# 经典合约解读——ERC20

**Published by:** [Confucian](https://paragraph.com/@confucian-2/)
**Published on:** 2022-10-31
**URL:** https://paragraph.com/@confucian-2/erc20-2

## Content

ERC20解读参考 OpenZeppelin文档 和 以太坊官方开发者文档，结合自己的理解。什么是ERC20ERC20（Ethereum Request for Comments 20）一种同质化代币标准。EIP-20 中提出。 ERC20 代币合约追踪同质化（可替代）代币：任何一个代币都完全等同于任何其他代币；没有任何代币具有与之相关的特殊权利或行为。这使得 ERC20 代币可用于交换货币、投票权、质押等。为什么要遵守ERC20EIP-20 中的动机：允许以太坊上的任何代币被其他应用程序（从钱包到去中心化交易所）重新使用的标准接口。以太坊上的所有应用都默认支持 ERC20 ，如果你想自己发币，那么你的代码必须遵循 ERC20 标准，这样钱包（如MetaMask）等应用才能将你的币显示出来。代码实现需要实现以下函数和事件：function name() public view returns (string) function symbol() public view returns (string) function decimals() public view returns (uint8) function totalSupply() public view returns (uint256) function balanceOf(address _owner) public view returns (uint256 balance) function transfer(address _to, uint256 _value) public returns (bool success) function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) function approve(address _spender, uint256 _value) public returns (bool success) function allowance(address _owner, address _spender) public view returns (uint256 remaining) event Transfer(address indexed _from, address indexed _to, uint256 _value) event Approval(address indexed _owner, address indexed _spender, uint256 _value) 使用 OpenZeppelin 提供的库能够轻松快速地构建 ERC20 Token 。快速构建这是一个 GLD token 。// contracts/GLDToken.sol // SPDX-License-Identifier: MIT pragma solidity ^0.8.0; import "@openzeppelin/contracts/token/ERC20/ERC20.sol"; contract GLDToken is ERC20 { constructor(uint256 initialSupply) ERC20("Gold", "GLD") { _mint(msg.sender, initialSupply); } } 通常，我们定义代币的发行量和代币名称及符号。IERC20先来看下 ERC20 的接口（IERC20），这方便我们在开发中直接定义 ERC20 代币。 同样地，OpenZeppelin 为我们提供了相应的库，方便开发者导入即用。import "@openzeppelin/contracts/token/ERC20/IERC20.sol"; EIP 中定义的 ERC20 标准接口：pragma solidity ^0.8.0; interface IERC20 { event Transfer(address indexed from, address indexed to, uint256 value); event Approval(address indexed owner, address indexed spender, uint256 value); function totalSupply() external view returns (uint256); function balanceOf(address account) external view returns (uint256); function transfer(address to, uint256 amount) external returns (bool); function allowance(address owner, address spender) external view returns (uint256); function approve(address spender, uint256 amount) external returns (bool); function transferFrom( address from, address to, uint256 amount ) external returns (bool); } 逐一分析函数：totalSupply() ：返回总共的代币数量。balanceOf(address account) ：返回 account 地址拥有的代币数量。transfer(address to, uint256 amount) ：将 amount 数量的代币发送给 to 地址，返回布尔值告知是否执行成功。触发 Transfer 事件。allowance(address owner, address spender) ：返回授权花费者 spender 通过 transferFrom 代表所有者花费的剩余代币数量。默认情况下为零。当 approve 和 transferFrom 被调用时，值将改变。approve(address spender, uint256 amount) ：授权 spender 可以花费 amount 数量的代币，返回布尔值告知是否执行成功。触发 Approval 事件。transferFrom(address from, address to, uint256 amount) ：将 amount 数量的代币从 from 地址发送到 to 地址，返回布尔值告知是否执行成功。触发 Transfer 事件。事件（定义中的 indexed 便于查找过滤）：Transfer(address from, address to, uint256 value) ：当代币被一个地址转移到另一个地址时触发。