第四天：Solidity 核心语法一次过——mapping、struct、modifier 与错误处理

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

contract Votes {
    struct Candidate {
        string name;
        uint256 voteCount;
    }

    mapping(address => uint256) public votedFor;   // 谁投给了谁
    mapping(uint256 => Candidate) public candidates; // 编号 => 候选人
    uint256 public candidateCount;

    function addCandidate(string calldata _name) external {
        candidates[candidateCount++] = Candidate(_name, 0);
    }

    function vote(uint256 _id) external {
        require(votedFor[msg.sender] == 0, "already voted");
        votedFor[msg.sender] = _id;
        candidates[_id].voteCount += 1;
    }
}

contract Owned {
    address public owner;

    error NotOwner(address caller);

    modifier onlyOwner() {
        if (msg.sender != owner) {
            revert NotOwner(msg.sender);
        }
        _;
    }

    constructor() {
        owner = msg.sender;
    }

    function transferOwnership(address _new) external onlyOwner {
        owner = _new;
    }
}