Pattern Steps

1. Check: Validate all conditions and requirements before making any state changes.
2. Effect: Update the contract's state.
3. Interaction: Interact with external contracts or send Ether.

Why use CEI?

• -Prevents reentrancy attacks by updating state before external calls.
• -Makes contracts easier to reason about and audit.
• -Encourages clear separation of logic.

Example

function withdraw(uint256 amount) public {
    require(balances[msg.sender] >= amount, "Insufficient balance"); // Check
    balances[msg.sender] -= amount; // Effect
    (bool sent, ) = msg.sender.call{value: amount}(""); // Interaction
    require(sent, "Failed to send Ether");
}