X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/767e5b9549d03685a601ccc2f32123f1dac618c6..a52087dc8f244861d144229b04e64f934ed1d03f:/src/part04.rs?ds=sidebyside diff --git a/src/part04.rs b/src/part04.rs index a4bd164..f2a6b6c 100644 --- a/src/part04.rs +++ b/src/part04.rs @@ -1,73 +1,77 @@ -// Rust-101, Part 04: Ownership, Borrowing -// ======================================= +// Rust-101, Part 04: Ownership, Borrowing, References +// =================================================== -use std::cmp; - -// Rust aims to be a "safe systems language". As a systems language, of course it -// provides *references* (or *pointers*). But as a safe language, it has to -// prevent bugs like this C++ snippet. +//@ Rust aims to be a "safe systems language". As a systems language, of course it +//@ provides *references* (or *pointers*). But as a safe language, it has to +//@ prevent bugs like this C++ snippet. /* void foo(std::vector v) { - int &first = v[0]; + int *first = &v[0]; v.push_back(42); - first = 1337; // This is bad! + *first = 1337; // This is bad! } */ -// What's going wrong here? `first` is a reference into the vector `v`. -// The operation `push_back` may re-allocate the storage for the vector, -// in case the old buffer was full. If that happens, `first` is now -// a dangling pointer, and accessing it can crash the program (or worse). -// -// It turns out that only the combination of two circumstances can lead to such a bug: -// *aliasing* and *mutation*. In the code above, we have `first` and the buffer of `v` -// being aliases, and when `push_back` is called, the latter is used to perform a mutation. -// Therefore, the central principle of the Rust typesystem is to *rule out mutation in the presence -// of aliasing*. The core tool to achieve that is the notion of *ownership*. +//@ What's going wrong here? `first` is a pointer into the vector `v`. The operation `push_back` +//@ may re-allocate the storage for the vector, in case the old buffer was full. If that happens, +//@ `first` is now a dangling pointer, and accessing it can crash the program (or worse). +//@ +//@ It turns out that only the combination of two circumstances can lead to such a bug: +//@ *aliasing* and *mutation*. In the code above, we have `first` and the buffer of `v` +//@ being aliases, and when `push_back` is called, the latter is used to perform a mutation. +//@ Therefore, the central principle of the Rust typesystem is to *rule out mutation in the presence +//@ of aliasing*. The core tool to achieve that is the notion of *ownership*. -// What does that mean in practice? Consider the following example. -fn take(v: Vec) { /* do something */ } -fn foo1() { +// ## Ownership +//@ What does that mean in practice? Consider the following example. +fn work_on_vector(v: Vec) { /* do something */ } +fn ownership_demo() { let v = vec![1,2,3,4]; - take(v); - /* println!("The first element is: {}", v[0]); */ + work_on_vector(v); + /* println!("The first element is: {}", v[0]); */ /* BAD! */ } -// Rust attaches additional meaning to the argument of `take`: The function can assume -// that it entirely *owns* `v`, and hence can do anything with it. When `take` ends, -// nobody needs `v` anymore, so it will be deleted (including its buffer on the heap). -// Passing a `Vec` to `take` is considered *transfer of ownership*: Someone used -// to own that vector, but now he gave it on to `take` and has no business with it anymore. -// -// If you give a book to your friend, you cannot some 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 `foo1`. 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. +//@ Rust attaches additional meaning to the argument of `work_on_vector`: The function can assume +//@ that it entirely *owns* `v`, and hence can do anything with it. When `work_on_vector` ends, +//@ nobody needs `v` anymore, so it will be deleted (including its buffer on the heap). +//@ Passing a `Vec` to `work_on_vector` is considered *transfer of ownership*: Someone used +//@ to own that vector, but now he gave it on to `take` and has no business with it anymore. +//@ +//@ 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! +//@ Essentially, ownership rules out aliasing, hence making the kind of problem discussed above +//@ impossible. -// If you go back to our example with `vec_min`, and try to call that function twice, you will -// get the same error. That's because `vec_min` demands that the caller transfers ownership of the -// 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 *borrowing* the vector, and it works a bit like borrowing does in the real world: -// If you borrow a book to your friend, 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 borrow 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 borrows. First of all, there's the *shared* borrow. -// This is where the book metaphor kind of breaks down... you can give a shared borrow of -// *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 does not allow -// mutation through a shared borrow. +// ## Borrowing a shared reference +//@ If you go back to our example with `vec_min`, and try to call that function twice, you will +//@ get the same error. That's because `vec_min` demands that the caller transfers ownership of the +//@ 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* the vector, and it considers references as *borrowing* ownership. This +//@ works a bit like borrowing does in the real