Modifiers

• Check some behavior before a function executes
• The placeholder statement (denoted by a single underscore character _) is used to denote where the body of the function being modified should be inserted.
• The _ symbol can appear in the modifier multiple times. Each occurrence is replaced with the function body, and the function returns the return value of the final occurrence.
• Modifiers cannot implicitly access or change the arguments and return values of functions they modify. Their values can only be passed to them explicitly at the point of invocation

contract owned {
    constructor() { owner = payable(msg.sender); }
    address payable owner;

    // This contract only defines a modifier but does not use
    // it: it will be used in derived contracts.
    // The function body is inserted where the special symbol
    // `_;` in the definition of a modifier appears.
    // This means that if the owner calls this function, the
    // function is executed and otherwise, an exception is
    // thrown.
    modifier onlyOwner {
        require(
            msg.sender == owner,
            "Only owner can call this function."
        );
        _;
    }
}

• modifier can receive arguments

contract priced {
    // Modifiers can receive arguments:
    modifier costs(uint price) {
        if (msg.value >= price) {
            _;
        }
    }
}

Usage

contract Register is priced, owned {
    mapping(address => bool) registeredAddresses;
    uint price;

    constructor(uint initialPrice) { price = initialPrice; }

    // It is important to also provide the
    // `payable` keyword here, otherwise the function will
    // automatically reject all Ether sent to it.
    function register() public payable **costs(price)** {
        registeredAddresses[msg.sender] = true;
    }

    // This contract inherits the `onlyOwner` modifier from
    // the `owned` contract. As a result, calls to `changePrice` will
    // only take effect if they are made by the stored owner.
    function changePrice(uint price_) public onlyOwner {
        price = price_;
    }
}