深入解析 ERC-721 标准：从源码实现到核心机制

ERC-721 是以太坊上非同质化代币（NFT）的核心标准，定义了 NFT 的所有权、转移和授权规则。本文将通过一个简化版的 ERC-721 合约源码（基于 Solidity），逐层剖析其实现逻辑、核心难点和关键设计思想。

ERC-721 的核心功能

所有权与余额管理

每个 NFT 通过唯一的 tokenId 标识所有权，合约通过以下数据结构实现：

mapping(uint256 => address) internal _ownerOf;     // TokenID → 所有者地址
mapping(address => uint256) internal _balanceOf;  // 地址 → 持有代币数量

• 所有权查询：ownerOf(uint256 id) 方法验证代币存在性后返回所有者。
• 余额统计：balanceOf(address owner) 确保查询地址非零后返回余额。

授权机制

ERC-721 支持两种授权模式：

• 单代币授权：允许特定地址操作单个代币（通过 approve() 设置）。
• 全局授权：允许操作者（Operator）管理所有者的全部代币（通过 setApprovalForAll() 设置）。

// 单代币授权映射
mapping(uint256 => address) internal _approvals; 

// 全局授权映射
mapping(address => mapping(address => bool)) public isApprovedForAll; 

代币转移

代币转移是 ERC-721 最核心的功能，分为两种模式：

• 普通转账：transferFrom() 直接更新所有权。
• 安全转账：safeTransferFrom() 额外检查接收方是否为合约，并验证其能否处理 NFT。

// 基础转账逻辑（简化版）
function transferFrom(address from, address to, uint256 id) public {
    require(_isApprovedOrOwner(from, msg.sender, id), "未授权");
    _balanceOf[from]--;
    _balanceOf[to]++;
    _ownerOf[id] = to;
    delete _approvals[id]; // 清除单代币授权
    emit Transfer(from, to, id);
}

实现中的核心难点

权限验证的复合逻辑

在转账或授权时，需验证调用者是否具备以下任一权限：

1. 代币所有者（Owner）
2. 被单代币授权的地址（Approved）
3. 被全局授权的操作者（Operator）

这一逻辑通过 _isApprovedOrOwner() 实现：

function _isApprovedOrOwner(address owner, address spender, uint256 id) 
    internal view returns (bool) 
{
    return spender == owner 
        || isApprovedForAll[owner][spender] 
        || spender == _approvals[id];
}

安全转账的接收方检查

当接收方（to）为合约时，必须调用其 onERC721Received 方法，并验证返回值是否为预定义的魔法值 0x150b7a02（即 IERC721Receiver.onERC721Received.selector）。这是为了防止代币误转入无法处理的合约。

// 安全转账逻辑片段
require(
    to.code.length == 0 || // 接收方为普通地址
    IERC721Receiver(to).onERC721Received(...) == IERC721Receiver.onERC721Received.selector,
    "不安全的接收方"
);

ERC-165 接口兼容性

ERC-721 必须实现 ERC-165 标准，声明支持的接口。这是为了与其他合约（如市场平台）兼容，使其能够自动检测合约功能。

function supportsInterface(bytes4 interfaceId) external pure returns (bool) {
    return interfaceId == type(IERC721).interfaceId 
        || interfaceId == type(IERC165).interfaceId;
}

关键实现细节与最佳实践

状态更新顺序：防止重入攻击

在 safeTransferFrom 方法中，遵循 Checks-Effects-Interactions 模式：

1. 检查：验证权限和输入有效性。
2. 更新：先修改合约状态（所有权和余额）。
3. 交互：最后与外部合约（接收方）交互。 这一顺序可有效避免重入攻击。

零地址处理

• 铸造代币：_mint() 方法要求接收方地址非零，防止无效铸造。
• 销毁代币：_burn() 将代币所有者设为零地址，并触发 Transfer 事件。

事件触发规则

所有关键操作必须触发对应事件，以便链下监听：

• 转账：Transfer(address from, address to, uint256 id)
• 单代币授权：Approval(address owner, address spender, uint256 id)
• 全局授权：ApprovalForAll(address owner, address operator, bool approved)

安全实践与扩展建议

输入验证

• 代币存在性：在 ownerOf() 和 getApproved() 中检查 owner != address(0)。
• 防重复铸造：_mint() 检查 _ownerOf[id] == address(0)。

权限隔离

• 内部方法保护：_mint 和 _burn 标记为 internal，需通过包装函数（如 MyNFT.mint()）调用，便于添加权限控制（如仅允许合约所有者调用）。

扩展功能

• 元数据扩展：实现 tokenURI(uint256 id) 方法以支持链上/链下元数据。
• 枚举扩展：添加 totalSupply() 和 tokenByIndex() 方法，支持代币遍历。
• 批量操作：优化批量转账和授权，降低 Gas 成本。

