rust/src/doc/guide-pointers.md

% The Rust Pointer Guide

Rust's pointers are one of its more unique and compelling features. Pointers are also one of the more confusing topics for newcomers to Rust. They can also be confusing for people coming from other languages that support pointers, such as C++. This guide will help you understand this important topic.

Be sceptical of non-reference pointers in Rust: use them for a deliberate purpose, not just to make the compiler happy. Each pointer type comes with an explanation about when they are appropriate to use. Default to references unless you're in one of those specific situations.

You may be interested in the cheat sheet, which gives a quick overview of the types, names, and purpose of the various pointers.

An introduction

If you aren't familiar with the concept of pointers, here's a short introduction. Pointers are a very fundamental concept in systems programming languages, so it's important to understand them.

Pointer Basics

When you create a new variable binding, you're giving a name to a value that's stored at a particular location on the stack. (If you're not familiar with the "heap" vs. "stack", please check out this Stack Overflow question, as the rest of this guide assumes you know the difference.) Like this:

let x = 5i;
let y = 8i;

location	value
0xd3e030	5
0xd3e028	8

We're making up memory locations here, they're just sample values. Anyway, the point is that x, the name we're using for our variable, corresponds to the memory location 0xd3e030, and the value at that location is 5. When we refer to x, we get the corresponding value. Hence, x is 5.

Let's introduce a pointer. In some languages, there is just one type of 'pointer,' but in Rust, we have many types. In this case, we'll use a Rust reference, which is the simplest kind of pointer.

let x = 5i;
let y = 8i;
let z = &y;

location	value
0xd3e030	5
0xd3e028	8
0xd3e020	0xd3e028

See the difference? Rather than contain a value, the value of a pointer is a location in memory. In this case, the location of y. x and y have the type int, but z has the type &int. We can print this location using the {:p} format string:

let x = 5i;
let y = 8i;
let z = &y;

println!("{:p}", z);

This would print 0xd3e028, with our fictional memory addresses.

Because int and &int are different types, we can't, for example, add them together:

let x = 5i;
let y = 8i;
let z = &y;

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

This gives us an error:

hello.rs:6:24: 6:25 error: mismatched types: expected `int` but found `&int` (expected int but found &-ptr)
hello.rs:6     println!("{}", x + z);
                                  ^

We can dereference the pointer by using the * operator. Dereferencing a pointer means accessing the value at the location stored in the pointer. This will work:

let x = 5i;
let y = 8i;
let z = &y;

println!("{}", x + *z);

It prints 13.

That's it! That's all pointers are: they point to some memory location. Not much else to them. Now that we've discussed the 'what' of pointers, let's talk about the 'why.'

Pointer uses

Rust's pointers are quite useful, but in different ways than in other systems languages. We'll talk about best practices for Rust pointers later in the guide, but here are some ways that pointers are useful in other languages:

In C, strings are a pointer to a list of chars, ending with a null byte. The only way to use strings is to get quite familiar with pointers.

Pointers are useful to point to memory locations that are not on the stack. For example, our example used two stack variables, so we were able to give them names. But if we allocated some heap memory, we wouldn't have that name available. In C, malloc is used to allocate heap memory, and it returns a pointer.

As a more general variant of the previous two points, any time you have a structure that can change in size, you need a pointer. You can't tell at compile time how much memory to allocate, so you've gotta use a pointer to point at the memory where it will be allocated, and deal with it at run time.

Pointers are useful in languages that are pass-by-value, rather than pass-by-reference. Basically, languages can make two choices (this is made up syntax, it's not Rust):

fn foo(x) {
    x = 5
}

fn main() {
    i = 1
    foo(i)
    // what is the value of i here?
}

In languages that are pass-by-value, foo will get a copy of i, and so the original version of i is not modified. At the comment, i will still be 1. In a language that is pass-by-reference, foo will get a reference to i, and therefore, can change its value. At the comment, i will be 5.

