rust/doc/tutorial-borrowed-ptr.md

% Rust Borrowed Pointers Tutorial

Introduction

Borrowed pointers are one of the more flexible and powerful tools available in Rust. A borrowed pointer can be used to point anywhere: into the managed and exchange heaps, into the stack, and even into the interior of another data structure. With regard to flexibility, it is comparable to a C pointer or C++ reference. However, unlike C and C++, the Rust compiler includes special checks that ensure that borrowed pointers are being used safely. Another advantage of borrowed pointers is that they are invisible to the garbage collector, so working with borrowed pointers helps keep things efficient.

Despite the fact that they are completely safe, at runtime, a borrowed pointer is “just a pointer”. They introduce zero overhead. All safety checks are done at compilation time.

Although borrowed pointers have rather elaborate theoretical underpinnings (region pointers), the core concepts will be familiar to anyone who worked with C or C++. Therefore, the best way to explain how they are used—and their limitations—is probably just to work through several examples.

By example

Borrowed pointers are called borrowed because they are only valid for a limit duration. Borrowed pointers never claim any kind of ownership over the data that they point at: instead, they are used for cases where you like to make use of data for a short time.

As an example, consider a simple struct type Point:

struct Point {x: float, y: float}

We can use this simple definition to allocate points in many ways. For example, in this code, each of these three local variables contains a point, but allocated in a different place:

# struct Point {x: float, y: float}
let on_the_stack :  Point =  Point {x: 3.0, y: 4.0};
let shared_box   : @Point = @Point {x: 5.0, y: 1.0};
let unique_box   : ~Point = ~Point {x: 7.0, y: 9.0};

Suppose we wanted to write a procedure that computed the distance between any two points, no matter where they were stored. For example, we might like to compute the distance between on_the_stack and shared_box, or between shared_box and unique_box. One option is to define a function that takes two arguments of type point—that is, it takes the points by value. But this will cause the points to be copied when we call the function. For points, this is probably not so bad, but often copies are expensive or, worse, if there are mutable fields, they can change the semantics of your program. So we’d like to define a function that takes the points by pointer. We can use borrowed pointers to do this:

# struct Point {x: float, y: float}
# fn sqrt(f: float) -> float { 0f }
fn compute_distance(p1: &Point, p2: &Point) -> float {
    let x_d = p1.x - p2.x;
    let y_d = p1.y - p2.y;
    sqrt(x_d * x_d + y_d * y_d)
}

Now we can call compute_distance() in various ways:

# struct Point {x: float, y: float}
# let on_the_stack :  Point =  Point{x: 3.0, y: 4.0};
# let shared_box   : @Point = @Point{x: 5.0, y: 1.0};
# let unique_box   : ~Point = ~Point{x: 7.0, y: 9.0};
# fn compute_distance(p1: &Point, p2: &Point) -> float { 0f }
compute_distance(&on_the_stack, shared_box);
compute_distance(shared_box, unique_box);

Here the & operator is used to take the address of the variable on_the_stack; this is because on_the_stack has the type Point (that is, a struct value) and we have to take its address to get a value. We also call this borrowing the local variable on_the_stack, because we are created an alias: that is, another route to the same data.

In the case of the boxes shared_box and unique_box, however, no explicit action is necessary. The compiler will automatically convert a box like @Point or ~Point to a borrowed pointer like &Point. This is another form of borrowing; in this case, the contents of the shared/unique box is being lent out.

Whenever a value is borrowed, there are some limitations on what you can do with the original. For example, if the contents of a variable have been lent out, you cannot send that variable to another task, nor will you be permitted to take actions that might cause the borrowed value to be freed or to change its type (I’ll get into what kinds of actions those are shortly). This rule should make intuitive sense: you must wait for a borrowed value to be returned (that is, for the borrowed pointer to go out of scope) before you can make full use of it again.

