導言

待補充....

程式碼

pragma solidity^0.4.25;

library SafeMath{
    function add(uint a, uint b) internal pure returns (uint c) {
        c = a + b;
        require(c >= a);
    }
    function sub(uint a, uint b) internal pure returns (uint c) {
        require(b <= a);
        c = a - b;
    }
    function mul(uint a, uint b) internal pure returns (uint c) {
        c = a * b;
        require(a == 0 || c / a == b);
    }
    function div(uint a, uint b) internal pure returns (uint c) {
        require(b > 0);
        c = a / b;
    }
}
contract Exchange{
    using SafeMath for uint256;
    
    mapping (address => mapping (address => uint256)) tokenLedger;
    mapping (address => uint256) lastActiveTx;
    address public feeAddress;

    event Deposit(address indexed tokenAddress, address indexed user, uint256 amount, uint256 balance);
    event Withdraw(address indexed tokenAddress, address indexed user, uint256 amount, uint256 balance);
    
    
    modifier onlyOwner{
        require(msg.sender == owner);
        _;
    }
    
    constructor() {
        owner = msg.sender;
    }
    
}

解說

(待補)