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Understanding Ownership1

To run the program:

$ cargo run --bin understanding-ownership
   Compiling understanding-ownership v0.1.0 ...

Lessons learned

Ownership is Rust's most unique feature, enabling memory safety without GC.

What is Ownership?

All programs have to manage the way they use a computer's memory while running.

Different approaches:

  • GC (garbage collection) that looks for no-longer used memory e.g. Dart, Java.
  • Explicitly allocate and free the memory e.g. C, C++.
  • Automatic reference counting e.g. Obj-C, Swift.
  • System of ownership with compiler checks e.g. Rust.

The Stack and the Heap

Stack and Heap = memory available during runtime, structured differently:

  • Stack stores values in LIFO, pushing and popping known fixed-size data.
  • Heap allocates memory and returns a pointer to the address of that location.
    • The pointer can be stored on the stack.
  • When your code calls a function, the values (and pointers) are pushed/popped.

Ownership addresses:

  • Keeping track of what parts of code are using what data on the heap.
  • Minimizing the amount of duplicate data on the heap.
  • Cleaning up unused data on the heap.

Ownership Rules

Basic rules:

  • Each value in Rust has an owner.
  • There can only be one owner at a time.
  • When the owner goes out of scope, the value will be dropped.

For example:

// s is not yet eclared.
{
   // s is valid within this scope.
   let s = "hello";
}
// scope is now over, s is no longer valid.

The String Type

The types described in "Data Types" are all of a known size (i.e. i32), and can be stored on the stack and popped off the stack when their scope is over, and quickly/trivially copied to use the same value in a different scope. However, what about data stored on the heap, i.e. String?

We've seen string literals, i.e. &str:

// Immutable hardcoded string literals (pushed on the stack).
let s: &str = "hello";

However, to take input, or compute strings, we need a second type, String:∂

// Immutable string object (allocated on the heap).
let s = String::from("hello");

// Mutable string object (also allocated on the heap).
let mut s = String::from("hello");
s.push_str(", world!");
println!("{}", s);

For the latter (mutable, growable piece of text):

  • The memory must be requested from the memory allocator at runtime.
  • We need a way to return this memory to the allocator when String is done.

The first is done by us, via String::from.

The second part is different; memory is automatically returned once the variable that owns it goes out of scope. Multiple variables can interact with the same data in different ways:

// Assigns 5 to x, and copies the value to y. Both are now 5.
let x = 5;
let y = x;

// Does not make a copy, they both are _pointers_ are copied, but not the value.
let x = String::from("Hello");
let y = x;

For example, x and y are now both pointers that look like this:

name value
ptr @0xFF (Example)
len 5
capacity 5

And the String located @0xFF looks like this:

index value
0 H
1 e
2 l
3 l
4 o

Rust avoids what is called a double free error:

let x = String::from("Hello");
let y = x;

// ERROR: Borrow of moved value: `x`
println!("{} World", y);

There are a couple of different ways to deal with it:

  • Clone: Deeply clone the heap data of the String. Often expensive.

    let x = String::from("Hello");
let y = x.clone();
println!("{} World", y);

  • Copy: (Stack Only) Shallow (trivial) copy of the stack data:

    // The "Copy" trait allows us to do this.
// All integer types, boolean types, floating point types, char implement it.
// Tuples, if they only contain types that implement Copy, i.e. (i32, i32).
let x = 5;
let y = x;
println!("x = {}, y = {}", x , y);

  • Move: Passing or returning a variable to/from a function:

    let x = String::from("Hello");

takes_ownership(x);

// If we tried to use "x" here, we would get a compile-time error.

fn takes_ownership(some_string: String) {
   println!("{}", some_string);
}

Rust can also use a value without transferring ownership, called a reference.

References and Borrowing

The issue with the above (takes_ownership) is that we have to return the String to the calling function so we can still use the String after the call to take_ownership. Instead, we can provide a reference to the String value:

  • A reference is like a pointer in that's an address we can follow.
  • Unlike a pointer, a reference is guaranteed to point to a valid value.

<Stack>:

name value
ptr @0x00FF

@0x00FF:

name value
ptr @0xFF00
len 5
capacity 5

@0xFF00:

index value
0 H
1 e
2 l
3 l
4 o

The &x syntax lets us create a reference that refers to x, but does not own it.

Because it does not own it, the value it points to will not be dropped when the reference stops being used:

// Here x goes into scope (is allocated).
let x = String::from("Hello");
let l = calculate_length(&x);

fn calculate_length(s: &String) -> usize {
   s.len();
   // Here s goes out of scope, but since it's a reference, it's not dropped.
}

// Here x goes out of scope, and it is dropped.

We call the action of creating a reference borrowing.

As in real life, you can borrow something from someone else, but when you're done, you have to give it back (you don't own it):

let x = String::from("Hello");
change(&x);

fn change(s: &String) {
   // ERROR: Cannot borrow `*some_string` as mutable
   s.push_str(" World");
}

To fix the code, we need a mutable reference:

let x = String::from("Hello");
change(&x);

fn change(s: &mut String) {
   // ERROR: Cannot borrow `*some_string` as mutable
   s.push_str(" World");
}

Mutable references have one big restriction: if you have a mutable reference to a value, you can have no other references to that value.

This code that attempts to create two mutable references to x will fail:

let x = String::from("Hello");
let a = &mut x;
// ERROR: Cannot borrow `x` as mutable more than once at a time.
let b = &mut x;
println!("{}, {}", a, b);

This prevents what is called a data race, or:

  • Two or more pointers accessing the same data at the same time.
  • At least one of the pointers is being used to write to the data.
  • There's no mechanism being used to synchronize across to the data.

One option is also creating a new scope:

let x = String::from("Hello");
{
   let a = &mut x;
}
let b = &mut x;

Dangling References

A dangling pointer references a location in memory given to someone else. In Rust, the compiler guarantees that references will never be dangling references; if you have a reference to some data, the compiler will ensure that the data will not go out of scope before the reference to the data does:

fn a() {
   let a = b();
}

// ERROR: Missing lifetime specifier.
fn b() -> &String {
   let b = String::from("Hello");
   &b
}

To fix this, return the String directly:

fn a() {
   let a = b();
}

fn b() -> String {
   let b = String::from("Hello");
   b
}

This works! Ownership is moved out, and nothing is deallocated.

tl;dr:

  1. References are always valid
  2. At any given time, you can have either one mutable reference or any number of immutable references.

The next kind of reference is a slice.

The Slice Type

A slice references a contiguous sequence of elements in a collection:

let x = String::from("Hello World");
let h = &x[/*0*/..5];
let w = &x[6../*11*/];

x:

name value
ptr @0xFF00
len 11
capacity 11

w:

name value
ptr @0xFF05
len 5

@0xFF00:

index value
0 H
1 e
2 l
3 l
4 o
5
6 W
7 o
8 r
9 l
10 d

Ultimately, string literals (&str) are immutable references!

Footnotes

  1. Source: https://doc.rust-lang.org/book/ch04-00-understanding-ownership.html ↩