什么是ERC-20代币标准？请详细说明ERC-20的接口规范和实现方法 - 面试题

ERC-20是以太坊上最广泛使用的代币标准，定义了同质化代币的接口规范。以下是ERC-20标准的详细解析：

ERC-20标准概述

ERC-20代表"Ethereum Request for Comments 20"，由Fabian Vogelsteller于2015年提出。它定义了一套标准接口，使不同代币能够在以太坊生态系统中互操作。

必须实现的方法

1. totalSupply()

返回代币的总供应量。

solidity
function totalSupply() external view returns (uint256) {
    return _totalSupply;
}

2. balanceOf(address account)

返回指定账户的代币余额。

solidity
function balanceOf(address account) external view returns (uint256) {
    return _balances[account];
}

3. transfer(address recipient, uint256 amount)

从调用者账户转移代币到接收者账户。

solidity
function transfer(address recipient, uint256 amount) external returns (bool) {
    _transfer(_msgSender(), recipient, amount);
    return true;
}

4. allowance(address owner, address spender)

返回授权给spender的代币数量。

solidity
function allowance(address owner, address spender) external view returns (uint256) {
    return _allowances[owner][spender];
}

5. approve(address spender, uint256 amount)

授权spender使用调用者的代币。

solidity
function approve(address spender, uint256 amount) external returns (bool) {
    _approve(_msgSender(), spender, amount);
    return true;
}

6. transferFrom(address sender, address recipient, uint256 amount)

使用授权额度从sender账户转移代币到recipient账户。

solidity
function transferFrom(address sender, address recipient, uint256 amount) external returns (bool) {
    _transfer(sender, recipient, amount);
    uint256 currentAllowance = _allowances[sender][_msgSender()];
    require(currentAllowance >= amount, "ERC20: transfer amount exceeds allowance");
    unchecked {
        _approve(sender, _msgSender(), currentAllowance - amount);
    }
    return true;
}

可选实现的方法

1. name()

返回代币名称。

solidity
function name() external view returns (string memory) {
    return _name;
}

2. symbol()

返回代币符号。

solidity
function symbol() external view returns (string memory) {
    return _symbol;
}

3. decimals()

返回代币的小数位数，通常为18。

solidity
function decimals() external view returns (uint8) {
    return _decimals;
}

事件（Events）

1. Transfer

代币转移时触发。

solidity
event Transfer(address indexed from, address indexed to, uint256 value);

2. Approval

授权时触发。

solidity
event Approval(address indexed owner, address indexed spender, uint256 value);

完整的ERC-20实现示例

solidity
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract MyToken {
    string private _name;
    string private _symbol;
    uint8 private _decimals;
    uint256 private _totalSupply;
    mapping(address => uint256) private _balances;
    mapping(address => mapping(address => uint256)) private _allowances;

    constructor(string memory name_, string memory symbol_, uint256 initialSupply) {
        _name = name_;
        _symbol = symbol_;
        _decimals = 18;
        _totalSupply = initialSupply;
        _balances[msg.sender] = initialSupply;
        emit Transfer(address(0), msg.sender, initialSupply);
    }

    function name() external view returns (string memory) {
        return _name;
    }

    function symbol() external view returns (string memory) {
        return _symbol;
    }

    function decimals() external view returns (uint8) {
        return _decimals;
    }

    function totalSupply() external view returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address account) external view returns (uint256) {
        return _balances[account];
    }

    function transfer(address recipient, uint256 amount) external returns (bool) {
        _transfer(msg.sender, recipient, amount);
        return true;
    }

    function allowance(address owner, address spender) external view returns (uint256) {
        return _allowances[owner][spender];
    }

    function approve(address spender, uint256 amount) external returns (bool) {
        _approve(msg.sender, spender, amount);
        return true;
    }

    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool) {
        _transfer(sender, recipient, amount);
        uint256 currentAllowance = _allowances[sender][msg.sender];
        require(currentAllowance >= amount, "ERC20: transfer amount exceeds allowance");
        unchecked {
            _approve(sender, msg.sender, currentAllowance - amount);
        }
        return true;
    }

    function _transfer(address sender, address recipient, uint256 amount) internal {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");
        uint256 senderBalance = _balances[sender];
        require(senderBalance >= amount, "ERC20: transfer amount exceeds balance");
        unchecked {
            _balances[sender] = senderBalance - amount;
            _balances[recipient] += amount;
        }
        emit Transfer(sender, recipient, amount);
    }

    function _approve(address owner, address spender, uint256 amount) internal {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");
        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }
}

使用OpenZeppelin库

OpenZeppelin提供了经过审计的ERC-20实现，推荐在生产环境中使用。

solidity
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract MyToken is ERC20, Ownable {
    constructor(uint256 initialSupply) ERC20("My Token", "MTK") {
        _mint(msg.sender, initialSupply);
    }

    function mint(address to, uint256 amount) public onlyOwner {
        _mint(to, amount);
    }
}

ERC-20的应用场景

1. 去中心化金融（DeFi）

流动性提供
借贷协议
衍生品交易

2. 治理代币

DAO投票
协议治理
社区激励

3. 稳定币

USDT、USDC、DAI
法币抵押
算法稳定

4. 实用代币

平台访问权限
服务支付
奖励机制

ERC-20的局限性

1. 授权机制问题

需要两次交易（approve + transferFrom）
可能导致授权额度泄露

2. 缺乏批量操作

每次转移都需要单独交易
Gas成本较高

3. 无法处理代币回调

不支持接收代币时的通知
可能导致代币丢失

其他ERC代币标准

ERC-721

非同质化代币（NFT）标准，每个代币都是唯一的。

ERC-1155

多代币标准，支持同质化和非同质化代币。

ERC-777

改进的ERC-20标准，支持代币回调和批量操作。

ERC-4626

金库代币标准，用于DeFi收益聚合器。

最佳实践

使用OpenZeppelin库：避免重复造轮子，使用经过审计的代码
添加访问控制：实现适当的权限管理
事件日志：记录所有重要操作
安全审计：在部署前进行专业审计
测试覆盖：确保充分的测试覆盖率

ERC-20标准是以太坊生态系统的基础，理解其工作原理对于开发区块链应用至关重要。