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.
Before we get to the details, two important notes about the ownership system.
Rust has a focus on safety and speed. It accomplishes these goals through many zero-cost abstractions, which means that in Rust, abstractions cost as little as possible in order to make them work. The ownership system is a prime example of a zero cost abstraction. All of the analysis we'll talk about in this guide is done at compile time. You do not pay any run-time cost for any of these features.
However, this system does have a certain cost: learning curve. Many new users to Rust experience something we like to call "fighting with the borrow checker," where the Rust compiler refuses to compile a program that the author thinks is valid. This often happens because the programmer's mental model of how ownership should work doesn't match the actual rules that Rust implements. You probably will experience similar things at first. There is good news, however: more experienced Rust developers report that once they work with the rules of the ownership system for a period of time, they fight the borrow checker less and less.
With that in mind, let's learn about 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::new(5);
}
The Box::new function creates a Box<T> (specifically Box<i32> in this
case) by allocating a small segment of memory on the heap with enough space to
fit an i32. 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::new(5); add_one(x); } fn add_one(mut num: Box<i32>) { *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::new(5); add_one(x); println!("{}", x); } fn add_one(mut num: Box<i32>) { *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::new(5); let y = add_one(x); println!("{}", y); } fn add_one(mut num: Box<i32>) -> Box<i32> { *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.
Here's the current state of our add_one function:
fn add_one(mut num: Box<i32>) -> Box<i32> { *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 i32) { *num += 1; }
This function borrows an i32 from its caller, and then increments it. When
the function is over, and num goes out of scope, the borrow is over.
We have to change our main a bit too:
fn main() { let mut x = 5; add_one(&mut x); println!("{}", x); } fn add_one(num: &mut i32) { *num += 1; }
We don't need to assign the result of add_one() anymore, because it doesn't
return anything anymore. This is because we're not passing ownership back,
since we just borrow, not take ownership.
Lending out a reference to a resource that someone else owns can be complicated, however. For example, imagine this set of operations:
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.
Remember the function that borrowed an i32? Let's look at it again.
fn add_one(num: &mut i32) { *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 mut i32) { *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 mut i32)
If you compare &mut i32 to &'a mut i32, they're the same, it's just that the
lifetime 'a has snuck in between the & and the mut i32. We read &mut i32 as "a
mutable reference to an i32" and &'a mut i32 as "a mutable reference to an i32 with the lifetime 'a.'"
Why do lifetimes matter? Well, for example, here's some code:
struct Foo<'a> { x: &'a i32, } fn main() { let y = &5; // 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> {
declares a lifetime, and
x: &'a i32,
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 i32 it contains.
A way to think about lifetimes is to visualize the scope that a reference is valid for. For example:
fn main() { let y = &5; // -+ y goes into scope // | // stuff // | // | } // -+ y goes out of scope
Adding in our Foo:
struct Foo<'a> { x: &'a i32, } fn main() { let y = &5; // -+ 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 i32, } fn main() { let x; // -+ x goes into scope // | { // | let y = &5; // ---+ 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.
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: i32 = 5; let x: &'static i32 = &FOO;
This adds an i32 to the data segment of the binary, and x is a reference
to it.
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: i32, owner: Car, } fn main() { let car = Car { name: "DeLorean".to_string() }; for _ in 0..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: i32, owner: Rc<Car>, } fn main() { let car = Car { name: "DeLorean".to_string() }; let car_owner = Rc::new(car); for _ in 0..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>.
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 lifetime 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.
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: u32, s: &str); // elided fn debug<'a>(lvl: u32, s: &'a str); // expanded // In the preceding 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: u32) -> &str; // elided fn substr<'a>(s: &'a str, until: u32) -> &'a str; // expanded fn get_str() -> &str; // ILLEGAL, no inputs fn frob(s: &str, t: &str) -> &str; // ILLEGAL, two inputs fn frob<'a, 'b>(s: &'a str, t: &'b str) -> &str; // Expanded: Output lifetime is unclear 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.