References and Snapshots

The issue with the tuple code in Listing 3-5 is that we have to return the Array to the calling function so we can still use the Array after the call to calculate_length, because the Array was moved into calculate_length.

Snapshots

Instead, we can provide a snapshot of the Array value. In Cairo, a snapshot is an immutable view of a value at a certain point in time. In the previous chapter, we talked about how Cairo's ownership system prevents us from using a value after we've moved it, protecting us from potentially writing twice to the same memory cell when appending values to arrays. However, it's not very convenient. Let's see how we can retain ownership of the value in the calling function using snapshots.

Here is how you would define and use a calculate_length function that takes a snapshot to an array as a parameter instead of taking ownership of the underlying value. In this example, the calculate_length function returns the length of the array passed as parameter. As we're passing it as a snapshot, which is an immutable view of the array, we can be sure that the calculate_length function will not mutate the array, and ownership of the array is kept in the main function.

use array::ArrayTrait;
use debug::PrintTrait;

fn main() {
    let mut arr1 = ArrayTrait::<u128>::new();
    let first_snapshot = @arr1; // Take a snapshot of `arr1` at this point in time
    arr1.append(1_u128); // Mutate `arr1` by appending a value
    let first_length = calculate_length(first_snapshot); // Calculate the length of the array when the snapshot was taken
    let second_length = calculate_length(@arr1); // Calculate the current length of the array
    first_length.print();
    second_length.print();
}

fn calculate_length(arr: @Array<u128>) -> usize {
    arr.len()
}

Note: It is only possible to call the len() method on an array snapshot because it is defined as such in the ArrayTrait trait. If you try to call a method that is not defined for snapshots on a snapshot, you will get a compilation error. However, you can call methods expecting a snapshot on non-snapshot types.

The output of this program is:

[DEBUG]	                               	(raw: 0)

[DEBUG]	                              	(raw: 1)

Run completed successfully, returning []

First, notice that all the tuple code in the variable declaration and the function return value is gone. Second, note that we pass @arr1 into calculate_length and, in its definition, we take @Array<u128> rather than Array<u128>.

Let’s take a closer look at the function call here:

let mut arr1 = ArrayTrait::<u128>::new();
let second_length = calculate_length(@arr1); // Calculate the current length of the array

The @arr1 syntax lets us create a snapshot of the value in arr1. Because a snapshot is an immutable view of a value, the value it points to cannot be modified through the snapshot, and the value it refers to will not be dropped once the snapshot stops being used.

Similarly, the signature of the function uses @ to indicate that the type of the parameter arr is a snapshot. Let’s add some explanatory annotations:

fn calculate_length(array_snapshot: @Array<u128>) -> usize { // array_snapshot is a snapshot of an Array
    array_snapshot.len()
} // Here, array_snapshot goes out of scope and is dropped.
// However, because it is only a view of what the original array `arr` contains, the original `arr` can still be used.

The scope in which the variable array_snapshot is valid is the same as any function parameter’s scope, but the underlying value of the snapshot is not dropped when array_snapshot stops being used. When functions have snapshots as parameters instead of the actual values, we won’t need to return the values in order to give back ownership of the original value, because we never had it.

Snapshots can be converted back into regular values using the desnap operator *, as long as the value type is copyable (which is not the case for Arrays, as they don't implement Copy). In the following example, we want to calculate the area of a rectangle, but we don't want to take ownership of the rectangle in the calculate_area function, because we might want to use the rectangle again after the function call. Since our function doesn't mutate the rectangle instance, we can pass the snapshot of the rectangle to the function, and then transform the snapshots back into values using the desnap operator *.

The snapshot type is always copyable and droppable, so that you can use it multiple times without worrying about ownership transfers.

use debug::PrintTrait;

#[derive(Copy,Drop)]
struct Rectangle {
    height: u64,
    width: u64,
}

fn main(){
    let rec = Rectangle{height:3_u64, width:10_u64};
    let area = calculate_area(@rec);
    area.print();

}

fn calculate_area(rec: @Rectangle) -> u64 {
    // As rec is a snapshot to a Rectangle, its fields are also snapshots of the fields types.
    // We need to transform the snapshots back into values using the desnap operator `*`.
    // This is only possible if the type is copyable, which is the case for u64.
    // Here, `*` is used for both multiplying the height and width and for desnapping the snapshots.
    *rec.height * *rec.width
}

But, what happens if we try to modify something we’re passing as snapshot? Try the code in Listing 3-6. Spoiler alert: it doesn’t work!

#[derive(Copy,Drop)]
struct Rectangle {
    height: u64,
    width: u64,
}

fn main(){
    let rec = Rectangle{height:3_u64, width:10_u64};
    flip(@rec);
}

fn flip(rec: @Rectangle) {
    let temp = rec.height;
    rec.height = rec.width;
    rec.width = temp;
}

Here’s the error:

error: Invalid left-hand side of assignment.
 --> ownership.cairo:15:5
    rec.height = rec.width;
    ^********^

The compiler prevents us from modifying values associated to snapshots.

Mutable References

We can achieve the behavior we want in Listing 3-6 by using a mutable reference instead of a snapshot. Mutable references are actually mutable values passed to a function that are implicitly returned at the end of the function, returning ownership to the calling context. By doing so, they allow you to mutate the value passed while keeping ownership of it by returning it automatically at the end of the execution. In Cairo, a parameter can be passed as mutable reference using the ref modifier.

Note: In Cairo, a parameter can only be passed as mutable reference using the ref modifier if the variable is declared as mutable with mut.

In Listing 3-7, we use a mutable reference to modify the value of the height and width fields of the Rectangle instance in the flip function.

use debug::PrintTrait;
#[derive(Copy, Drop)]
struct Rectangle {
    height: u64,
    width: u64,
}

fn main() {
    let mut rec = Rectangle { height: 3_u64, width: 10_u64 };
    flip(ref rec);
    rec.height.print();
    rec.width.print();
}

fn flip(ref rec: Rectangle) {
    let temp = rec.height;
    rec.height = rec.width;
    rec.width = temp;
}

First, we change rec to be mut. Then we pass a mutable reference of rec into flip with ref rec, and update the function signature to accept a mutable reference with ref rec: Rectangle. This makes it very clear that the flip function will mutate the value of the Rectangle instance passed as parameter.

The output of the program is:

[DEBUG]
                                (raw: 10)

[DEBUG]	                        (raw: 3)

As expected, the height and width fields of the rec variable have been swapped.

Small recap

Let’s recap what we’ve discussed about ownership, snapshots, and references:

• At any given time, a variable can only have one owner.
• You can pass a variable by-value, by-snapshot, or by-reference to a function.
• If you pass-by-value, ownership of the variable is transferred to the function.
• If you want to keep ownership of the variable and know that your function won’t mutate it, you can pass it as a snapshot with @.
• If you want to keep ownership of the variable and know that your function will mutate it, you can pass it as a mutable reference with ref.