RareSkills Solidity Interview Question #29 Answered: What does a modifier do?

This series will provide answers to the list of Solidity interview questions that were published by RareSkills..

Question #29 (Easy): What does a modifier do?

Answer: A modifier executes code before and/or after the modified function’s logic is executed. When a modifier is specified on a function, it is invoked when the function is called and passes execution control to the function where _; is found. Modifiers are commonly used as access control mechanisms and allow for precondition and postcondition checks.

Demonstration:

// SPDX-License-Identifier: MIT
pragma solidity 0.8.24;

// SimpleAccessControl demonstrates basic access control mechanisms
contract SimpleAccessControl {
    address public owner;

    // Modifier to restrict function access to the contract's owner
    modifier onlyOwner() {
        // This check will run before the `changeOwner` logic is executed
        require(msg.sender == owner, "Caller is not the owner");
        
        // This will execute the `changeOwner` logic.
        _;
    }

    // Constructor sets the deploying address as the contract's owner
    constructor() {
        owner = msg.sender;
    }

    // Function to change the owner, restricted to the current owner
    function changeOwner(address newOwner) public onlyOwner {
        owner = newOwner;
    }
}

Further Discussion:

The _; syntax in a Solidity modifier is a placeholder that represents the point at which the modified function’s code is executed. For instance, if _; is placed at the beginning of a modifier, the function’s code runs before any other code in the modifier. If _; is placed at the end, the modifier’s code will run before the function’s code.

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Previous articles:

• Question #19 (Easy): What is the difference between fallback and receive?
• Question #20 (Easy): What is reentrancy?
• Question #21 (Easy): As of the Shanghai upgrade, what is the gas limit per block?
• Question #22 (Easy): What prevents infinite loops from running forever?
• Question #23 (Easy): What is the difference between tx.origin and msg.sender?
• Question #24 (Easy): How do you send Ether to a contract that does not have payable functions, or a receive or fallback?
• Question #25 (Easy): What is the difference between view and pure?
• Question #26 (Easy): What is the difference between transferFrom and safeTransferFrom in ERC721?
• Question #27 (Easy): How can an ERC1155 token be made into a non-fungible token?
• Question #28 (Easy): What is access control and why is it important?

Disclosure: Some content may have been generated with the help of artificial intelligence.