So what do pointers have to do with this? Well, since pointers point to a location in memory...

fn foo(&int x) {
    *x = 5
}

fn main() {
    i = 1
    foo(&i)
    // what is the value of i here?
}

Even in a language which is pass by value, i will be 5 at the comment. You see, because the argument x is a pointer, we do send a copy over to foo, but because it points at a memory location, which we then assign to, the original value is still changed. This pattern is called 'pass-reference-by-value.' Tricky!

Common pointer problems

We've talked about pointers, and we've sung their praises. So what's the downside? Well, Rust attempts to mitigate each of these kinds of problems, but here are problems with pointers in other languages:

Uninitialized pointers can cause a problem. For example, what does this program do?

&int x;
*x = 5; // whoops!

Who knows? We just declare a pointer, but don't point it at anything, and then set the memory location that it points at to be 5. But which location? Nobody knows. This might be harmless, and it might be catastrophic.

When you combine pointers and functions, it's easy to accidentally invalidate the memory the pointer is pointing to. For example:

fn make_pointer(): &int {
    x = 5;

    return &x;
}

fn main() {
    &int i = make_pointer();
    *i = 5; // uh oh!
}

x is local to the make_pointer function, and therefore, is invalid as soon as make_pointer returns. But we return a pointer to its memory location, and so back in main, we try to use that pointer, and it's a very similar situation to our first one. Setting invalid memory locations is bad.

As one last example of a big problem with pointers, aliasing can be an issue. Two pointers are said to alias when they point at the same location in memory. Like this:

fn mutate(&int i, int j) {
    *i = j;
}

fn main() {
  x = 5;
  y = &x;
  z = &x; //y and z are aliased


  run_in_new_thread(mutate, y, 1);
  run_in_new_thread(mutate, z, 100);

  // what is the value of x here?
}

In this made-up example, run_in_new_thread spins up a new thread, and calls the given function name with its arguments. Since we have two threads, and they're both operating on aliases to x, we can't tell which one finishes first, and therefore, the value of x is actually non-deterministic. Worse, what if one of them had invalidated the memory location they pointed to? We'd have the same problem as before, where we'd be setting an invalid location.

Conclusion

That's a basic overview of pointers as a general concept. As we alluded to before, Rust has different kinds of pointers, rather than just one, and mitigates all of the problems that we talked about, too. This does mean that Rust pointers are slightly more complicated than in other languages, but it's worth it to not have the problems that simple pointers have.

References

The most basic type of pointer that Rust has is called a 'reference.' Rust references look like this:

let x = 5i;
let y = &x;

println!("{}", *y);
println!("{:p}", y);
println!("{}", y);

We'd say "y is a reference to x." The first println! prints out the value of y's referent by using the dereference operator, *. The second one prints out the memory location that y points to, by using the pointer format string. The third println! also prints out the value of y's referent, because println! will automatically dereference it for us.

Here's a function that takes a reference:

fn succ(x: &int) -> int { *x + 1 }

You can also use & as an operator to create a reference, so we can call this function in two different ways:

fn succ(x: &int) -> int { *x + 1 }

fn main() {

    let x = 5i;
    let y = &x;

    println!("{}", succ(y));
    println!("{}", succ(&x));
}

Both of these println!s will print out 6.

Of course, if this were real code, we wouldn't bother with the reference, and just write:

fn succ(x: int) -> int { x + 1 }

References are immutable by default:

let x = 5i;
let y = &x;

*y = 5; // error: cannot assign to immutable dereference of `&`-pointer `*y`

They can be made mutable with mut, but only if its referent is also mutable. This works:

let mut x = 5i;
let y = &mut x;

This does not:

let x = 5i;
let y = &mut x; // error: cannot borrow immutable local variable `x` as mutable

Immutable pointers are allowed to alias:

let x = 5i;
let y = &x;
let z = &x;

Mutable ones, however, are not:

let x = 5i;
let y = &mut x;
let z = &mut x; // error: cannot borrow `x` as mutable more than once at a time

Despite their complete safety, a reference's representation at runtime is the same as that of an ordinary pointer in a C program. They introduce zero overhead. The compiler does all safety checks at compile time. The theory that allows for this was originally called region pointers. Region pointers evolved into what we know today as lifetimes.

