Ownable contract

contract Ownable {    address public owner;    event OwnershipTransferred(        address indexed previousOwner,        address indexed newOwner    );    /**     * @dev The Ownable constructor sets the original `owner` of the contract to the sender     * account.     */    constructor() public {        owner = msg.sender;    }    /**     * @dev Throws if called by any account other than the owner.     */    modifier onlyOwner() {        require(msg.sender == owner);        _;    }    /**     * @dev Allows the current owner to transfer control of the contract to a newOwner.     * @param _newOwner The address to transfer ownership to.     */    function transferOwnership(address _newOwner) public onlyOwner {        _transferOwnership(_newOwner);    }    /**     * @dev Transfers control of the contract to a newOwner.     * @param _newOwner The address to transfer ownership to.     */    function _transferOwnership(address _newOwner) internal {        require(_newOwner != address(0));        emit OwnershipTransferred(owner, _newOwner);        owner = _newOwner;    }}

Constructor

As a constructor defined in smart contract runs only once while deploying we can set our owner variable to the msg.sender , or it you want to define a custom owner to your smart contract you can directly get address as an argument from the constructor

 address public owner;constructor() public {        owner = msg.sender;    } // this sets the contract deployer as the owner of contract/* constructor(address _owner) public {        owner = _owner;    } // this sets the custom address(_owner) as the owner of smart contract*/

Modifier

 modifier onlyOwner() {        require(msg.sender == owner);        _;    }

We can use this modifier to restrict the function access to only owner and not for public. we can directly define modifier in function viewports

function WithDrawToken() public payable Onlyowner  {} // this function is only acessasible to the msg.sender who has the ower account

Transfer Ownership 

function transferOwnership(address _newOwner) public onlyOwner {        _transferOwnership(_newOwner);    }    /**     * @dev Transfers control of the contract to a newOwner.     * @param _newOwner The address to transfer ownership to.     */    function _transferOwnership(address _newOwner) internal {        require(_newOwner != address(0));        emit OwnershipTransferred(owner, _newOwner);        owner = _newOwner;    }

The function transferOwnership is used to change the owner , since its secured with modifier only existing owner can change the owner of the smart contract.