Other uses for the & operator

In the previous example, the value on_the_stack was defined like so:

# struct Point {x: float, y: float}
let on_the_stack: Point = Point {x: 3.0, y: 4.0};

This results in a by-value variable. As a consequence, we had to explicitly take the address of on_the_stack to get a borrowed pointer. Sometimes however it is more convenient to move the & operator into the definition of on_the_stack:

# struct Point {x: float, y: float}
let on_the_stack2: &Point = &Point {x: 3.0, y: 4.0};

Applying & to an rvalue (non-assignable location) is just a convenient shorthand for creating a temporary and taking its address:

# struct Point {x: float, y: float}
let tmp = Point {x: 3.0, y: 4.0};
let on_the_stack2 : &Point = &tmp;

Taking the address of fields

As in C, the & operator is not limited to taking the address of local variables. It can also be used to take the address of fields or individual array elements. For example, consider this type definition for rectangle:

struct Point {x: float, y: float} // as before
struct Size {w: float, h: float} // as before
struct Rectangle {origin: Point, size: Size}

Now again I can define rectangles in a few different ways:

# struct Point {x: float, y: float}
# struct Size {w: float, h: float} // as before
# struct Rectangle {origin: Point, size: Size}
let rect_stack   = &Rectangle {origin: Point {x: 1f, y: 2f},
                               size: Size {w: 3f, h: 4f}};
let rect_managed = @Rectangle {origin: Point {x: 3f, y: 4f},
                               size: Size {w: 3f, h: 4f}};
let rect_unique  = ~Rectangle {origin: Point {x: 5f, y: 6f},
                               size: Size {w: 3f, h: 4f}};

In each case I can use the & operator to extact out individual subcomponents. For example, I could write:

# struct Point {x: float, y: float} // as before
# struct Size {w: float, h: float} // as before
# struct Rectangle {origin: Point, size: Size}
# let rect_stack  = &{origin: Point {x: 1f, y: 2f}, size: Size {w: 3f, h: 4f}};
# let rect_managed = @{origin: Point {x: 3f, y: 4f}, size: Size {w: 3f, h: 4f}};
# let rect_unique = ~{origin: Point {x: 5f, y: 6f}, size: Size {w: 3f, h: 4f}};
# fn compute_distance(p1: &Point, p2: &Point) -> float { 0f }
compute_distance(&rect_stack.origin, &rect_managed.origin);

which would borrow the field origin from the rectangle on the stack from the managed box and then compute the distance between them.

Borrowing managed boxes and rooting

We’ve seen a few examples so far where heap boxes (both managed and unique) are borrowed. Up till this point, we’ve glossed over issues of safety. As stated in the introduction, at runtime a borrowed pointer is simply a pointer, nothing more. Therefore, if we wish to avoid the issues that C has with dangling pointers (and we do!), a compile-time safety check is required.

The basis for the check is the notion of lifetimes. A lifetime is basically a static approximation of the period in which the pointer is valid: it always corresponds to some expression or block within the program. Within that expression, the pointer can be used freely, but if the pointer somehow leaks outside of that expression, the compiler will report an error. We’ll be discussing lifetimes more in the examples to come, and a more thorough introduction is also available.

When a borrowed pointer is created, the compiler must ensure that it will remain valid for its entire lifetime. Sometimes this is relatively easy, such as when taking the address of a local variable or a field that is stored on the stack:

struct X { f: int }
fn example1() {
    let mut x = X { f: 3 };
    let y = &mut x.f;  // -+ L
    ...                //  |
}                      // -+

Here, the lifetime of the borrowed pointer is simply L, the remainder of the function body. No extra work is required to ensure that x.f will not be freed. This is true even if x is mutated.

The situation gets more complex when borrowing data that resides in heap boxes:

# struct X { f: int }
fn example2() {
    let mut x = @X { f: 3 };
    let y = &x.f;      // -+ L
    ...                //  |
}                      // -+

In this example, the value x is in fact a heap box, and y is therefore a pointer into that heap box. Again the lifetime of y will be L, the remainder of the function body. But there is a crucial difference: suppose x were reassigned during the lifetime L? If we’re not careful, that could mean that the managed box would become unrooted and therefore be subject to garbage collection

***Note:***In our current implementation, the garbage collector is implemented using reference counting and cycle detection.

For this reason, whenever the interior of a managed box stored in a mutable location is borrowed, the compiler will insert a temporary that ensures that the managed box remains live for the entire lifetime. So, the above example would be compiled as:

# struct X { f: int }
fn example2() {
    let mut x = @X {f: 3};
    let x1 = x;
    let y = &x1.f;     // -+ L
    ...                //  |
}                      // -+

Now if x is reassigned, the pointer y will still remain valid. This process is called rooting.

