This topic is about Solidity – Function Modifiers.
Function Modifiers are used to modify the behaviour of a function. For example to add a prerequisite to a function.
First we create a modifier with or without parameter.
contract Owner {
modifier onlyOwner {
require(msg.sender == owner);
_;
}
modifier costs(uint price) {
if (msg.value >= price) {
_;
}
}
}
The function body is inserted where the special symbol “_;” appears in the definition of a modifier. So if condition of modifier is satisfied while calling this function, the function is executed and otherwise, an exception is thrown.
See the example below −
pragma solidity ^0.5.0;
contract Owner {
address owner;
constructor() public {
owner = msg.sender;
}
modifier onlyOwner {
require(msg.sender == owner);
_;
}
modifier costs(uint price) {
if (msg.value >= price) {
_;
}
}
}
contract Register is Owner {
mapping (address => bool) registeredAddresses;
uint price;
constructor(uint initialPrice) public { price = initialPrice; }
function register() public payable costs(price) {
registeredAddresses[msg.sender] = true;
}
function changePrice(uint _price) public onlyOwner {
price = _price;
}
}
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