1 % The Rust Ownership Guide
3 This guide presents Rust's ownership system. This is one of Rust's most unique
4 and compelling features, with which Rust developers should become quite
5 acquainted. Ownership is how Rust achieves its largest goal, memory safety.
6 The ownership system has a few distinct concepts: **ownership**, **borrowing**,
7 and **lifetimes**. We'll talk about each one in turn.
11 Before we get to the details, two important notes about the ownership system.
13 Rust has a focus on safety and speed. It accomplishes these goals through many
14 "zero cost abstractions," which means that in Rust, abstractions cost as little
15 as possible in order to make them work. The ownership system is a prime example
16 of a zero cost abstraction. All of the analysis we'll talk about in this guide
17 is _done at compile time_. You do not pay any run-time cost for any of these
20 However, this system does have a certain cost: learning curve. Many new users
21 to Rust experience something we like to call "fighting with the borrow
22 checker," where the Rust compiler refuses to compile a program that the author
23 thinks is valid. This often happens because the programmer's mental model of
24 how ownership should work doesn't match the actual rules that Rust implements.
25 You probably will experience similar things at first. There is good news,
26 however: more experienced Rust developers report that once they work with the
27 rules of the ownership system for a period of time, they fight the borrow
28 checker less and less.
30 With that in mind, let's learn about ownership.
34 At its core, ownership is about 'resources.' For the purposes of the vast
35 majority of this guide, we will talk about a specific resource: memory. The
36 concept generalizes to any kind of resource, like a file handle, but to make it
37 more concrete, we'll focus on memory.
39 When your program allocates some memory, it needs some way to deallocate that
40 memory. Imagine a function `foo` that allocates four bytes of memory, and then
41 never deallocates that memory. We call this problem 'leaking' memory, because
42 each time we call `foo`, we're allocating another four bytes. Eventually, with
43 enough calls to `foo`, we will run our system out of memory. That's no good. So
44 we need some way for `foo` to deallocate those four bytes. It's also important
45 that we don't deallocate too many times, either. Without getting into the
46 details, attempting to deallocate memory multiple times can lead to problems.
47 In other words, any time some memory is allocated, we need to make sure that we
48 deallocate that memory once and only once. Too many times is bad, not enough
49 times is bad. The counts must match.
51 There's one other important detail with regards to allocating memory. Whenever
52 we request some amount of memory, what we are given is a handle to that memory.
53 This handle (often called a 'pointer', when we're referring to memory) is how
54 we interact with the allocated memory. As long as we have that handle, we can
55 do something with the memory. Once we're done with the handle, we're also done
56 with the memory, as we can't do anything useful without a handle to it.
58 Historically, systems programming languages require you to track these
59 allocations, deallocations, and handles yourself. For example, if we want some
60 memory from the heap in a language like C, we do this:
64 int *x = malloc(sizeof(int));
66 // we can now do stuff with our handle x
73 The call to `malloc` allocates some memory. The call to `free` deallocates the
74 memory. There's also bookkeeping about allocating the correct amount of memory.
76 Rust combines these two aspects of allocating memory (and other resources) into
77 a concept called 'ownership.' Whenever we request some memory, that handle we
78 receive is called the 'owning handle.' Whenever that handle goes out of scope,
79 Rust knows that you cannot do anything with the memory anymore, and so
80 therefore deallocates the memory for you. Here's the equivalent example in
89 The `box` keyword creates a `Box<T>` (specifically `Box<int>` in this case) by
90 allocating a small segment of memory on the heap with enough space to fit an
91 `int`. But where in the code is the box deallocated? We said before that we
92 must have a deallocation for each allocation. Rust handles this for you. It
93 knows that our handle, `x`, is the owning reference to our box. Rust knows that
94 `x` will go out of scope at the end of the block, and so it inserts a call to
95 deallocate the memory at the end of the scope. Because the compiler does this
96 for us, it's impossible to forget. We always have exactly one deallocation paired
97 with each of our allocations.
99 This is pretty straightforward, but what happens when we want to pass our box
100 to a function? Let's look at some code:
109 fn add_one(mut num: Box<int>) {
114 This code works, but it's not ideal. For example, let's add one more line of
115 code, where we print out the value of `x`:
126 fn add_one(mut num: Box<int>) {
131 This does not compile, and gives us an error:
134 error: use of moved value: `x`
139 Remember, we need one deallocation for every allocation. When we try to pass
140 our box to `add_one`, we would have two handles to the memory: `x` in `main`,
141 and `num` in `add_one`. If we deallocated the memory when each handle went out
142 of scope, we would have two deallocations and one allocation, and that's wrong.