Here's the simple explanation: would you expect this code to compile?

fn main() {
    println!("{}", x);
    let x = 5;
}

Probably not. That's because you know that the name x is valid from where it's declared to when it goes out of scope. In this case, that's the end of the main function. So you know this code will cause an error. We call this duration a 'lifetime'. Let's try a more complex example:

fn main() {
    let x = &mut 5i;

    if *x < 10 {
        let y = &x;

        println!("Oh no: {}", y);
        return;
    }

    *x -= 1;

    println!("Oh no: {}", x);
}

Here, we're borrowing a pointer to x inside of the if. The compiler, however, is able to determine that that pointer will go out of scope without x being mutated, and therefore, lets us pass. This wouldn't work:

fn main() {
    let x = &mut 5i;

    if *x < 10 {
        let y = &x;
        *x -= 1;

        println!("Oh no: {}", y);
        return;
    }

    *x -= 1;

    println!("Oh no: {}", x);
}

It gives this error:

test.rs:5:8: 5:10 error: cannot assign to `*x` because it is borrowed
test.rs:5         *x -= 1;
                  ^~
test.rs:4:16: 4:18 note: borrow of `*x` occurs here
test.rs:4         let y = &x;
                          ^~

As you might guess, this kind of analysis is complex for a human, and therefore hard for a computer, too! There is an entire guide devoted to references and lifetimes that goes into lifetimes in great detail, so if you want the full details, check that out.

Best practices

In general, prefer stack allocation over heap allocation. Using references to stack allocated information is preferred whenever possible. Therefore, references are the default pointer type you should use, unless you have specific reason to use a different type. The other types of pointers cover when they're appropriate to use in their own best practices sections.

Use references when you want to use a pointer, but do not want to take ownership. References just borrow ownership, which is more polite if you don't need the ownership. In other words, prefer:

fn succ(x: &int) -> int { *x + 1 }

to

fn succ(x: Box<int>) -> int { *x + 1 }

As a corollary to that rule, references allow you to accept a wide variety of other pointers, and so are useful so that you don't have to write a number of variants per pointer. In other words, prefer:

fn succ(x: &int) -> int { *x + 1 }

to

fn box_succ(x: Box<int>) -> int { *x + 1 }

fn rc_succ(x: std::rc::Rc<int>) -> int { *x + 1 }

Boxes

Box<T> is Rust's 'boxed pointer' type. Boxes provide the simplest form of heap allocation in Rust. Creating a box looks like this:

let x = box(std::boxed::HEAP) 5i;

box is a keyword that does 'placement new,' which we'll talk about in a bit. box will be useful for creating a number of heap-allocated types, but is not quite finished yet. In the meantime, box's type defaults to std::boxed::HEAP, and so you can leave it off:

let x = box 5i;

As you might assume from the HEAP, boxes are heap allocated. They are deallocated automatically by Rust when they go out of scope:

{
    let x = box 5i;

    // stuff happens

} // x is destructed and its memory is free'd here

However, boxes do not use reference counting or garbage collection. Boxes are what's called an affine type. This means that the Rust compiler, at compile time, determines when the box comes into and goes out of scope, and inserts the appropriate calls there. Furthermore, boxes are a specific kind of affine type, known as a region. You can read more about regions in this paper on the Cyclone programming language.

You don't need to fully grok the theory of affine types or regions to grok boxes, though. As a rough approximation, you can treat this Rust code:

{
    let x = box 5i;

    // stuff happens
}

As being similar to this C code:

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

    // stuff happens

    free(x);
}

Of course, this is a 10,000 foot view. It leaves out destructors, for example. But the general idea is correct: you get the semantics of malloc/free, but with some improvements:

1. It's impossible to allocate the incorrect amount of memory, because Rust figures it out from the types.
2. You cannot forget to free memory you've allocated, because Rust does it for you.
3. Rust ensures that this free happens at the right time, when it is truly not used. Use-after-free is not possible.
4. Rust enforces that no other writeable pointers alias to this heap memory, which means writing to an invalid pointer is not possible.

See the section on references or the lifetimes guide for more detail on how lifetimes work.

