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::interface]
trait IConstructorExample<TContractState> {
    fn get_a(self: @TContractState) -> u128;
    fn get_b(self: @TContractState) -> u8;
    fn get_c(self: @TContractState) -> u256;
}

#[starknet::contract]
mod ConstructorExample {
    use starknet::storage::{StoragePointerReadAccess, StoragePointerWriteAccess};

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

    #[constructor]
    fn constructor(ref self: ContractState, a: u128, b: u8, c: u256) {
        // Initialize storage variables with constructor parameters
        self.a.write(a);
        self.b.write(b);
        self.c.write(c);
    }

    #[abi(embed_v0)]
    impl ConstructorExample of super::IConstructorExample<ContractState> {
        fn get_a(self: @ContractState) -> u128 {
            self.a.read()
        }

        fn get_b(self: @ContractState) -> u8 {
            self.b.read()
        }

        fn get_c(self: @ContractState) -> u256 {
            self.c.read()
        }
    }
}
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.