Constructor

A constructor is a special function that initializes a contract’s state during deployment. It has several key characteristics:

Runs exactly once when the contract is deployed
Must be annotated with #[constructor]
Up to one constructor per contract
Function is conventionally named constructor

Here’s an example that shows how to initialize storage variables during contract deployment:

#[starknet::contract]
mod ConstructorContract {
    // This trait is necessary to be able to write to a specific storage variable
    use starknet::storage::StoragePointerWriteAccess;

    #[storage]
    struct Storage {
        a: u128,
        b: u8,
        c: u256,
    }

    // The constructor is decorated with a `#[constructor]` attribute.
    // It is not inside an `impl` block.
    #[constructor]
    fn constructor(ref self: ContractState, a: u128, b: u8, c: u256) {
        self.a.write(a);
        self.b.write(b);
        self.c.write(c);
    }
}

In this example:

The constructor takes three parameters: a, b, and c
Each parameter corresponds to a storage variable of the same name, but you can specify any argument variable name
The values are written to storage using the write() method. You need to import the StoragePointerWriteAccess trait to be able to write to a specific storage pointer

Constructors are ideal for:

Setting initial contract state
Storing deployment-time parameters
Initializing access control (e.g., setting an owner)

Constructor values cannot be changed after deployment unless you specifically implement functions to modify them.