world: If you borrow a book to your friend, 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 borrow 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. -// So, let's re-write `vec_min` to work on a shared borrow of a vector. In fact, the only -// thing we have to change is the type of the function. The `e` in the loop now gets type -// `&i32`, hence we have to deference it. +//@ So, let's re-write `vec_min` to work on a shared reference to a vector, written `&Vec`. +//@ I also took the liberty to convert the function from `SomethingOrNothing` to the standard +//@ library type `Option`. fn vec_min(v: &Vec) -> Option { + use std::cmp; + let mut min = None; - for e in v { + // 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 { None => *e, Some(n) => cmp::min(n, *e) @@ -77,71 +81,66 @@ fn vec_min(v: &Vec) -> Option { } // Now that `vec_min` does not acquire ownership of the vector anymore, we can call it multiple times on the same vector and also do things like -fn foo2() { +fn shared_ref_demo() { let v = vec![5,4,3,2,1]; let first = &v[0]; vec_min(&v); vec_min(&v); println!("The first element is: {}", *first); } -// What's going on here? First, `&` is how you create a shared borrow to something. This code creates three -// shared borrows to `v`: The borrow for `first` begins in the 2nd line of the function and lasts all the way to -// the end. The other two borrows, created for calling `vec_min`, only last for the duration of that -// call. -// -// Technically, of course, borrows are pointers. Notice that since `vec_min` only gets a shared -// borrow, Rust knows that it cannot mutate `v` in any way. Hence the pointer created before calling -// `vec_min` remains valid. +//@ What's going on here? First, `&` is how you borrow 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 +//@ reference, Rust knows that it cannot mutate `v`. Hence the pointer into the buffer of `v` +//@ that was created before calling `vec_min` remains valid. -// There is a second kind of borrow, a *mutable borrow*. As the name suggests, such a borrow permits -// mutation, and hence has to prevent aliasing. There can only ever be one mutable borrow to a -// particular piece of data. +// ## 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. +//@ 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. +//@ As an example, consider a function which increments every element of a vector by 1. +//@ The type `&mut Vec` is the type of mutable references to `vec`. Because the reference is +//@ mutable, we can use a mutable iterator, providing mutable references to the elements. fn vec_inc(v: &mut Vec) { - for e in v { + for e in v.iter_mut() { *e += 1; } } -// The type `&mut Vec` is the type of mutable borrows of `vec`. Because the borrow is -// mutable, we can change `e` in the loop. How can we call this function? -fn foo3() { +// Here's an example of calling `vec_inc`. +fn mutable_ref_demo() { let mut v = vec![5,4,3,2,1]; /* let first = &v[0]; */ vec_inc(&mut v); vec_inc(&mut v); - /* println!("The first element is: {}", *first); */ -} -// `&mut` is the operator to create a mutable borrow. We have to mark `v` as mutable in order -// to create such a borrow. Because the borrow 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 borrows do not overlap, so we never have more than one mutable borrow. -// However, we can *not* create a shared borrow that spans a call to `vec_inc`. Just try -// enabling the commented-out lines. This is because `vec_inc` could mutate the vector structurally -// (i.e., it could add or remove elements), and hence the pointer `first` could become invalid. -// -// Above, I said that having a mutable borrow excludes aliasing. But if you look at the code above carefully, -// you may say: "Wait! Don't the `v` in `foo3` and the `v` in `vec_inc` alias?" And you are right, -// they do. However, the `v` in `foo3` is not actually usable, it is not *active*: As long as there is an -// outstanding borrow, Rust will not allow you to do anything with `v`. This is, in fact, what -// prevents the creation of a mutable borrow when there already is a shared one. - -// This also works the other way around: In `foo4`, there is already a mutable borrow active in the `vec_min` -// line, so the attempt to create another shared borrow is rejected by the compiler. -fn foo4() { - let mut v = vec![5,4,3,2,1]; - let first = &mut v[0]; - /* vec_min(&v); */ - println!("The first element is: {}", *first); + /* 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. +//@ 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 +//@ 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++ snipped 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. -// So, to summarize: The ownership and borrowing system of Rust enforces the following three rules: +// ## 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 borrow, then nobody else can have active access to the data -// * If there is an active shared borrow, then every other active access to the data is also a shared borrow +// * 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 // // 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. +// to tackle many problems beyond basic memory safety. You will see some examples for this soon. -// [index](main.html) | [previous](part03.html) | [next](main.html) +//@ [index](main.html) | [previous](part03.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part04.rs) | [next](part05.html)