Steve Klabnik a56e7aee81 Add lifetime elision information to the ownership guide.

Fixes #19662.

2015-01-05 11:51:52 -05:00

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% The Rust Ownership Guide

This guide presents Rust's ownership system. This is one of Rust's most unique and compelling features, with which Rust developers should become quite acquainted. Ownership is how Rust achieves its largest goal, memory safety. The ownership system has a few distinct concepts: ownership, borrowing, and lifetimes. We'll talk about each one in turn.

Ownership

At its core, ownership is about 'resources.' For the purposes of the vast majority of this guide, we will talk about a specific resource: memory. The concept generalizes to any kind of resource, like a file handle, but to make it more concrete, we'll focus on memory.

When your program allocates some memory, it needs some way to deallocate that memory. Imagine a function foo that allocates four bytes of memory, and then never deallocates that memory. We call this problem 'leaking' memory, because each time we call foo, we're allocating another four bytes. Eventually, with enough calls to foo, we will run our system out of memory. That's no good. So we need some way for foo to deallocate those four bytes. It's also important that we don't deallocate too many times, either. Without getting into the details, attempting to deallocate memory multiple times can lead to problems. In other words, any time some memory is allocated, we need to make sure that we deallocate that memory once and only once. Too many times is bad, not enough times is bad. The counts must match.

There's one other important detail with regards to allocating memory. Whenever we request some amount of memory, what we are given is a handle to that memory. This handle (often called a 'pointer', when we're referring to memory) is how we interact with the allocated memory. As long as we have that handle, we can do something with the memory. Once we're done with the handle, we're also done with the memory, as we can't do anything useful without a handle to it.

Historically, systems programming languages require you to track these allocations, deallocations, and handles yourself. For example, if we want some memory from the heap in a language like C, we do this:

{
    int *x = malloc(sizeof(int));

    // we can now do stuff with our handle x
    *x = 5;

    free(x);
}

The call to malloc allocates some memory. The call to free deallocates the memory. There's also bookkeeping about allocating the correct amount of memory.

Rust combines these two aspects of allocating memory (and other resources) into a concept called 'ownership.' Whenever we request some memory, that handle we receive is called the 'owning handle.' Whenever that handle goes out of scope, Rust knows that you cannot do anything with the memory anymore, and so therefore deallocates the memory for you. Here's the equivalent example in Rust:

{
    let x = box 5i;
}

The box keyword creates a Box<T> (specifically Box<int> in this case) by allocating a small segment of memory on the heap with enough space to fit an int. But where in the code is the box deallocated? We said before that we must have a deallocation for each allocation. Rust handles this for you. It knows that our handle, x, is the owning reference to our box. Rust knows that x will go out of scope at the end of the block, and so it inserts a call to deallocate the memory at the end of the scope. Because the compiler does this for us, it's impossible to forget. We always have exactly one deallocation paired with each of our allocations.

This is pretty straightforward, but what happens when we want to pass our box to a function? Let's look at some code:

fn main() {
    let x = box 5i;

    add_one(x);
}

fn add_one(mut num: Box<int>) {
    *num += 1;
}

This code works, but it's not ideal. For example, let's add one more line of code, where we print out the value of x:

fn main() {
    let x = box 5i;

    add_one(x);

    println!("{}", x);
}

fn add_one(mut num: Box<int>) {
    *num += 1;
}

This does not compile, and gives us an error:

error: use of moved value: `x`
   println!("{}", x);
                  ^

Remember, we need one deallocation for every allocation. When we try to pass our box to add_one, we would have two handles to the memory: x in main, and num in add_one. If we deallocated the memory when each handle went out of scope, we would have two deallocations and one allocation, and that's wrong. So when we call add_one, Rust defines num as the owner of the handle. And so, now that we've given ownership to num, x is invalid. x's value has "moved" from x to num. Hence the error: use of moved value x.

To fix this, we can have add_one give ownership back when it's done with the box:

fn main() {
    let x = box 5i;

    let y = add_one(x);

    println!("{}", y);
}

fn add_one(mut num: Box<int>) -> Box<int> {
    *num += 1;

    num
}

This code will compile and run just fine. Now, we return a box, and so the ownership is transferred back to y in main. We only have ownership for the duration of our function before giving it back. This pattern is very common, and so Rust introduces a concept to describe a handle which temporarily refers to something another handle owns. It's called "borrowing," and it's done with "references", designated by the & symbol.