143 So when we call `add_one`, Rust defines `num` as the owner of the handle. And
144 so, now that we've given ownership to `num`, `x` is invalid. `x`'s value has
145 "moved" from `x` to `num`. Hence the error: use of moved value `x`.
147 To fix this, we can have `add_one` give ownership back when it's done with the
159 fn add_one(mut num: Box<int>) -> Box<int> {
166 This code will compile and run just fine. Now, we return a `box`, and so the
167 ownership is transferred back to `y` in `main`. We only have ownership for the
168 duration of our function before giving it back. This pattern is very common,
169 and so Rust introduces a concept to describe a handle which temporarily refers
170 to something another handle owns. It's called "borrowing," and it's done with
171 "references", designated by the `&` symbol.
175 Here's the current state of our `add_one` function:
178 fn add_one(mut num: Box<int>) -> Box<int> {
185 This function takes ownership, because it takes a `Box`, which owns its
186 contents. But then we give ownership right back.
188 In the physical world, you can give one of your possessions to someone for a
189 short period of time. You still own your possession, you're just letting someone
190 else use it for a while. We call that 'lending' something to someone, and that
191 person is said to be 'borrowing' that something from you.
193 Rust's ownership system also allows an owner to lend out a handle for a limited
194 period. This is also called 'borrowing.' Here's a version of `add_one` which
195 borrows its argument rather than taking ownership:
198 fn add_one(num: &mut int) {
203 This function borrows an `int` from its caller, and then increments it. When
204 the function is over, and `num` goes out of scope, the borrow is over.
208 Lending out a reference to a resource that someone else owns can be
209 complicated, however. For example, imagine this set of operations:
211 1. I acquire a handle to some kind of resource.
212 2. I lend you a reference to the resource.
213 3. I decide I'm done with the resource, and deallocate it, while you still have
215 4. You decide to use the resource.
217 Uh oh! Your reference is pointing to an invalid resource. This is called a
218 "dangling pointer" or "use after free," when the resource is memory.
220 To fix this, we have to make sure that step four never happens after step
221 three. The ownership system in Rust does this through a concept called
222 "lifetimes," which describe the scope that a reference is valid for.
224 Let's look at that function which borrows an `int` again:
227 fn add_one(num: &int) -> int {
232 Rust has a feature called 'lifetime elision,' which allows you to not write
233 lifetime annotations in certain circumstances. This is one of them. We will
234 cover the others later. Without eliding the lifetimes, `add_one` looks like
238 fn add_one<'a>(num: &'a int) -> int {
243 The `'a` is called a **lifetime**. Most lifetimes are used in places where
244 short names like `'a`, `'b` and `'c` are clearest, but it's often useful to
245 have more descriptive names. Let's dig into the syntax in a bit more detail:
251 This part _declares_ our lifetimes. This says that `add_one` has one lifetime,
252 `'a`. If we had two, it would look like this:
255 fn add_two<'a, 'b>(...)
258 Then in our parameter list, we use the lifetimes we've named:
261 ...(num: &'a int) -> ...
264 If you compare `&int` to `&'a int`, they're the same, it's just that the
265 lifetime `'a` has snuck in between the `&` and the `int`. We read `&int` as "a
266 reference to an int" and `&'a int` as "a reference to an int with the lifetime 'a.'"
268 Why do lifetimes matter? Well, for example, here's some code:
276 let y = &5i; // this is the same as `let _y = 5; let y = &_y;
277 let f = Foo { x: y };
283 As you can see, `struct`s can also have lifetimes. In a similar way to functions,
291 declares a lifetime, and
299 uses it. So why do we need a lifetime here? We need to ensure that any reference
300 to a `Foo` cannot outlive the reference to an `int` it contains.
302 ## Thinking in scopes
304 A way to think about lifetimes is to visualize the scope that a reference is
305 valid for. For example:
309 let y = &5i; // -+ y goes into scope
313 } // -+ y goes out of scope
324 let y = &5i; // -+ y goes into scope
325 let f = Foo { x: y }; // -+ f goes into scope
328 } // -+ f and y go out of scope
331 Our `f` lives within the scope of `y`, so everything works. What if it didn't?
332 This code won't work:
340 let x; // -+ x goes into scope
343 let y = &5i; // ---+ y goes into scope
344 let f = Foo { x: y }; // ---+ f goes into scope
345 x = &f.x; // | | error here
346 } // ---+ f and y go out of scope
348 println!("{}", x); // |
349 } // -+ x goes out of scope
352 Whew! As you can see here, the scopes of `f` and `y` are smaller than the scope
353 of `x`. But when we do `x = &f.x`, we make `x` a reference to something that's
354 about to go out of scope.