Borrowing unique boxes

The previous example demonstrated rooting, the process by which the compiler ensures that managed boxes remain live for the duration of a borrow. Unfortunately, rooting does not work if the data being borrowed is a unique box, as it is not possible to have two references to a unique box.

For unique boxes, therefore, the compiler will only allow a borrow if the compiler can guarantee that the unique box will not be reassigned or moved for the lifetime of the pointer. This does not necessarily mean that the unique box is stored in immutable memory. For example, the following function is legal:

# fn some_condition() -> bool { true }
fn example3() -> int {
    let mut x = ~{f: 3};
    if some_condition() {
        let y = &x.f;      // -+ L
        return *y;         //  |
    }                      // -+
    x = ~{f: 4};
    ...
# return 0;
}

Here, as before, the interior of the variable x is being borrowed and x is declared as mutable. However, the compiler can clearly see that x is not assigned anywhere in the lifetime L of the variable y. Therefore, it accepts the function, even though x is mutable and in fact is mutated later in the function.

It may not be clear why we are so concerned about the variable which was borrowed being mutated. The reason is that unique boxes are freed as soon as their owning reference is changed or goes out of scope. Therefore, a program like this is illegal (and would be rejected by the compiler):

fn example3() -> int {
    let mut x = ~X {f: 3};
    let y = &x.f;
    x = ~{f: 4};  // Error reported here.
    *y
}

To make this clearer, consider this diagram showing the state of memory immediately before the re-assignment of x:

    Stack               Exchange Heap

  x +----------+
    | ~{f:int} | ----+
  y +----------+     |
    | &int     | ----+
    +----------+     |    +---------+
                     +--> |  f: 3   |
                          +---------+

Once the reassignment occurs, the memory will look like this:

    Stack               Exchange Heap

  x +----------+          +---------+
    | ~{f:int} | -------> |  f: 4   |
  y +----------+          +---------+
    | &int     | ----+
    +----------+     |    +---------+
                     +--> | (freed) |
                          +---------+

Here you can see that the variable y still points at the old box, which has been freed.

In fact, the compiler can apply this same kind of reasoning can be applied to any memory which is (uniquely) owned by the stack frame. So we could modify the previous example to introduce additional unique pointers and structs, and the compiler will still be able to detect possible mutations:

fn example3() -> int {
    struct R { g: int }
    struct S { mut f: ~R }

    let mut x = ~S {mut f: ~R {g: 3}};
    let y = &x.f.g;
    x = ~S {mut f: ~R {g: 4}}; // Error reported here.
    x.f = ~R {g: 5};           // Error reported here.
    *y
}

In this case, two errors are reported, one when the variable x is modified and another when x.f is modified. Either modification would cause the pointer y to be invalidated.

Things get tricker when the unique box is not uniquely owned by the stack frame (or when the compiler doesn’t know who the owner is). Consider a program like this:

struct R { g: int }
struct S { mut f: ~R }
fn example5a(x: @S ...) -> int {
    let y = &x.f.g;   // Error reported here.
    ...
#   return 0;
}

Here the heap looks something like:

     Stack            Managed Heap       Exchange Heap

  x +------+        +-------------+       +------+
    | @... | ---->  | mut f: ~... | --+-> | g: 3 |
  y +------+        +-------------+   |   +------+
    | &int | -------------------------+
    +------+

In this case, the owning reference to the value being borrowed is in fact x.f. Moreover, x.f is both mutable and aliasable. Aliasable means that it is possible that there are other pointers to that same managed box, so even if the compiler were to prevent x.f from being mutated, the field might still be changed through some alias of x. Therefore, to be safe, the compiler only accepts pure actions during the lifetime of y. We’ll have a final example on purity but inn unique fields, as in the following example:

Besides ensuring purity, the only way to borrow the interior of a unique found in aliasable memory is to ensure that it is stored within unique fields, as in the following example:

struct R { g: int }
struct S { f: ~R }
fn example5b(x: @S) -> int {
    let y = &x.f.g;
    ...
# return 0;
}

Here, the field f is not declared as mutable. But that is enough for the compiler to know that, even if aliases to x exist, the field f cannot be changed and hence the unique box g will remain valid.