Borrowing

Here's the current state of our add_one function:

fn add_one(mut num: Box<int>) -> Box<int> {
    *num += 1;

    num
}

This function takes ownership, because it takes a Box, which owns its contents. But then we give ownership right back.

In the physical world, you can give one of your possessions to someone for a short period of time. You still own your possession, you're just letting someone else use it for a while. We call that 'lending' something to someone, and that person is said to be 'borrowing' that something from you.

Rust's ownership system also allows an owner to lend out a handle for a limited period. This is also called 'borrowing.' Here's a version of add_one which borrows its argument rather than taking ownership:

fn add_one(num: &mut int) {
    *num += 1;
}

This function borrows an int from its caller, and then increments it. When the function is over, and num goes out of scope, the borrow is over.

Lifetimes

Lending out a reference to a resource that someone else owns can be complicated, however. For example, imagine this set of operations:

I acquire a handle to some kind of resource.
I lend you a reference to the resource.
I decide I'm done with the resource, and deallocate it, while you still have your reference.
You decide to use the resource.

Uh oh! Your reference is pointing to an invalid resource. This is called a "dangling pointer" or "use after free," when the resource is memory.

To fix this, we have to make sure that step four never happens after step three. The ownership system in Rust does this through a concept called "lifetimes," which describe the scope that a reference is valid for.

Let's look at that function which borrows an int again:

fn add_one(num: &int) -> int {
    *num + 1
}

Rust has a feature called 'lifetime elision,' which allows you to not write lifetime annotations in certain circumstances. This is one of them. We will cover the others later. Without eliding the lifetimes, add_one looks like this:

fn add_one<'a>(num: &'a int) -> int {
    *num + 1
}

The 'a is called a lifetime. Most lifetimes are used in places where short names like 'a, 'b and 'c are clearest, but it's often useful to have more descriptive names. Let's dig into the syntax in a bit more detail:

fn add_one<'a>(...)

This part declares our lifetimes. This says that add_one has one lifetime, 'a. If we had two, it would look like this:

fn add_two<'a, 'b>(...)

Then in our parameter list, we use the lifetimes we've named:

...(num: &'a int) -> ...

If you compare &int to &'a int, they're the same, it's just that the lifetime 'a has snuck in between the & and the int. We read &int as "a reference to an int" and &'a int as "a reference to an int with the lifetime 'a.'"

Why do lifetimes matter? Well, for example, here's some code:

struct Foo<'a> {
    x: &'a int,
}

fn main() {
    let y = &5i; // this is the same as `let _y = 5; let y = &_y;
    let f = Foo { x: y };

    println!("{}", f.x);
}

As you can see, structs can also have lifetimes. In a similar way to functions,

struct Foo<'a> {
# x: &'a int,
# }

declares a lifetime, and

# struct Foo<'a> {
x: &'a int,
# }

uses it. So why do we need a lifetime here? We need to ensure that any reference to a Foo cannot outlive the reference to an int it contains.

Thinking in scopes

A way to think about lifetimes is to visualize the scope that a reference is valid for. For example:

fn main() {
    let y = &5i;    // -+ y goes into scope
                    //  |
    // stuff        //  |
                    //  |
}                   // -+ y goes out of scope

Adding in our Foo:

struct Foo<'a> {
    x: &'a int,
}

fn main() {
    let y = &5i;          // -+ y goes into scope
    let f = Foo { x: y }; // -+ f goes into scope
    // stuff              //  |
                          //  |
}                         // -+ f and y go out of scope

Our f lives within the scope of y, so everything works. What if it didn't? This code won't work:

struct Foo<'a> {
    x: &'a int,
}

fn main() {
    let x;                    // -+ x goes into scope
                              //  |
    {                         //  |
        let y = &5i;          // ---+ y goes into scope
        let f = Foo { x: y }; // ---+ f goes into scope
        x = &f.x;             //  | | error here
    }                         // ---+ f and y go out of scope
                              //  |
    println!("{}", x);        //  |
}                             // -+ x goes out of scope

Whew! As you can see here, the scopes of f and y are smaller than the scope of x. But when we do x = &f.x, we make x a reference to something that's about to go out of scope.

Named lifetimes are a way of giving these scopes a name. Giving something a name is the first step towards being able to talk about it.

