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