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Permissions: Add Allowances for External Roles

In this step we are adding a mapping so we can store address => uint amounts. This will be like an array that stores [0x123546...] an address, to a specific number. So, we always know how much someone can withdraw. We also add a new modifier that checks: Is it the owner itself or just someone with allowance?

//SPDX-License-Identifier: MIT

pragma solidity 0.8.1;

import "";

contract SharedWallet is Ownable {
    function isOwner() internal view returns(bool) {
        return owner() == msg.sender;

    mapping(address => uint) public allowance;

    function addAllowance(address _who, uint _amount) public onlyOwner {
        allowance[_who] = _amount;

    modifier ownerOrAllowed(uint _amount) {
        require(isOwner() || allowance[msg.sender] >= _amount, "You are not allowed!");

    function withdrawMoney(address payable _to, uint _amount) public ownerOrAllowed(_amount) {
        require(_amount <= address(this).balance, "Contract doesn't own enough money");

    receive() external payable {


Did you catch the bug?

Have a look at the withdrawMoney functionality and think it through!

In the next lecture we're going to improve our smart contract a little bit and avoid double spending.

Last update: April 29, 2022