Clarified exercise 11.1.

[rust-101.git] / src / part04.rs

// Rust-101, Part 04: Ownership, Borrowing, References
// ===================================================

//@ 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<int> v) {
      int *first = &v[0];
      v.push_back(42);
      *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<i32>) { /* do something */ }
fn ownership_demo() {
    let v = vec![1,2,3,4];
    work_on_vector(v);
    /* 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<i32>` 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.

// ## 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* 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.

//@ 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
//@ library type `Option`.
fn vec_min(v: &Vec<i32>) -> Option<i32> {
    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.
    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)
        });
    }
    min
}

// 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_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 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
//@ 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.

// ## 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.
//@ 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;
    }
}
// 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); */             /* 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++ 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.

// ## 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
// 
// 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.

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