Where to use function modifiers

Function modifiers are simple snippets that can change the behavior of a contract. In this recipe, you will learn how to create and use function modifiers in solidity.

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...