Lab #6. NFT Exchange

NFT.sol

pragma solidity ^0.4.19;

contract ERC20 {
    string public name;
    string public symbol;
    string public website;
    address public base_address;
    address public owner_address;

    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, string _name, string _symbol, string _website, address _owner_address) public {  
        totalSupply_ = total;
        balances[msg.sender] = totalSupply_;
        name =  _name;
        symbol = _symbol;
    	website = _website;
    	owner_address = this;
    	owner_address = _owner_address;
    }  

    function totalSupply() public view returns (uint256) {
	return totalSupply_;
    }
    
    function balanceOf(address tokenOwner) public view returns (uint) {
        return balances[tokenOwner];
    }

    function transfer(address receiver, uint numTokens) public 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, uint numTokens) public returns (bool) {
        allowed[msg.sender][delegate] = numTokens;
        Approval(msg.sender, delegate, numTokens);
        return true;
    }

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

    function transferFrom(address owner, address buyer, uint numTokens) public 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);
        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 contractsHolder  {
    mapping(address => ERC20) public addresses;
    
    function createToken (uint256 total, string name, string symbol, string website) public {
        ERC20 new_contract = new ERC20(total, name, symbol, website, msg.sender);
        addresses[msg.sender] = new_contract;
    }
    
    
    function transferOwnership (address _new_owner) public payable {
        require(msg.value == 0.5 * 10 ** 18, "Minimal cost is 0.5 ETH");
        
        ERC20 contract_entity = addresses[msg.sender];
        addresses[msg.sender] = addresses[_new_owner];
        addresses[_new_owner] = contract_entity;
    }
}

Successfully deployed:
https://ropsten.etherscan.io/tx/0xea2a22759b4ee5e69f30248f9869787256a79d508f9a5712664400da3591adbe