lesson-03chapter-02ownable.sol

pragma solidity ^0.4.25;

contract Ownable {
  address private _owner;

  event OwnershipTransferred(
    address indexed previousOwner,
    address indexed newOwner
  );

 
  constructor() internal {
    _owner = msg.sender;
    emit OwnershipTransferred(address(0), _owner);
  }

  
  function owner() public view returns(address) {
    return _owner;
  }

  
  modifier onlyOwner() {
    require(isOwner());
    _;
  }

  
  function isOwner() public view returns(bool) {
    return msg.sender == _owner;
  }

 
  function renounceOwnership() public onlyOwner {
    emit OwnershipTransferred(_owner, address(0));
    _owner = address(0);
  }

 
  function transferOwnership(address newOwner) public onlyOwner {
    _transferOwnership(newOwner);
  }

 
  function _transferOwnership(address newOwner) internal {
    require(newOwner != address(0));
    emit OwnershipTransferred(_owner, newOwner);
    _owner = newOwner;
  }
}