Page 4: Error Handling, Events, and Security Basics

4. Error Handling, Events, and Security Basics

Since smart contracts cannot be modified once deployed, robust error handling and built-in security are essential from the start.

4.1. Error Handling (require, revert, assert)

• require(condition, "message"): Checks the validity of user input or state variables. Most frequently used. If the condition is not met, it reverts the transaction and refunds the remaining gas to the user.
• revert("message"): Similar to require, it reverts the transaction but is used explicitly for more complex conditions.
• assert(condition): Used to check for internal errors or bugs. (Generally, require is recommended for user/external condition errors).

4.2. Events

Events log data onto the blockchain in log form, allowing the front-end of a DApp to subscribe to state changes in the contract. This is a gas-efficient method.

event ValueChanged(address indexed user, uint256 newValue); 

function updateValue(uint256 x) public { 
    // ... logic ... 
    emit ValueChanged(msg.sender, x); 
}

4.3. Basic Security Pattern: Preventing Reentrancy

One of the most dangerous vulnerabilities related to external calls is the Reentrancy Attack. To prevent this, you must use the 'Checks-Effects-Interactions' pattern.

• Checks: Execute all require statements.
• Effects: Update state variables (most crucial).
• Interactions: Make external calls (last step).