cairo by example

snapshots

Snapshots provide read-only instances of an object without taking ownership when passed to a function. Every type T has a snapshot version @T, which is always droppable and duplicatable.

To create a snapshot of a value x of type T, simply use @x.

use array::ArrayTrait;
use option::OptionTrait;
use box::BoxTrait;

// Receives an array snapshot
fn sum_starting_two(data: @Array<u32>) -> u32 {
    //data.append(5_u32); <- This will fail, as data is read-only
    let first = *data.get(0).unwrap().unbox();
    let second = *data.get(1).unwrap().unbox();
    first + second

    //*data[0] + *data[1] <- also works
}

fn main() -> u32 {
    let mut data: Array<u32> = ArrayTrait::new();
    data.append(1_u32);
    data.append(2_u32);
    data.append(3_u32);
    data.append(4_u32);
    data.get(0);
    sum_starting_two(@data) // Using a snapshot instead of the mut variable
}

Try it out!
  1. Install the toolchain:
    • For macOS and Linux, run our script:
      • curl -sL https://raw.githubusercontent.com/lambdaclass/cairo-by-example/main/build/installer.sh | bash -s 2.2.0
    • For Windows and others, please see the official guide
  2. Run the example:
    1. Copy the example into a snapshots.cairo file and run with:
      2. %!s(<nil>) snapshots.cairo

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