_You can follow along with the video course from here._ ### Introduction In this lesson, we will explore **modifiers** and how they can simplify code writing and management in Solidity. Modifiers enable developers to create reusable code snippets that can be applied to multiple functions, enhancing code readability, maintainability, and security. ### Repeated Conditions If we build a contract with multiple _administrative functions_, that should only be executed by the contract owner, we might repeatedly check the caller identity: ```solidity require(msg.sender == owner, "Sender is not owner"); ``` However, repeating this line in every function clutters the contract, making it harder to read, maintain, and debug. ### Modifiers Modifiers in Solidity allow embedding **custom lines of code** within any function to modify its behaviour. Here's how to create a modifier: ```solidity modifier onlyOwner { require(msg.sender == owner, "Sender is not owner"); _; } ``` > 🗒️ **NOTE**:br > The modifier is named `onlyOwner` to reflect the condition it checks. ### The `_` (underscore) The underscore `_` placed in the body is a placeholder for the modified function's code. When the function with the modifier is called, the code before `_` runs first, and if it succeeds, the function's code executes next. For example, the `onlyOwner` modifier can be applied to the `withdraw` function like this: ```solidity function withdraw(uint amount) public onlyOwner { // Function logic } ``` When `withdraw` is called, the contract first executes the `onlyOwner` modifier. If the `require` statement passes, the rest of the `withdraw` function executes. If the underscore `_` were placed before the `require` statement, the function's logic would execute first, followed by the `require` check, which is not the intended use case. ### Conclusion Using modifiers like `onlyOwner` simplifies contract development by centralizing common conditions, reducing code repetition, and enhancing contract readability and maintainability. ### 🧑‍💻 Test yourself 1. 📕 Why is it beneficial to use `modifiers` for access control? 2. 🧑‍💻 Implement a modifier named `onlyAfter(uint256 _time)` that ensures a function can only be executed after a specified time.