constructor-patterns

Constructor & Initializer Patterns

Constructor for Non-Upgradeable Contracts

Constructors run once at deployment. Validate all parameters and set immutables.

contract Vault {
    error ZeroAddress();
    error InvalidFee(uint256 fee);

    address public immutable TOKEN;
    address public immutable ORACLE;
    uint256 public immutable FEE_BPS;
    uint256 public immutable DEPLOYMENT_BLOCK;

    constructor(address token, address oracle, uint256 feeBps) {
        if (token == address(0)) revert ZeroAddress();
        if (oracle == address(0)) revert ZeroAddress();
        if (feeBps > 10_000) revert InvalidFee(feeBps);

        TOKEN = token;
        ORACLE = oracle;
        FEE_BPS = feeBps;
        DEPLOYMENT_BLOCK = block.number;
    }
}

Initializer for Upgradeable Contracts

Proxies never call the implementation's constructor. Use initializer from OpenZeppelin.

import {Initializable} from "@openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol";
import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
import {PausableUpgradeable} from "@openzeppelin/contracts-upgradeable/utils/PausableUpgradeable.sol";

contract VaultV1 is Initializable, OwnableUpgradeable, PausableUpgradeable {
    uint256 public depositCap;
    address public feeRecipient;

    /// @custom:oz-upgrades-unsafe-allow constructor
    constructor() {
        _disableInitializers();
    }

    function initialize(
        address admin,
        address _feeRecipient,
        uint256 _depositCap
    ) external initializer {
        if (admin == address(0)) revert ZeroAddress();
        if (_feeRecipient == address(0)) revert ZeroAddress();

        __Ownable_init(admin);
        __Pausable_init();

        feeRecipient = _feeRecipient;
        depositCap = _depositCap;
    }
}

Critical: Disable Initializers in Implementation

Without _disableInitializers(), an attacker can call initialize() on the implementation contract directly and potentially selfdestruct it (pre-Dencun) or manipulate its state.