immutable-constants

Immutable & Constant Variables

When to Use Each

Type	Set At	Stored In	SLOAD Cost	Use Case
constant	Compile time	Bytecode (inlined)	0 gas	Known at write time: math constants, hashes, selectors
immutable	Deploy time (constructor)	Bytecode (appended)	0 gas	Known at deploy: addresses, chain IDs, config
Storage	Runtime	Storage slot	2,100 cold / 100 warm	Must change after deployment

Constant Variables

Must be known at compile time. Value is inlined everywhere it's used.

contract Constants {
    // Numeric constants
    uint256 public constant MAX_BPS = 10_000;
    uint256 public constant WAD = 1e18;
    uint256 public constant RAY = 1e27;
    uint256 public constant PRECISION = 1e18;

    // Hash constants — computed at compile time
    bytes32 public constant ADMIN_ROLE = keccak256("ADMIN_ROLE");
    bytes32 public constant MINTER_ROLE = keccak256("MINTER_ROLE");

    // EIP-712 type hashes
    bytes32 public constant PERMIT_TYPEHASH = keccak256(
        "Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)"
    );

    // EIP-1967 slots
    bytes32 internal constant _IMPLEMENTATION_SLOT =
        bytes32(uint256(keccak256("eip1967.proxy.implementation")) - 1);

    // Selector constants
    bytes4 public constant TRANSFER_SELECTOR = IERC20.transfer.selector;
}

What Can Be Constant

• Literal values (uint256, int256, bool, address, bytes1-bytes32)
• keccak256() of literal values
• Expressions evaluated at compile time
• Cannot be constant: values depending on runtime state, block.timestamp, msg.sender

Immutable Variables

Set once in the constructor, stored in bytecode. Cannot be changed after deployment.

contract ImmutableConfig {
    address public immutable WETH;
    address public immutable FACTORY;
    address public immutable ORACLE;
    uint256 public immutable DEPLOYMENT_CHAIN_ID;
    uint256 public immutable CREATED_AT;
    uint8 public immutable UNDERLYING_DECIMALS;

    constructor(
        address weth,
        address factory,
        address oracle,
        address underlying
    ) {
        if (weth == address(0)) revert ZeroAddress();
        if (factory == address(0)) revert ZeroAddress();
        if (oracle == address(0)) revert ZeroAddress();

        WETH = weth;
        FACTORY = factory;
        ORACLE = oracle;
        DEPLOYMENT_CHAIN_ID = block.chainid;
        CREATED_AT = block.timestamp;
        UNDERLYING_DECIMALS = IERC20Metadata(underlying).decimals();
    }
}