If you do have a unique box in a mutable field, and you wish to borrow it, one option is to use the swap operator to bring that unique box onto your stack:

struct R { g: int }
struct S { mut f: ~R }
fn example5c(x: @S) -> int {
    let mut v = ~R {g: 0};
    v <-> x.f;         // Swap v and x.f
    { // Block constrains the scope of `y`:
        let y = &v.g;
        ...
    }
    x.f <- v;          // Replace x.f
    ...
# return 0;
}

Of course, this has the side effect of modifying your managed box for the duration of the borrow, so it only works when you know that you won’t be accessing that same box for the duration of the loan. Note also that sometimes it is necessary to introduce additional blocks to constrain the scope of the loan. In this example, the borrowed pointer y would still be in scope when you moved the value v back into x.f, and hence moving v would be considered illegal. You cannot move values if they are outstanding loans which are still valid. By introducing the block, the scope of y is restricted and so the move is legal.

Borrowing and enums

The previous example showed that borrowing unique boxes found in aliasable, mutable memory is not permitted, so as to prevent pointers into freed memory. There is one other case where the compiler must be very careful to ensure that pointers remain valid: pointers into the interior of an enum.

As an example, let’s look at the following shape type that can represent both rectangles and circles:

struct Point {x: float, y: float}; // as before
struct Size {w: float, h: float}; // as before
enum Shape {
    Circle(Point, float),   // origin, radius
    Rectangle(Point, Size)  // upper-left, dimensions
}

Now I might write a function to compute the area of a shape. This function takes a borrowed pointer to a shape to avoid the need of copying them.

# struct Point {x: float, y: float}; // as before
# struct Size {w: float, h: float}; // as before
# enum Shape {
#     Circle(Point, float),   // origin, radius
#     Rectangle(Point, Size)  // upper-left, dimensions
# }
# const tau: float = 6.28f;
fn compute_area(shape: &Shape) -> float {
    match *shape {
        Circle(_, radius) => 0.5 * tau * radius * radius,
        Rectangle(_, ref size) => size.w * size.h
    }
}

The first case matches against circles. Here the radius is extracted from the shape variant and used to compute the area of the circle (Like any up-to-date engineer, we use the tau circle constant and not that dreadfully outdated notion of pi).

The second match is more interesting. Here we match against a rectangle and extract its size: but rather than copy the size struct, we use a by-reference binding to create a pointer to it. In other words, a pattern binding like ref size in fact creates a pointer of type &size into the interior of the enum.

To make this more clear, let’s look at a diagram of how things are laid out in memory in the case where shape points at a rectangle:

Stack             Memory

+-------+         +---------------+
| shape | ------> | rectangle(    |
+-------+         |   {x: float,  |
| size  | -+      |    y: float}, |
+-------+  +----> |   {w: float,  |
                  |    h: float}) |
                  +---------------+

Here you can see that rectangular shapes are composed of five words of memory. The first is a tag indicating which variant this enum is (rectangle, in this case). The next two words are the x and y fields for the point and the remaining two are the w and h fields for the size. The binding size is then a pointer into the inside of the shape.

Perhaps you can see where the danger lies: if the shape were somehow to be reassigned, perhaps to a circle, then although the memory used to store that shape value would still be valid, it would have a different type! This is shown in the following diagram, depicting what the state of memory would be if shape were overwritten with a circle:

Stack             Memory

+-------+         +---------------+
| shape | ------> | circle(       |
+-------+         |   {x: float,  |
| size  | -+      |    y: float}, |
+-------+  +----> |   float)      |
                  |               |
                  +---------------+

As you can see, the size pointer would not be pointing at a float and not a struct. This is not good.

So, in fact, for every ref binding, the compiler will impose the same rules as the ones we saw for borrowing the interior of a unique box: it must be able to guarantee that the enum will not be overwritten for the duration of the borrow. In fact, the example I gave earlier would be considered safe. This is because the shape pointer has type &Shape, which means “borrowed pointer to immutable memory containing a shape”. If however the type of that pointer were &const Shape or &mut Shape, then the ref binding would not be permitted. Just as with unique boxes, the compiler will permit ref bindings into data owned by the stack frame even if it is mutable, but otherwise it requires that the data reside in immutable memory.

