solidity-patterns

Solidity Design Patterns

Checks-Effects-Interactions (CEI)

The most critical pattern in Solidity. All functions that interact with external contracts must follow this order:

1. Checks — validate inputs and preconditions
2. Effects — update state
3. Interactions — call external contracts

function withdraw(uint256 amount) external {
    // CHECKS
    if (amount == 0) revert ZeroAmount();
    if (balances[msg.sender] < amount) revert InsufficientBalance();

    // EFFECTS
    balances[msg.sender] -= amount;

    // INTERACTIONS
    (bool success,) = msg.sender.call{value: amount}("");
    if (!success) revert TransferFailed();

    emit Withdrawn(msg.sender, amount);
}

Violating CEI is the root cause of reentrancy. Even with ReentrancyGuard, always write CEI-compliant code as defense in depth.

Pull-Over-Push

Never iterate over recipients to push funds. Let users withdraw (pull) their own funds.

// BAD: Push pattern — one revert blocks everyone
function distributeRewards(address[] calldata recipients, uint256[] calldata amounts) external {
    for (uint256 i; i < recipients.length; ++i) {
        payable(recipients[i]).transfer(amounts[i]); // reverts if any fails
    }
}

// GOOD: Pull pattern — each user claims independently
mapping(address => uint256) public pendingRewards;

function claimReward() external {
    uint256 amount = pendingRewards[msg.sender];
    if (amount == 0) revert NothingToClaim();

    pendingRewards[msg.sender] = 0;

    (bool success,) = msg.sender.call{value: amount}("");
    if (!success) revert TransferFailed();

    emit RewardClaimed(msg.sender, amount);
}

Guard Check Pattern

Centralize validation in modifiers or internal functions for reuse and consistency.

error Unauthorized();
error InvalidAmount();
error Expired(uint256 deadline);

modifier onlyWhitelisted() {
    if (!whitelist[msg.sender]) revert Unauthorized();
    _;
}

modifier validAmount(uint256 amount) {
    if (amount == 0 || amount > MAX_AMOUNT) revert InvalidAmount();
    _;
}

modifier beforeDeadline(uint256 deadline) {
    if (block.timestamp > deadline) revert Expired(deadline);
    _;
}

function deposit(uint256 amount, uint256 deadline)
    external
    onlyWhitelisted
    validAmount(amount)
    beforeDeadline(deadline)
{
    // core logic only — all checks in modifiers
}