Modifiers in solidity

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:

1. Restrict access
2. Validate inputs
3. Guard against reentrancy hack

• use the modifier in 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);
   }
 }