356 Named lifetimes are a way of giving these scopes a name. Giving something a
357 name is the first step towards being able to talk about it.
361 The lifetime named 'static' is a special lifetime. It signals that something
362 has the lifetime of the entire program. Most Rust programmers first come across
363 `'static` when dealing with strings:
366 let x: &'static str = "Hello, world.";
369 String literals have the type `&'static str` because the reference is always
370 alive: they are baked into the data segment of the final binary. Another
374 static FOO: int = 5i;
375 let x: &'static int = &FOO;
378 This adds an `int` to the data segment of the binary, and FOO is a reference to
383 In all the examples we've considered so far, we've assumed that each handle has
384 a singular owner. But sometimes, this doesn't work. Consider a car. Cars have
385 four wheels. We would want a wheel to know which car it was attached to. But
399 let car = Car { name: "DeLorean".to_string() };
401 for _ in range(0u, 4) {
402 Wheel { size: 360, owner: car };
407 We try to make four `Wheel`s, each with a `Car` that it's attached to. But the
408 compiler knows that on the second iteration of the loop, there's a problem:
411 error: use of moved value: `car`
412 Wheel { size: 360, owner: car };
414 note: `car` moved here because it has type `Car`, which is non-copyable
415 Wheel { size: 360, owner: car };
419 We need our `Car` to be pointed to by multiple `Wheel`s. We can't do that with
420 `Box<T>`, because it has a single owner. We can do it with `Rc<T>` instead:
435 let car = Car { name: "DeLorean".to_string() };
437 let car_owner = Rc::new(car);
439 for _ in range(0u, 4) {
440 Wheel { size: 360, owner: car_owner.clone() };
445 We wrap our `Car` in an `Rc<T>`, getting an `Rc<Car>`, and then use the
446 `clone()` method to make new references. We've also changed our `Wheel` to have
447 an `Rc<Car>` rather than just a `Car`.
449 This is the simplest kind of multiple ownership possible. For example, there's
450 also `Arc<T>`, which uses more expensive atomic instructions to be the
451 thread-safe counterpart of `Rc<T>`.
455 Earlier, we mentioned 'lifetime elision,' a feature of Rust which allows you to
456 not write lifetime annotations in certain circumstances. All references have a
457 lifetime, and so if you elide a lifetime (like `&T` instead of `&'a T`), Rust
458 will do three things to determine what those lifetimes should be.
460 When talking about lifetime elision, we use the term 'input lifetime' and
461 'output lifetime'. An 'input lifetime' is a lifetime associated with a parameter
462 of a function, and an 'output lifetime' is a lifetime associated with the return
463 value of a function. For example, this function has an input lifetime:
466 fn foo<'a>(bar: &'a str)
469 This one has an output lifetime:
472 fn foo<'a>() -> &'a str
475 This one has a lifetime in both positions:
478 fn foo<'a>(bar: &'a str) -> &'a str
481 Here are the three rules:
483 * Each elided lifetime in a function's arguments becomes a distinct lifetime
486 * If there is exactly one input lifetime, elided or not, that lifetime is
487 assigned to all elided lifetimes in the return values of that function..
489 * If there are multiple input lifetimes, but one of them is `&self` or `&mut
490 self`, the lifetime of `self` is assigned to all elided output lifetimes.
492 Otherwise, it is an error to elide an output lifetime.
496 Here are some examples of functions with elided lifetimes, and the version of
497 what the elided lifetimes are expand to:
500 fn print(s: &str); // elided
501 fn print<'a>(s: &'a str); // expanded
503 fn debug(lvl: uint, s: &str); // elided
504 fn debug<'a>(lvl: uint, s: &'a str); // expanded
506 // In the preceeding example, `lvl` doesn't need a lifetime because it's not a
507 // reference (`&`). Only things relating to references (such as a `struct`
508 // which contains a reference) need lifetimes.
510 fn substr(s: &str, until: uint) -> &str; // elided
511 fn substr<'a>(s: &'a str, until: uint) -> &'a str; // expanded
513 fn get_str() -> &str; // ILLEGAL, no inputs
515 fn frob(s: &str, t: &str) -> &str; // ILLEGAL, two inputs
517 fn get_mut(&mut self) -> &mut T; // elided
518 fn get_mut<'a>(&'a mut self) -> &'a mut T; // expanded
520 fn args<T:ToCStr>(&mut self, args: &[T]) -> &mut Command // elided
521 fn args<'a, 'b, T:ToCStr>(&'a mut self, args: &'b [T]) -> &'a mut Command // expanded
523 fn new(buf: &mut [u8]) -> BufWriter; // elided
524 fn new<'a>(buf: &'a mut [u8]) -> BufWriter<'a> // expanded