comes with the promise
-//@ that nobody else has *any kind of access* to the referee - in contrast to shared references, there is no aliasing with mutable references. It is thus always safe to perform mutation through such a reference.
+
+//@ There is a second way to borrow something, a second kind of reference: The *mutable reference*.
+//@ This is a reference that comes with the promise that nobody else has *any kind of access* to
+//@ the referee - in contrast to shared references, there is no aliasing with mutable references.
+//@ It is thus always safe to perform mutation through such a reference.
//@ Because there cannot be another reference to the same data, we could also call it a *unique* reference, but that is not their official name.
//@ As an example, consider a function which increments every element of a vector by 1.
-//@ The type `&mut Vec<i32>` is the type of mutable references to `vec<i32>`. Because the reference is
-//@ mutable, we can use a mutable iterator, providing mutable references to the elements.
+//@ The type `&mut Vec<i32>` is the type of mutable references to `vec<i32>`. Because the reference
+//@ is mutable, we can use a mutable iterator, providing mutable references to the elements.
fn vec_inc(v: &mut Vec<i32>) {
for e in v.iter_mut() {
*e += 1;
vec_inc(&mut v);
/* println!("The first element is: {}", *first); */ /* BAD! */
}
-//@ `&mut` is the operator to create a mutable reference. We have to mark `v` as mutable in order to create such a
-//@ reference: Even though we completely own `v`, Rust tries to protect us from accidentally mutating things.
+
+//@ `&mut` is the operator to create a mutable reference. We have to mark `v` as mutable in order
+//@ to create such a reference: Even though we completely own `v`, Rust tries to protect us from
+//@ accidentally mutating things.
//@ Hence owned variables that you intend to mutate have to be annotated with `mut`.
-//@ Because the reference passed to `vec_inc` only lasts as long as the function call, we can still call
-//@ `vec_inc` on the same vector twice: The durations of the two references do not overlap, so we never have more
-//@ than one mutable reference - we only ever borrow `v` once at a time. However, we can *not* create a shared reference that spans a call to `vec_inc`. Just try
+//@ Because the reference passed to `vec_inc` only lasts as long as the function call, we can still
+//@ call `vec_inc` on the same vector twice: The durations of the two references do not overlap, so
+//@ we never have more than one mutable reference - we only ever borrow `v` once at a time.
+//@ However, we can *not* create a shared reference that spans a call to `vec_inc`. Just try
//@ enabling the commented-out lines, and watch Rust complain. This is because `vec_inc` could mutate
//@ the vector structurally (i.e., it could add or remove elements), and hence the reference `first`
-//@ could become invalid. In other words, Rust keeps us safe from bugs like the one in the C++ snippet above.
+//@ could become invalid. In other words, Rust keeps us safe from bugs like the one in the C++
+//@ snippet above.
//@
-//@ Above, I said that having a mutable reference excludes aliasing. But if you look at the code above carefully,
-//@ you may say: "Wait! Don't the `v` in `mutable_ref_demo` and the `v` in `vec_inc` alias?" And you are right,
-//@ they do. However, the `v` in `mutable_ref_demo` is not actually usable, it is not *active*: As long as `v` is
-//@ borrowed, Rust will not allow you to do anything with it.
+//@ Above, I said that having a mutable reference excludes aliasing. But if you look at the code
+//@ above carefully, you may say: "Wait! Don't the `v` in `mutable_ref_demo` and the `v` in
+//@ `vec_inc` alias?" And you are right, they do. However, the `v` in `mutable_ref_demo` is not
+//@ actually usable, it is not *active*: As long as `v` is borrowed, Rust will not allow you to do
+//@ anything with it.
// ## Summary
// The ownership and borrowing system of Rust enforces the following three rules:
//
// * There is always exactly one owner of a piece of data
// * If there is an active mutable reference, then nobody else can have active access to the data
-// * If there is an active shared reference, then every other active access to the data is also a shared reference
+// * If there is an active shared reference, then every other active access to the data is also a
+// shared reference
//
-// As it turns out, combined with the abstraction facilities of Rust, this is a very powerful mechanism
-// to tackle many problems beyond basic memory safety. You will see some examples for this soon.
+// As it turns out, combined with the abstraction facilities of Rust, this is a very powerful
+// mechanism to tackle many problems beyond basic memory safety. You will see some examples for
+// this soon.
-//@ [index](main.html) | [previous](part03.html) | [raw source](workspace/src/part04.rs) | [next](part05.html)
+//@ [index](main.html) | [previous](part03.html) | [raw source](workspace/src/part04.rs) |
+//@ [next](part05.html)
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