_You can follow along with the video course from here._ ### Introduction In the previous lesson, we learned how to make our contracts more gas efficient. In this lesson, we will further enhance their efficiency. ### Require One way to improve gas efficiency is by optimizing our `require` statements. Currently, the `require` statement forces us to store the string 'sender is not an owner'. Each character in this string is stored individually, making the logic to manage it complex and expensive. ### Custom Errors Introduced in **Solidity 0.8.4**, custom errors can be used in `revert` statements. These errors should be declared at the top of the code and used in `if` statements. The cheaper error code is then called in place of the previous error message string, reducing gas costs. We can start by creating a custom error: ```solidity error NotOwner(); ``` Then, we can replace the `require` function with an `if` statement, using the `revert` function with the newly created error: ```solidity if (msg.sender != i_owner) { revert NotOwner(); } ``` By implementing custom errors, we reduce gas costs and simplify error handling in our smart contracts. ### Conclusion In this lesson, we have learned how to further optimize gas efficiency in Solidity contracts by using custom errors instead of traditional require statements with strings. ### 🧑‍💻 Test yourself 1. 📕 What are the benefits of declaring custom errors instead of using the `require` keyword? 2. 🧑‍💻 Create a custom error that is triggered when msg.sender is address(0) and then convert it into an equivalent if statement with a `revert` function.