'static

The lifetime named 'static' is a special lifetime. It signals that something has the lifetime of the entire program. Most Rust programmers first come across 'static when dealing with strings:

let x: &'static str = "Hello, world.";

String literals have the type &'static str because the reference is always alive: they are baked into the data segment of the final binary. Another example are globals:

static FOO: int = 5i;
let x: &'static int = &FOO;

This adds an int to the data segment of the binary, and FOO is a reference to it.

Shared Ownership

In all the examples we've considered so far, we've assumed that each handle has a singular owner. But sometimes, this doesn't work. Consider a car. Cars have four wheels. We would want a wheel to know which car it was attached to. But this won't work:

struct Car {
    name: String,
}

struct Wheel {
    size: int,
    owner: Car,
}

fn main() {
    let car = Car { name: "DeLorean".to_string() };

    for _ in range(0u, 4) {
        Wheel { size: 360, owner: car };
    }
}

We try to make four Wheels, each with a Car that it's attached to. But the compiler knows that on the second iteration of the loop, there's a problem:

error: use of moved value: `car`
    Wheel { size: 360, owner: car };
                              ^~~
note: `car` moved here because it has type `Car`, which is non-copyable
    Wheel { size: 360, owner: car };
                              ^~~

We need our Car to be pointed to by multiple Wheels. We can't do that with Box<T>, because it has a single owner. We can do it with Rc<T> instead:

use std::rc::Rc;

struct Car {
    name: String,
}

struct Wheel {
    size: int,
    owner: Rc<Car>,
}

fn main() {
    let car = Car { name: "DeLorean".to_string() };

    let car_owner = Rc::new(car);

    for _ in range(0u, 4) {
        Wheel { size: 360, owner: car_owner.clone() };
    }
}

We wrap our Car in an Rc<T>, getting an Rc<Car>, and then use the clone() method to make new references. We've also changed our Wheel to have an Rc<Car> rather than just a Car.

This is the simplest kind of multiple ownership possible. For example, there's also Arc<T>, which uses more expensive atomic instructions to be the thread-safe counterpart of Rc<T>.

Lifetime Elision

Earlier, we mentioned 'lifetime elision,' a feature of Rust which allows you to not write lifetime annotations in certain circumstances. All references have a lifetime, and so if you elide a lifetime (like &T instead of &'a T), Rust will do three things to determine what those lifetimes should be.

When talking about lifetime elision, we use the term 'input lifetime' and 'output lifetime'. An 'input liftime' is a lifetime associated with a parameter of a function, and an 'output lifetime' is a lifetime associated with the return value of a function. For example, this function has an input lifetime:

fn foo<'a>(bar: &'a str)

This one has an output lifetime:

fn foo<'a>() -> &'a str

This one has a lifetime in both positions:

fn foo<'a>(bar: &'a str) -> &'a str

Here are the three rules:

Each elided lifetime in a function's arguments becomes a distinct lifetime parameter.
If there is exactly one input lifetime, elided or not, that lifetime is assigned to all elided lifetimes in the return values of that function..
If there are multiple input lifetimes, but one of them is &self or &mut self, the lifetime of self is assigned to all elided output lifetimes.

Otherwise, it is an error to elide an output lifetime.

Examples

Here are some examples of functions with elided lifetimes, and the version of what the elided lifetimes are expand to:

fn print(s: &str);                                      // elided
fn print<'a>(s: &'a str);                               // expanded

fn debug(lvl: uint, s: &str);                           // elided
fn debug<'a>(lvl: uint, s: &'a str);                    // expanded

// In the preceeding example, `lvl` doesn't need a lifetime because it's not a
// reference (`&`). Only things relating to references (such as a `struct`
// which contains a reference) need lifetimes.

fn substr(s: &str, until: uint) -> &str;                // elided
fn substr<'a>(s: &'a str, until: uint) -> &'a str;      // expanded

fn get_str() -> &str;                                   // ILLEGAL, no inputs

fn frob(s: &str, t: &str) -> &str;                      // ILLEGAL, two inputs

fn get_mut(&mut self) -> &mut T;                        // elided
fn get_mut<'a>(&'a mut self) -> &'a mut T;              // expanded

fn args<T:ToCStr>(&mut self, args: &[T]) -> &mut Command                  // elided
fn args<'a, 'b, T:ToCStr>(&'a mut self, args: &'b [T]) -> &'a mut Command // expanded

fn new(buf: &mut [u8]) -> BufWriter;                    // elided
fn new<'a>(buf: &'a mut [u8]) -> BufWriter<'a>          // expanded

Coming Soon.

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