What Is Ownership?

Cairo implements an ownership system to ensure the safety and correctness of its compiled code. The ownership mechanism complements the linear type system, which enforces that objects are used exactly once. This helps prevent common operations that can produce runtime errors, such as illegal memory address references or multiple writes to the same memory address, and ensures the soundness of Cairo programs by checking at compile time that all the dictionaries are squashed.

Now that we’re past basic Cairo syntax, we won’t include all the fn main() { code in examples, so if you’re following along, make sure to put the following examples inside a main function manually. As a result, our examples will be a bit more concise, letting us focus on the actual details rather than boilerplate code.

Ownership Rules

First, let’s take a look at the ownership rules. Keep these rules in mind as we work through the examples that illustrate them:

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

Variable Scope

As a first example of ownership, we’ll look at the scope of some variables. A scope is the range within a program for which an item is valid. Take the following variable:

let s = 'hello';

The variable s refers to a short string, where the value of the string is hardcoded into the text of our program. The variable is valid from the point at which it’s declared until the end of the current scope. Listing 4-1 shows a program with comments annotating where the variable s would be valid.

//TAG: ignore_fmt
fn main() {
    {                      // s is not valid here, it’s not yet declared
        let s = 'hello';   // s is valid from this point forward

        // do stuff with s
    }                      // this scope is now over, and s is no longer valid

Listing 4-1: A variable and the scope in which it is valid

In other words, there are two important points in time here:

  • When s comes into scope, it is valid.
  • It remains valid until it goes out of scope.

At this point, the relationship between scopes and when variables are valid is similar to that in other programming languages. Now we’ll build on top of this understanding by using the Array type we introduced in the previous chapter.

Ownership with the Array Type

To illustrate the rules of ownership, we need a data type that is more complex. The types covered in the Data Types section of Chapter 2 are of a known size, can be quickly and trivially copied to make a new, independent instance if another part of code needs to use the same value in a different scope, and can easily be dropped when they're no longer used. But what is the behavior with the Array type whose size is unknown at compile time and which can't be trivially copied?

Here is a short reminder of what an array looks like:

fn main() {
    let mut arr = ArrayTrait::<u128>::new();

So, how does the ownership system ensure that each cell is never written to more than once? Consider the following code, where we try to pass the same instance of an array in two consecutive function calls:

fn foo(arr: Array<u128>) {}

fn bar(arr: Array<u128>) {}

fn main() {
    let mut arr = ArrayTrait::<u128>::new();

In this case, we try to pass the same array instance arr by value to the functions foo and bar, which means that the parameter used in both function calls is the same instance of the array. If you append a value to the array in foo, and then try to append another value to the same array in bar, what would happen is that you would attempt to try to write to the same memory cell twice, which is not allowed in Cairo. To prevent this, the ownership of the arr variable moves from the main function to the foo function. When trying to call bar with arr as a parameter, the ownership of arr was already moved to the first call. The ownership system thus prevents us from using the same instance of arr in foo.

Running the code above will result in a compile-time error:

error: Variable was previously moved. Trait has no implementation in context: core::traits::Copy::<core::array::Array::<core::integer::u128>>
 --> array.cairo:6:9
    let mut arr = ArrayTrait::<u128>::new();

The Copy Trait

If a type implements the Copy trait, passing it to a function will not move the ownership of the value to the function called, but will instead pass a copy of the value. You can implement the Copy trait on your type by adding the #[derive(Copy)] annotation to your type definition. However, Cairo won't allow a type to be annotated with Copy if the type itself or any of its components don't implement the Copy trait. While Arrays and Dictionaries can't be copied, custom types that don't contain either of them can be.

#[derive(Copy, Drop)]
struct Point {
    x: u128,
    y: u128,

fn main() {
    let p1 = Point { x: 5, y: 10 };

fn foo(p: Point) { // do something with p

In this example, we can pass p1 twice to the foo function because the Point type implements the Copy trait. This means that when we pass p1 to foo, we are actually passing a copy of p1, and the ownership of p1 remains with the main function. If you remove the Copy trait derivation from the Point type, you will get a compile-time error when trying to compile the code.

Don't worry about the Struct keyword. We will introduce this in Chapter 5.

The Drop Trait

You may have noticed that the Point type in the previous example also implements the Drop trait. In Cairo, a value cannot go out of scope unless it has been previously moved. For example, the following code will not compile, because the struct A is not moved before it goes out of scope:

struct A {}

fn main() {
    A {}; // error: Value not dropped.

This is to ensure the soundness of Cairo programs. Soundness refers to the fact that if a statement during the execution of the program is false, no cheating prover can convince an honest verifier that it is true. In our case, we want to ensure the consistency of consecutive dictionary key updates during program execution, which is only checked when the dictionaries are squashed - which moves the ownership of the dictionary to the squash method, thus allowing the dictionary to go out of scope. Unsquashed dictionaries are dangerous, as a malicious prover could prove the correctness of inconsistent updates.

However, types that implement the Drop trait are allowed to go out of scope without being explicitly moved. When a value of a type that implements the Drop trait goes out of scope, the Drop implementation is called on the type, which moves the value to the drop function, allowing it to go out of scope - This is what we call "dropping" a value. It is important to note that the implementation of drop is a "no-op", meaning that it doesn't perform any actions other than allowing the value to go out of scope.

The Drop implementation can be derived for all types, allowing them to be dropped when going out of scope, except for dictionaries (Felt252Dict) and types containing dictionaries. For example, the following code compiles:

struct A {}

fn main() {
    A {}; // Now there is no error.