Note: Right now, pattern bindings not explicitly annotated with ref or copy use a special mode of "implicit by reference". This is changing as soon as we finish updating all the existing code in the compiler that relies on the current settings.

Returning borrowed pointers

So far, all of the examples we’ve looked at use borrowed pointers in a “downward” direction. That is, the borrowed pointer is created and then used during the method or code block which created it. It is also possible to return borrowed pointers to the caller, but as we'll see this requires some explicit annotation.

For example, we could write a subroutine like this:

struct Point {x: float, y: float}
fn get_x(p: &r/Point) -> &r/float { &p.x }

Here, the function get_x() returns a pointer into the structure it was given. The type of the parameter (&r/Point) and return type (&r/float) both make use of a new syntactic form that we have not seen so far. Here the identifier r serves as an explicit name for the lifetime of the pointer. So in effect this function is declaring that it takes in a pointer with lifetime r and returns a pointer with that same lifetime.

In general, it is only possible to return borrowed pointers if they are derived from a borrowed pointer which was given as input to the procedure. In that case, they will always have the same lifetime as one of the parameters; named lifetimes are used to indicate which parameter that is.

In the examples before, function parameter types did not include a lifetime name. In this case, the compiler simply creates a new, anonymous name, meaning that the parameter is assumed to have a distinct lifetime from all other parameters.

Named lifetimes that appear in function signatures are conceptually the same as the other lifetimes we've seen before, but they are a bit abstract: they don’t refer to a specific expression within get_x(), but rather to some expression within the caller of get_x(). The lifetime r is actually a kind of lifetime parameter: it is defined by the caller to get_x(), just as the value for the parameter p is defined by that caller.

In any case, whatever the lifetime r is, the pointer produced by &p.x always has the same lifetime as p itself, as a pointer to a field of a struct is valid as long as the struct is valid. Therefore, the compiler is satisfied with the function get_x().

To drill in this point, let’s look at a variation on the example, this time one which does not compile:

struct Point {x: float, y: float}
fn get_x_sh(p: @Point) -> &float {
    &p.x // Error reported here
}

Here, the function get_x_sh() takes a managed box as input and returns a borrowed pointer. As before, the lifetime of the borrowed pointer that will be returned is a parameter (specified by the caller). That means that effectively get_x_sh() is promising to return a borrowed pointer that is valid for as long as the caller would like: this is subtly different from the first example, which promised to return a pointer that was valid for as long as the pointer it was given.

Within get_x_sh(), we see the expression &p.x which takes the address of a field of a managed box. This implies that the compiler must guarantee that, so long as the resulting pointer is valid, the managed box will not be reclaimed by the garbage collector. But recall that get_x_sh() also promised to return a pointer that was valid for as long as the caller wanted it to be. Clearly, get_x_sh() is not in a position to make both of these guarantees; in fact, it cannot guarantee that the pointer will remain valid at all once it returns, as the parameter p may or may not be live in the caller. Therefore, the compiler will report an error here.

In general, if you borrow a managed (or unique) box to create a borrowed pointer, the pointer will only be valid within the function and cannot be returned. This is why the typical way to return borrowed pointers is to take borrowed pointers as input (the only other case in which it can be legal to return a borrowed pointer is if the pointer points at a static constant).

Named lifetimes

Let's look at named lifetimes in more detail. In effect, the use of named lifetimes allows you to group parameters by lifetime. For example, consider this function:

# struct Point {x: float, y: float}; // as before
# struct Size {w: float, h: float}; // as before
# enum Shape {
#     Circle(Point, float),   // origin, radius
#     Rectangle(Point, Size)  // upper-left, dimensions
# }
# fn compute_area(shape: &Shape) -> float { 0f }
fn select<T>(shape: &r/Shape, threshold: float,
             a: &r/T, b: &r/T) -> &r/T {
    if compute_area(shape) > threshold {a} else {b}
}

This function takes three borrowed pointers and assigns each the same lifetime r. In practice, this means that, in the caller, the lifetime r will be the intersection of the lifetime of the three region parameters. This may be overly conservative, as in this example:

