Ethereum Cookbook

How to do it...

1. Modifiers are declared with the modifier keyword as follows:

modifier modifierName {    // modifier definition}

2. The function body is inserted where the _ symbol appears in the definition of a modifier:

modifier onlyOwner {    require(msg.sender == owner);    _;}

3. Modifiers can accept parameters like functions do. The following example creates a generic modifier to verify the caller address:

contract Test {        address owner;        constructor() public {        owner = msg.sender;    }        modifier onlyBy(address user) {        require(msg.sender == user);        _;    }        function donate() onlyBy(owner) public {        // do something    }}

4. Multiple modifiers can be specified in a function. They will be evaluated in the order given:

contract modifierContract { address owner; ...