References and Snapshots

The issue with the tuple code in previous Listing 4-3 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

In the previous chapter, we talked about how Cairo's ownership system prevents us from using a variable after we've moved it, protecting us from potentially writing twice to the same memory cell. However, it's not very convenient. Let's see how we can retain ownership of the variable in the calling function using snapshots.

In Cairo, a snapshot is an immutable view of a value at a certain point in the execution of the program. Recall that memory is immutable, so modifying a variable actually fills a new memory cell. The old memory cell still exists, and snapshots are variables that refer to that "old" value. In this sense, snapshots are a view "into the past".

Here is how you would define and use a calculate_area function that takes a snapshot of a Rectangle struct as a parameter instead of taking ownership of the underlying value. In this example, the calculate_area function returns the area of the Rectangle passed as a snapshot. Since we’re passing it as an immutable view, we can be sure that calculate_area will not mutate the Rectangle, and ownership remains in the main function.

Filename: src/lib.cairo

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

#[executable]
fn main() {
    let mut rec = Rectangle { height: 3, width: 10 };
    let first_snapshot = @rec; // Take a snapshot of `rec` at this point in time
    rec.height = 5; // Mutate `rec` by changing its height
    let first_area = calculate_area(first_snapshot); // Calculate the area of the snapshot
    let second_area = calculate_area(@rec); // Calculate the current area
    println!("The area of the rectangle when the snapshot was taken is {}", first_area);
    println!("The current area of the rectangle is {}", second_area);
}

fn calculate_area(rec: @Rectangle) -> u64 {
    *rec.height * *rec.width
}

Note: Accessing fields of a snapshot (e.g., rec.height) yields snapshots of those fields, which we desnap with * to get the values. This works here because u64 implements Copy. You’ll learn more about desnapping in the next section.

The output of this program is:

$ scarb execute 
   Compiling no_listing_09_snapshots v0.1.0 (listings/ch04-understanding-ownership/no_listing_09_snapshots/Scarb.toml)
    Finished `dev` profile target(s) in 1 second
   Executing no_listing_09_snapshots
The area of the rectangle when the snapshot was taken is 30
The current area of the rectangle is 50

First, notice that all the tuple code in the variable declaration and the function return value is gone. Second, note that we pass @rec into calculate_area and, in its definition, we take @Rectangle rather than Rectangle.

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

let second_length = calculate_length(@arr1); // Calculate the current length of the array

The @rec syntax lets us create a snapshot of the value in rec. Because a snapshot is an immutable view of a value at a specific point in execution, the usual rules of the linear type system are not enforced. In particular, snapshot variables always implement the Drop trait, never the Destruct trait, even dictionary snapshots.

It’s worth noting that @T is not a pointer—snapshots are passed by value to functions, just like regular variables. This means that the size of @T is the same as the size of T, and when you pass @rec to calculate_area, the entire struct (in this case, a Rectangle with two u64 fields) is copied to the function’s stack. For large data structures, this copying can be avoided by using Box<T>—provided that there's no need to mutate the value, which we’ll explore in Chapter 12, but for now, understand that snapshots rely on this by-value mechanism.

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_area(
    rec_snapshot: @Rectangle // rec_snapshot is a snapshot of a Rectangle
) -> u64 {
    *rec_snapshot.height * *rec_snapshot.width
} // Here, rec_snapshot goes out of scope and is dropped.
// However, because it is only a view of what the original `rec` contains, the original `rec` can still be used.

The scope in which the variable rec_snapshot is valid is the same as any function parameter’s scope, but the underlying value of the snapshot is not dropped when rec_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.

Desnap Operator

To convert a snapshot back into a regular variable, you can use the desnap operator *, which serves as the opposite of the @ operator.

Only Copy types can be desnapped. However, in the general case, because the value is not modified, the new variable created by the desnap operator reuses the old value, and so desnapping is a completely free operation, just like 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 *.

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

#[executable]
fn main() {
    let rec = Rectangle { height: 3, width: 10 };
    let area = calculate_area(@rec);
    println!("Area: {}", area);
}

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 a snapshot? Try the code in Listing 4-4. Spoiler alert: it doesn’t work!

Filename: src/lib.cairo

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

#[executable]
fn main() {
    let rec = Rectangle { height: 3, width: 10 };
    flip(@rec);
}

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

Listing 4-4: Attempting to modify a snapshot value

Here’s the error:

$ scarb execute 
   Compiling listing_04_04 v0.1.0 (listings/ch04-understanding-ownership/listing_04_attempt_modifying_snapshot/Scarb.toml)
error: Invalid left-hand side of assignment.
 --> listings/ch04-understanding-ownership/listing_04_attempt_modifying_snapshot/src/lib.cairo:16:5
    rec.height = rec.width;
    ^^^^^^^^^^

error: Invalid left-hand side of assignment.
 --> listings/ch04-understanding-ownership/listing_04_attempt_modifying_snapshot/src/lib.cairo:17:5
    rec.width = temp;
    ^^^^^^^^^

error: could not compile `listing_04_04` due to 2 previous errors
error: `scarb` command exited with error

The compiler prevents us from modifying values associated to snapshots.

Mutable References

We can achieve the behavior we want in Listing 4-4 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 4-5, we use a mutable reference to modify the value of the height and width fields of the Rectangle instance in the flip function.

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

#[executable]
fn main() {
    let mut rec = Rectangle { height: 3, width: 10 };
    flip(ref rec);
    println!("height: {}, width: {}", rec.height, rec.width);
}

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

Listing 4-5: Use of a mutable reference to modify a value

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.

Unlike snapshots, mutable references allow mutation, but like snapshots, ref arguments are not pointers—they are also passed by value. When you pass ref rec, the entire Rectangle type is copied to the function’s stack, regardless of whether it implements Copy. This ensures the function operates on its own local version of the data, which is then implicitly returned to the caller. To avoid this copying for large types, Cairo provides the Box<T> type introduced in Chapter 12 as an alternative, but for this example, the ref modifier suits our needs perfectly.

The output of the program is:

$ scarb execute 
   Compiling listing_04_05 v0.1.0 (listings/ch04-understanding-ownership/listing_05_mutable_reference/Scarb.toml)
    Finished `dev` profile target(s) in 1 second
   Executing listing_04_05
height: 10, width: 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 the linear type system, 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.