# struct Point {x: float, y: float}; // as before
# struct Size {w: float, h: float}; // as before
# enum Shape {
#     Circle(Point, float),   // origin, radius
#     Rectangle(Point, Size)  // upper-left, dimensions
# }
# fn compute_area(shape: &Shape) -> float { 0f }
# fn select<T>(shape: &Shape, threshold: float,
#              a: &r/T, b: &r/T) -> &r/T {
#     if compute_area(shape) > threshold {a} else {b}
# }
                                                  // -+ r
fn select_based_on_unit_circle<T>(                //  |-+ B
    threshold: float, a: &r/T, b: &r/T) -> &r/T { //  | |
                                                  //  | |
    let shape = Circle(Point {x: 0., y: 0.}, 1.); //  | |
    select(&shape, threshold, a, b)               //  | |
}                                                 //  |-+
                                                  // -+

In this call to select(), the lifetime of the first parameter shape is B, the function body. Both of the second two parameters a and b share the same lifetime, r, which is a lifetime parameter of select_based_on_unit_circle(). The caller will infer the intersection of these two lifetimes as the lifetime of the returned value, and hence the return value of shape() will be assigned a lifetime of B. This will in turn lead to a compilation error, because select_based_on_unit_circle() is supposed to return a value with the lifetime r.

To address this, we can modify the definition of select() to distinguish the lifetime of the first parameter from the lifetime of the latter two. After all, the first parameter is not being returned. Here is how the new select() might look:

# struct Point {x: float, y: float}; // as before
# struct Size {w: float, h: float}; // as before
# enum Shape {
#     Circle(Point, float),   // origin, radius
#     Rectangle(Point, Size)  // upper-left, dimensions
# }
# fn compute_area(shape: &Shape) -> float { 0f }
fn select<T>(shape: &tmp/Shape, threshold: float,
             a: &r/T, b: &r/T) -> &r/T {
    if compute_area(shape) > threshold {a} else {b}
}

Here you can see the lifetime of shape is now being called tmp. The parameters a, b, and the return value are all given the lifetime r. However, since the lifetime tmp is not returned, it would be shorter to just omit the named lifetime for shape altogether:

# struct Point {x: float, y: float}; // as before
# struct Size {w: float, h: float}; // as before
# enum Shape {
#     Circle(Point, float),   // origin, radius
#     Rectangle(Point, Size)  // upper-left, dimensions
# }
# fn compute_area(shape: &Shape) -> float { 0f }
fn select<T>(shape: &Shape, threshold: float,
             a: &r/T, b: &r/T) -> &r/T {
    if compute_area(shape) > threshold {a} else {b}
}

This is equivalent to the previous definition.

Purity

As mentioned before, the Rust compiler offers a kind of escape hatch that permits borrowing of any data, but only if the actions that occur during the lifetime of the borrow are pure. Pure actions are those which only modify data owned by the current stack frame. The compiler can therefore permit arbitrary pointers into the heap, secure in the knowledge that no pure action will ever cause them to become invalidated (the compiler must still track data on the stack which is borrowed and enforce those rules normally, of course).

Let’s revisit a previous example and show how purity can affect the compiler’s result. Here is example5a(), which borrows the interior of a unique box found in an aliasable, mutable location, only now we’ve replaced the ... with some specific code:

struct R { g: int }
struct S { mut f: ~R }
fn example5a(x: @S ...) -> int {
    let y = &x.f.g;   // Unsafe
    *y + 1        
}

The new code simply returns an incremented version of y. This clearly doesn’t do mutate anything in the heap, so the compiler is satisfied.

But suppose we wanted to pull the increment code into a helper, like this:

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

We can now update example5a() to use add_one():

# struct R { g: int }
# struct S { mut f: ~R }
# pure fn add_one(x: &int) -> int { *x + 1 }
fn example5a(x: @S ...) -> int {
    let y = &x.f.g;
    add_one(y)        // Error reported here
}

But now the compiler will report an error again. The reason is that it only considers one function at a time (like most type checkers), and so it does not know that add_one() only takes pure actions. We can help the compiler by labeling add_one() as pure:

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

With this change, the modified version of example5a() will again compile.

Conclusion

So there you have it. A (relatively) brief tour of borrowed pointer system. For more details, I refer to the (yet to be written) reference document on borrowed pointers, which will explain the full notation and give more examples.