bartubozkurt

/* Bad */
contract Bad{}
contract Tom is Bad{
    constructor() public Bad(){}
}

/* Better */
contract Good{}
contract Tom is Good{
    constructor() public {} // remove the constructor call.

bartubozkurt

/* Bad */
contract A {
    uint x;
    constructor() public {
        x = 0;
    }
    function A() public {
        x = 1;
    }
    

bartubozkurt

/* Bad */
    modifier myModif(){
        if(..){
           _;
        }
    }
    function get() myModif returns(uint){}

/* Better */
    modidfier myModif(){

bartubozkurt

/* Bad */
contract BadGuy {
    function isLove(address _addr) external returns(bool) {}
}

contract BadGirl {
    BadGuy badguy;
    modifier isCheck(address _addr) {
        require(badguy.isLove(_addr));
        _;

bartubozkurt

/* Bad */
contract Bad {
	function badDelegate(address _yourContract, bytes calldata _data) payable public returns (bytes memory) {
	    (bool success, bytes memory data) =  _yourContract.delegatecall(_data);
	    require(success);
	    return data;
	}
}
/* Vulnerability
Anyone can destroy the Bad contract using by “selfdestruct” 

bartubozkurt

/* Bad */
contract SendEth{
	mapping(address => uint256) public balanceOf;

	function withdraw(address user, uint256 numTokens) public {
	    require(balanceOf[user] >= numTokens);
	    balanceOf[user] -= numTokens;
	    user.transfer(numTokens * 1 ether);
	}
}

bartubozkurt

/* Bad */
address owner;
function setOwner() public {
	owner = msg.sender;
}

/* Better */
contract Buggy{
/* 
modifier onlyOwner() {

bartubozkurt

/* Bad */
// Sample1.sol
pragma solidity =0.8.4;
// Sample2.sol
pragma solidity =0.8.0;


/* Better */
// Sample1.sol
pragma solidity =0.8.4;

bartubozkurt

/* Bad */
pragma solidity ^0.8.4;

/* Better */
pragma solidity =0.8.4;

bartubozkurt

/* Bad */
pragma solidity ^0.4.25;

/* Better */
pragma solidity 0.8.17;