erc20_token.sol

// SPDX-License-Identifier: MIT
pragma solidity >=0.5.0 <0.9.0;

interface ERC20Interface {
  // Standard ERC-20 interface.
  function allowance(address owner, address spender) external view returns (uint256);
  function approve(address spender, uint256 value) external returns (bool);
  function transferFrom(address from, address to, uint256 value) external returns (bool);

  function totalSupply() external view returns (uint256);
  function balanceOf(address who) external view returns (uint256);  
  function transfer(address to, uint256 value) external returns (bool);

  event Transfer(address indexed from, address indexed to, uint tokens);
  event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
  // Extension of ERC-20 interface to support supply adjustment.
//   function mint(address to, uint256 value) external returns (bool);
//   function burn(address from, uint256 value) external returns (bool);
}

contract CACrypto is ERC20Interface
{
    string public name = "CACrypto";
    string public symbol = "CAC";
    uint public decimals = 0; //18 by default
    uint public override totalSupply;

    address public founder;
    mapping(address => uint) public balances;
    mapping(address => mapping(address => uint)) allowed;

    constructor() 
    {
      totalSupply = 1000000;
      founder = msg.sender;
      balances[founder] = totalSupply;
    }

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

    function transfer(address to, uint256 value) public override returns (bool success)
    {
      require(balances[msg.sender] >= value);
      balances[to] += value;
      balances[msg.sender] -= value;
      emit Transfer(msg.sender, to, value);
      return true;
    }

    function allowance(address owner, address spender) public view override returns (uint256)
    {
      return allowed[owner][spender];
    }

    function approve(address spender, uint value) public override returns (bool success)
    {
      require(balances[msg.sender] >= value);
      require(value > 0);
      allowed[msg.sender][spender] = value;

      emit Approval(msg.sender, spender, value);
      return true;
    }

    function transferFrom(address from, address to, uint256 value) external override returns (bool success)
    {
      require(allowed[from][msg.sender] >= value);
      require(balances[from] >= value);
      balances[from] -= value;
      allowed[from][msg.sender] -= value;
      balances[to] += value;

      emit Transfer(from, to, value);
      return true;
    }

}