X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/229b86d07e94cd3ec175051a44b3f3cb45b40b65..53b9ab56f7b712e6710fd873d456a0fe1f13b785:/workspace/src/part04.rs diff --git a/workspace/src/part04.rs b/workspace/src/part04.rs index 894ce24..e0d522e 100644 --- a/workspace/src/part04.rs +++ b/workspace/src/part04.rs @@ -1,11 +1,6 @@ -// ***Remember to enable/add this part in `main.rs`!*** +// Rust-101, Part 04: Ownership, Borrowing, References +// =================================================== -// Rust-101, Part 04: Ownership, Borrowing -// ======================================= - -// 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]; @@ -13,64 +8,24 @@ *first = 1337; // This is bad! } */ -// 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*. // ## 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]; work_on_vector(v); - /* println!("The first element is: {}", v[0]); */ + /* println!("The first element is: {}", v[0]); */ /* BAD! */ } -// 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. -// ## Shared borrowing -// 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 -// So, let's re-write `vec_min` to work on a shared borrow of 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, @@ -81,64 +36,37 @@ 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 shared_borrow_demo() { +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. All borrows are created like -// this - there is no way to have something like a NULL pointer. 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 respective 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 into the buffer of `v` -// that was created before calling `vec_min` remains valid. -// ## Mutable borrowing -// 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 -// As an example, consider a function which increments every element of a vector by 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. fn vec_inc(v: &mut Vec) { - for e in v { + for e in v.iter_mut() { *e += 1; } } // Here's an example of calling `vec_inc`. -fn mutable_borrow_demo() { +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); */ + /* println!("The first element is: {}", *first); */ /* BAD! */ } -// `&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, 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 pointer `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 borrow excludes aliasing. But if you look at the code above carefully, -// you may say: "Wait! Don't the `v` in `mutable_borrow_demo` and the `v` in `vec_inc` alias?" And you are right, -// they do. However, the `v` in `mutable_borrow_demo` 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`. // ## 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. You will see some examples for this soon. -// [index](main.html) | [previous](part03.html) | [next](part05.html)