bartubozkurt

/* Bad */
modifier isNotContract(address _a){
    uint len;
    assembly { len := extcodesize(_a) }
    require(len == 0);
    _;
}
/* Better */
modifier isNotContract(address _a){
		(tx.origin == msg.sender,"No Contract");

bartubozkurt

address owner;
/* Bad */
function sendTo(address receiver, uint amount) public {
  require(tx.origin == owner);
  receiver.transfer(amount);
}

/* Better */
function sendTo(address receiver, uint amount) public {
  require(msg.sender == owner);

bartubozkurt

/* Bad */
contract BadVault{
    function receive() payable public{
    }
}

/* Better */
contract GoodVault{
  // add withdraw function.
  function withdraw() public onlyOwner {

bartubozkurt

/* Bad */
contract Bad{
  function fund_reached() public returns(bool){
      return this.balance == 100 ether; // strict equalities
  }
}

/* Better */
contract Good{
  function fund_reached() public returns(bool){

bartubozkurt

/* Bad */
uint256 constant private targetEther = 1000 ether;
function join() public payable {    
    require(msg.value == 5 ether); // each play is 5 ether
    ...doSomething;
}

function claimReward(address _to) public {
	require(this.balance == targetEther);
	_to.transfer(targetEther);

bartubozkurt

pragma solidity =0.4.22;
/* Bad */
contract BadERC721Token{
   function ownerOf(uint256 _tokenId) external view returns (bool);
    //...
}

/* Better */
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/SafeERC721.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/IERC721.sol";

bartubozkurt

pragma solidity =0.4.22;
/* Bad */
contract BadERC20Token{
    function transfer(address to, uint value) external;
    //...
}

/* Better */
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/SafeERC20.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/IERC20.sol";

bartubozkurt

/* Bad */
function unlock(
  address _to,
  uint256 _amount,
  uint8[] _v,
  bytes32[] _r,
  bytes32[] _s
)
  external
{

bartubozkurt

/* Bad */
function approve(address _spender, uint256 _value) public returns (bool success) {
    _allowances[msg.sender][_spender] = _value
}
/* Better */
import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
SafeERC20 for IERC20;
IERC20 token;

function addAllowance(IERC20 _token,address _spender, uint256 _value) public returns (bool success) {

bartubozkurt

address bob = 0x237C8Aea434dE4784d23d145069c6D0Bef629A19 // Give Me Ether
IERC20 token;
uint N = 3 ether
uint M = 1 ether
/* 👩 Alice, 👨 Bob */

/* Bad */
/* FROM  | 🕛  | ORDER　　 | BLOCK |  GAS    */
/*  👩   | 0s  |   1    |   N   | 100,000 */ token.approve(bob,N); 
/*  👩   | 20s |   2    |  N+1  | 100,000 */ token.approve(bob,M);