When I try to compile the code on Ethereum's own site, I get the following error. Can you help me?

line: 99

pragma solidity ^0.6.0;

interface IERC20 {

    function totalSupply() external view returns (uint256);
    function balanceOf(address account) external view returns (uint256);
    function allowance(address owner, address spender) external view returns (uint256);

    function transfer(address recipient, uint256 amount) external returns (bool);
    function approve(address spender, uint256 amount) external returns (bool);
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);

contract ERC20Basic is IERC20 {

    string public constant name = "Pubice";
    string public constant symbol = "PBE";
    uint8 public constant decimals = 18;

    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
    event Transfer(address indexed from, address indexed to, uint tokens);

    mapping(address => uint256) balances;

    mapping(address => mapping (address => uint256)) allowed;

    uint256 totalSupply_;

    using SafeMath for uint256;

    constructor(uint256 total) public {
        totalSupply_ = total;
        balances[msg.sender] = totalSupply_;

    function totalSupply() public override view returns (uint256) {
        return totalSupply_;

    function balanceOf(address tokenOwner) public override view returns (uint256) {
        return balances[tokenOwner];

    function transfer(address receiver, uint256 numTokens) public override returns (bool) {
        require(numTokens <= balances[msg.sender]);
        balances[msg.sender] = balances[msg.sender].sub(numTokens);
        balances[receiver] = balances[receiver].add(numTokens);
        emit Transfer(msg.sender, receiver, numTokens);
        return true;

    function approve(address delegate, uint256 numTokens) public override returns (bool) {
        allowed[msg.sender][delegate] = numTokens;
        emit Approval(msg.sender, delegate, numTokens);
        return true;

    function allowance(address owner, address delegate) public override view returns (uint) {
        return allowed[owner][delegate];

    function transferFrom(address owner, address buyer, uint256 numTokens) public override returns (bool) {
        require(numTokens <= balances[owner]);
        require(numTokens <= allowed[owner][msg.sender]);

        balances[owner] = balances[owner].sub(numTokens);
        allowed[owner][msg.sender] = allowed[owner][msg.sender].sub(numTokens);
        balances[buyer] = balances[buyer].add(numTokens);
        emit Transfer(owner, buyer, numTokens);
        return true;

library SafeMath {
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        assert(b <= a);
        return a - b;

    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        assert(c >= a);
        return c;

contract DEX {

    IERC20 public token;

    event Bought(uint256 amount);
    event Sold(uint256 amount);

    constructor() public {
        token = new ERC20Basic();

    function buy() payable public {
        uint256 amountTobuy = msg.value;
        uint256 dexBalance = token.balanceOf(address(this));
        require(amountTobuy > 0, "You need to send some ether");
        require(amountTobuy <= dexBalance, "Not enough tokens in the reserve");
        token.transfer(msg.sender, amountTobuy);
        emit Bought(amountTobuy);

    function sell(uint256 amount) public {
        require(amount > 0, "You need to sell at least some tokens");
        uint256 allowance = token.allowance(msg.sender, address(this));
        require(allowance >= amount, "Check the token allowance");
        token.transferFrom(msg.sender, address(this), amount);
        emit Sold(amount);



You have a constructor in your ERC20Basic contract that expects a totalSupply, you need to pass that value as a parameter when you're instantiating the class in your DEX contract. So you should have for example

constructor() public {
    token = new ERC20Basic(1000000000);

depending on how you want to implement it. Hope this helps

  • Thank you. I had overlooked it and ran it directly. Sep 20 at 14:38
  • can i ask something? When transferring, I cut off a certain percentage of the incoming value. But it rolls automatically. How can I prevent this? serviceProviderFee = (amount * serviceProviderFeePercent) / 100; Sep 20 at 14:39

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