The Destruct Trait

Manually calling the squash method on a dictionary is not very convenient, and it is easy to forget to do so. To make it easier to use dictionaries, Cairo provides the Destruct trait, which allows you to specify the behavior of a type when it goes out of scope. While Dictionaries don't implement the Drop trait, they do implement the Destruct trait, which allows them to automatically be squashed when they go out of scope. This means that you can use dictionaries without having to manually call the squash method.

Consider the following example, in which we define a custom type that contains a dictionary:

struct A {
    dict: Felt252Dict<u128>

fn main() {
    A { dict: Default::default() };

If you try to run this code, you will get a compile-time error:

error: Variable not dropped. Trait has no implementation in context: core::traits::Drop::<temp7::temp7::A>. Trait has no implementation in context: core::traits::Destruct::<temp7::temp7::A>.
 --> temp7.cairo:7:5
    A {

When A goes out of scope, it can't be dropped as it implements neither the Drop (as it contains a dictionary and can't derive(Drop)) nor the Destruct trait. To fix this, we can derive the Destruct trait implementation for the A type:

struct A {
    dict: Felt252Dict<u128>

fn main() {
    A { dict: Default::default() }; // No error here

Now, when A goes out of scope, its dictionary will be automatically squashed, and the program will compile.

Copy Array data with Clone

If we do want to deeply copy the data of an Array, we can use a common method called clone. We’ll discuss method syntax in Chapter 6, but because methods are a common feature in many programming languages, you’ve probably seen them before.

Here’s an example of the clone method in action.

Note: in the following example, we need to import the Clone trait from the corelib clone module, and its implementation for the array type from the array module.

use clone::Clone;
use array::ArrayTCloneImpl;
fn main() {
    let arr1 = ArrayTrait::<u128>::new();
    let arr2 = arr1.clone();

Note: you will need to run scarb cairo-run with the --available-gas=2000000 option to run this example, because it uses a loop and must be ran with a gas limit.

When you see a call to clone, you know that some arbitrary code is being executed and that code may be expensive. It’s a visual indicator that something different is going on.

Ownership and Functions

Passing a variable to a function will either move it or copy it. As seen in the Array section, passing an Array as a function parameter transfers its ownership; let's see what happens with other types.

Listing 4-3 has an example with some annotations showing where variables go into and out of scope.

Filename: src/lib.cairo

struct MyStruct{}

fn main() {
    let my_struct = MyStruct{};  // my_struct comes into scope

    takes_ownership(my_struct);     // my_struct's value moves into the function...
                                    // ... and so is no longer valid here

    let x = 5;                 // x comes into scope

    makes_copy(x);                  // x would move into the function,
                                    // but u128 implements Copy, so it is okay to still
                                    // use x afterward

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

fn takes_ownership(some_struct: MyStruct) { // some_struct comes into scope
} // Here, some_struct goes out of scope and `drop` is called.

fn makes_copy(some_uinteger: u128) { // some_uinteger comes into scope
} // Here, some_integer goes out of scope and is dropped.

Listing 4-3: Functions with ownership and scope annotated

If we tried to use my_struct after the call to takes_ownership, Cairo would throw a compile-time error. These static checks protect us from mistakes. Try adding code to main that uses my_struct and x to see where you can use them and where the ownership rules prevent you from doing so.

Return Values and Scope

Returning values can also transfer ownership. Listing 4-4 shows an example of a function that returns some value, with similar annotations as those in Listing 4-3.

Filename: src/lib.cairo

struct A {}

fn main() {
    let a1 = gives_ownership();           // gives_ownership moves its return
                                          // value into a1

    let a2 = A {};                        // a2 comes into scope

    let a3 = takes_and_gives_back(a2);    // a2 is moved into
                                          // takes_and_gives_back, which also
                                          // moves its return value into a3

} // Here, a3 goes out of scope and is dropped. a2 was moved, so nothing
  // happens. a1 goes out of scope and is dropped.

fn gives_ownership() -> A {               // gives_ownership will move its
                                          // return value into the function
                                          // that calls it

    let some_a = A {};                    // some_a comes into scope

    some_a                                // some_a is returned and
                                          // moves ownership to the calling
                                          // function

// This function takes an instance some_a of A and returns it
fn takes_and_gives_back(some_a: A) -> A { // some_a comes into
                                          // scope

    some_a                               // some_a is returned and moves
                                         // ownership to the calling
                                         // function

Listing 4-4: Transferring ownership of return values

When a variable goes out of scope, its value is dropped, unless ownership of the value has been moved to another variable.

While this works, taking ownership and then returning ownership with every function is a bit tedious. What if we want to let a function use a value but not take ownership? It’s quite annoying that anything we pass in also needs to be passed back if we want to use it again, in addition to any data resulting from the body of the function that we might want to return as well.

Cairo does let us return multiple values using a tuple, as shown in Listing 4-5.

Filename: src/lib.cairo

fn main() {
    let arr1 = ArrayTrait::<u128>::new();

    let (arr2, len) = calculate_length(arr1);

fn calculate_length(arr: Array<u128>) -> (Array<u128>, usize) {
    let length = arr.len(); // len() returns the length of an array

    (arr, length)

Listing 4-5: Returning ownership of parameters

But this is too much ceremony and a lot of work for a concept that should be common. Luckily for us, Cairo has two features for using a value without transferring ownership, called references and snapshots.

Last change: 2023-09-20, commit: cbb0049