参考学习

• OpenZeppelin ERC-721 实现
• ERC-721 官方标准
• NFT 安全最佳实践

完整示例代码

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.26;

interface IERC165 {
    function supportsInterface(bytes4 interfaceID)
        external
        view
        returns (bool);
}

interface IERC721 is IERC165 {
    function balanceOf(address owner) external view returns (uint256 balance);
    function ownerOf(uint256 tokenId) external view returns (address owner);
    function safeTransferFrom(address from, address to, uint256 tokenId)
        external;
    function safeTransferFrom(
        address from,
        address to,
        uint256 tokenId,
        bytes calldata data
    ) external;
    function transferFrom(address from, address to, uint256 tokenId) external;
    function approve(address to, uint256 tokenId) external;
    function getApproved(uint256 tokenId)
        external
        view
        returns (address operator);
    function setApprovalForAll(address operator, bool _approved) external;
    function isApprovedForAll(address owner, address operator)
        external
        view
        returns (bool);
}

interface IERC721Receiver {
    function onERC721Received(
        address operator,
        address from,
        uint256 tokenId,
        bytes calldata data
    ) external returns (bytes4);
}

contract ERC721 is IERC721 {
   // 定义事件
    event Transfer(
        address indexed from, address indexed to, uint256 indexed id
    );
    event Approval(
        address indexed owner, address indexed spender, uint256 indexed id
    );
    event ApprovalForAll(
        address indexed owner, address indexed operator, bool approved
    );

    // 记录每个NFT与其所有者的映射关系
    mapping(uint256 => address) internal _ownerOf;

    // 记录每个地址所持有的代币数量
    mapping(address => uint256) internal _balanceOf;

    // 记录每个代币被授权的地址
    mapping(uint256 => address) internal _approvals;

    // 全局授权映射
    mapping(address => mapping(address => bool)) public isApprovedForAll;

    // 实现 IERC165 接口的方法
    function supportsInterface(bytes4 interfaceId)
        external
        pure
        returns (bool)
    {
        return interfaceId == type(IERC721).interfaceId
            || interfaceId == type(IERC165).interfaceId;
    }
   // 根据tokenId 获取其所有者
    function ownerOf(uint256 id) external view returns (address owner) {
        owner = _ownerOf[id];
        require(owner != address(0), "token doesn't exist");
    }
    // 获取账户余额
    function balanceOf(address owner) external view returns (uint256) {
        require(owner != address(0), "owner = zero address");
        return _balanceOf[owner];
    }

    function setApprovalForAll(address operator, bool approved) external {
        isApprovedForAll[msg.sender][operator] = approved;
        emit ApprovalForAll(msg.sender, operator, approved); // 触发ApprovalForAll 事件
    }

    function approve(address spender, uint256 id) external {
        address owner = _ownerOf[id];
        require(
            msg.sender == owner || isApprovedForAll[owner][msg.sender],
            "not authorized"
        );

        _approvals[id] = spender; //更新代币授权

        emit Approval(owner, spender, id); // 触发 Approval 事件
    }

    function getApproved(uint256 id) external view returns (address) {
        require(_ownerOf[id] != address(0), "token doesn't exist");
        return _approvals[id];
    }

    function _isApprovedOrOwner(address owner, address spender, uint256 id)
        internal
        view
        returns (bool)
    {
        return (
            spender == owner || isApprovedForAll[owner][spender]
                || spender == _approvals[id]
        );
    }

    function transferFrom(address from, address to, uint256 id) public {
        require(from == _ownerOf[id], "from != owner");
        require(to != address(0), "transfer to zero address");

        require(_isApprovedOrOwner(from, msg.sender, id), "not authorized");

        _balanceOf[from]--;
        _balanceOf[to]++;
        _ownerOf[id] = to;

        delete _approvals[id];

        emit Transfer(from, to, id);
    }

    function safeTransferFrom(address from, address to, uint256 id) external {
        transferFrom(from, to, id);

        require(
            to.code.length == 0
                || IERC721Receiver(to).onERC721Received(msg.sender, from, id, "")
                    == IERC721Receiver.onERC721Received.selector,
            "unsafe recipient"
        );
    }

    function safeTransferFrom(
        address from,
        address to,
        uint256 id,
        bytes calldata data
    ) external {
        transferFrom(from, to, id);

        require(
            to.code.length == 0
                || IERC721Receiver(to).onERC721Received(msg.sender, from, id, data)
                    == IERC721Receiver.onERC721Received.selector,
            "unsafe recipient"
        );
    }

    function _mint(address to, uint256 id) internal {
        require(to != address(0), "mint to zero address");
        require(_ownerOf[id] == address(0), "already minted");

        _balanceOf[to]++;
        _ownerOf[id] = to;

        emit Transfer(address(0), to, id);
    }

    function _burn(uint256 id) internal {
        address owner = _ownerOf[id];
        require(owner != address(0), "not minted");

        _balanceOf[owner] -= 1;

        delete _ownerOf[id];
        delete _approvals[id];

        emit Transfer(owner, address(0), id);
    }
}

contract MyNFT is ERC721 {
    function mint(address to, uint256 id) external {
        _mint(to, id);
    }

    function burn(uint256 id) external {
        require(msg.sender == _ownerOf[id], "not owner");
        _burn(id);
    }
}