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

4. 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);
    }
   }