Modifiers can be used to change the behavior of functions in a declarative way.
For example, you can use a modifier to automatically check a condition prior to executing the function. they can be used for:
- Restrict access
- Validate inputs
- Guard against reentrancy hack
- use the
modifierin the declaration of a function.
Modifier to check that the caller is the owner of the contract
modifier onlyOwner {
require(msg.sender == owner);
// Underscore is a special character only used inside
// a function modifier and it tells Solidity to
// execute the rest of the code.
_;
}
Modifiers can take inputs. This modifier checks that the address passed in is not the zero address.
modifier validAddress(address _addr) {
require(_addr != address(0), "Not valid address");
_;
}
function changeOwner(address _newOwner) public onlyOwner validAddress(_newOwner) {
owner = _newOwner;
}
Modifiers can be called before and / or after a function.This modifier prevents a function from being called while,it is still executing.
modifier noReentrancy() {
require(!locked, "No reentrancy");
locked = true;
_;
locked = false;
}
function decrement(uint i) public noReentrancy {
x -= i;
if (i > 1) {
decrement(i - 1);
}
}
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