Using boxes and references together is very common. For example:

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

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

    println!("{}", add_one(&*x));
}

In this case, Rust knows that x is being 'borrowed' by the add_one() function, and since it's only reading the value, allows it.

We can borrow x multiple times, as long as it's not simultaneous:

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

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

    println!("{}", add_one(&*x));
    println!("{}", add_one(&*x));
    println!("{}", add_one(&*x));
}

Or as long as it's not a mutable borrow. This will error:

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

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

    println!("{}", add_one(&*x)); // error: cannot borrow immutable dereference 
                                  // of `&`-pointer as mutable
}

Notice we changed the signature of add_one() to request a mutable reference.

Best practices

Boxes are appropriate to use in two situations: Recursive data structures, and occasionally, when returning data.

Recursive data structures

Sometimes, you need a recursive data structure. The simplest is known as a 'cons list':

#[deriving(Show)]
enum List<T> {
    Cons(T, Box<List<T>>),
    Nil,
}

fn main() {
    let list: List<int> = Cons(1, box Cons(2, box Cons(3, box Nil)));
    println!("{}", list);
}

This prints:

Cons(1, box Cons(2, box Cons(3, box Nil)))

The reference to another List inside of the Cons enum variant must be a box, because we don't know the length of the list. Because we don't know the length, we don't know the size, and therefore, we need to heap allocate our list.

Working with recursive or other unknown-sized data structures is the primary use-case for boxes.

Returning data

This is important enough to have its own section entirely. The TL;DR is this: you don't generally want to return pointers, even when you might in a language like C or C++.

See Returning Pointers below for more.

Rc and Arc

This part is coming soon.

Best practices

This part is coming soon.

Gc

The Gc<T> type exists for historical reasons, and is still used internally by the compiler. It is not even a 'real' garbage collected type at the moment.

In the future, Rust may have a real garbage collected type, and so it has not yet been removed for that reason.

Best practices

There is currently no legitimate use case for the Gc<T> type.

Raw Pointers

This part is coming soon.

Best practices

This part is coming soon.

Returning Pointers

In many languages with pointers, you'd return a pointer from a function so as to avoid a copying a large data structure. For example:

struct BigStruct {
    one: int,
    two: int,
    // etc
    one_hundred: int,
}

fn foo(x: Box<BigStruct>) -> Box<BigStruct> {
    return box *x;
}

fn main() {
    let x = box BigStruct {
        one: 1,
        two: 2,
        one_hundred: 100,
    };

    let y = foo(x);
}

The idea is that by passing around a box, you're only copying a pointer, rather than the hundred ints that make up the BigStruct.

This is an antipattern in Rust. Instead, write this:

struct BigStruct {
    one: int,
    two: int,
    // etc
    one_hundred: int,
}

fn foo(x: Box<BigStruct>) -> BigStruct {
    return *x;
}

fn main() {
    let x = box BigStruct {
        one: 1,
        two: 2,
        one_hundred: 100,
    };

    let y = box foo(x);
}

This gives you flexibility without sacrificing performance.

You may think that this gives us terrible performance: return a value and then immediately box it up ?! Isn't that the worst of both worlds? Rust is smarter than that. There is no copy in this code. main allocates enough room for the `box , passes a pointer to that memory into foo as x, and then foo writes the value straight into that pointer. This writes the return value directly into the allocated box.

This is important enough that it bears repeating: pointers are not for optimizing returning values from your code. Allow the caller to choose how they want to use your output.

Creating your own Pointers

This part is coming soon.

Best practices

This part is coming soon.

Cheat Sheet

Here's a quick rundown of Rust's pointer types:

Type	Name	Summary
&T	Reference	Allows one or more references to read T
&mut T	Mutable Reference	Allows a single reference to read and write T
Box<T>	Box	Heap allocated T with a single owner that may read and write T.
Rc<T>	"arr cee" pointer	Heap allocated T with many readers
Arc<T>	Arc pointer	Same as above, but safe sharing across threads
*const T	Raw pointer	Unsafe read access to T
*mut T	Mutable raw pointer	Unsafe read and write access to T

Related resources

• API documentation for Box
• Lifetimes guide
• Cyclone paper on regions, which inspired Rust's lifetime system