readme_en.md

WTF Cairo: 15. Constructor

We are learning Cairo, and writing WTF Cairo Tutorials for Starknet newbies. The tutorials are based on Cairo 2.2.0.

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In this chapter, we will explore constructor function in Cairo, a special function that initializes the state variables of a contract.

Constructor

Same as Solidity, the constructor in Cairo is a special function which will automatically run once during the contract deployment. It is often used to initialize the parameters of a contract, such as setting the owner address:

#[starknet::contract]
mod owner{
    // import contract address related libraries
    use starknet::ContractAddress;
    use starknet::get_caller_address;

    // Define storage variable
    #[storage]
    struct Storage{
        owner: ContractAddress,
    }

    // Set owner address during deployment
    #[constructor]
    fn constructor(ref self: ContractState) {
        self.owner.write(get_caller_address());
    }
}

In the above contract, we've defined a storage variable owner within the Storage struct. This owner is then initialized in the constructor function to the address of the caller.

Rules

1. The constructor function must be marked with the #[constructor] attribute.
2. Each contract can have, at most, one constructor.

Summary

In this chapter, we introduced the constructor function in Cairo. This special function will automatically run once during contract deployment, setting the stage for state variables of the contract.