注意：转移的值可能是 0 。Approval(address owner, address spender, uint256 value) ：当代币所有者授权别人使用代币时触发，即调用 approve 方法。元数据一般除了上述必须实现的函数外，还有一些别的方法：name() ：返回代币名称symbol() ：返回代币符号decimals() 返回代币小数点后位数ERC20来看下 ERC20 代币具体是怎么写的。 同样，OpenZepplin 提供了现成的合约代码：import "@openzeppelin/contracts/token/ERC20/ERC20.sol"; 这里贴一个GitHub源码链接： https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/ERC20.sol函数概览constructor(name_, symbol_) name() symbol() decimals() totalSupply() balanceOf(account) transfer(to, amount) allowance(owner, spender) approve(spender, amount) transferFrom(from, to, amount) increaseAllowance(spender, addedValue) decreaseAllowance(spender, subtractedValue) _transfer(from, to, amount) _mint(account, amount) _burn(account, amount) _approve(owner, spender, amount) _spendAllowance(owner, spender, amount) _beforeTokenTransfer(from, to, amount) _afterTokenTransfer(from, to, amount) 事件（同 IERC20）Transfer(from, to, value) Approval(owner, spender, value) 逐一分析constructor(string name, string symbol) ：设定代币的名称和符号。decimals 默认是 18 ，要修改成不同的值你应该重载它。这两个值是不变的，只在构造时赋值一次。name() ：返回代币的名称。symbol() ：返回代币的符号，通常是名称的缩写。decimals() ：返回小数点后位数，通常是 18 ，模仿 Ether 和 wei 。要更改就重写它。totalSupply()、balanceOf(address account)、transfer(address to, uint256 amount)、 allowance(address owner, address spender)、approve(address spender, uint256 amount)、transferFrom(address from, address to, uint256 amount) 都参考 IERC20 。increaseAllowance(address spender, uint256 addedValue) ：以原子的方式增加 spender 额度。返回布尔值告知是否执行成功，触发 Approval 事件。_transfer(address from, address to, uint256 amount) ：转账。这个内部函数相当于 transfer ，可以用于例如实施自动代币费用，削减机制等。触发 Transfer 事件。_mint(address account, uint256 amount) ：铸造 amount 数量的代币给 account 地址，增加总发行量。触发 Transfer 事件，其中参数 from 是零地址。_burn(address account, uint256 amount) ：从 account 地址中烧毁 amount 数量的代币，减少总发行量。触发 Transfer 事件，其中参数 to 是零地址。_approve(address owner, uint256 spender, uint256 amount) ：设定允许 spender 花费 owner 的代币数量。这个内部函数相当于 approve ，可以用于例如为某些子系统设置自动限额等。spendAllowance(address owner, address spender, uint256 amount) ：花费 amount 数量的 owner 授权 spender 的代币。在无限 allowance 的情况下不更新 allowance 金额。如果没有足够的余量，则恢复。可能触发 Approval 事件。_beforeTokenTransfer(address from, address to, uint256 amount) ：在任何代币转账前的 Hook 。它包括铸币和烧毁。调用条件：当 from 和 to 都不是零地址时，from 手里 amount 数量的代币将发送给 to 。当 from 是零地址时，将给 to 铸造 amount 数量的代币。当 to 是零地址时，from 手里 amount 数量的代币将被烧毁。from 和 to 不能同时为零地址。_afterTokenTransfer(address from, address to, uint256 amount) ：在任何代币转账后的 Hook 。它包括铸币和烧毁。调用条件：当 from 和 to 都不是零地址时，from 手里 amount 数量的代币将发送给 to 。当 from 是零地址时，将给 to 铸造 amount 数量的代币。当 to 是零地址时，from 手里 amount 数量的代币将被烧毁。from 和 to 不能同时为零地址。小结ERC20 代码中的 _transfer、_mint、_burn、_approve、_spendAllowance、_beforeTokenTransfer、_afterTokenTransfer 都是 internal 函数（其余为 public ），也就是说它们只能被派生合约调用。从零开始，自己动手1.编写IERC20// SPDX-License-Identifier: MIT pragma solidity ^0.8.0; interface IERC20 { /// @dev 总发行量 function totoalSupply() external view returns (uint256); /// @dev 查看地址余额 function balanceOf(address account) external view returns (uint256); /// @dev 单地址转账 function transfer(address account, uint256 amount) external returns (bool); /// @dev 查看被授权人代表所有者花费的代币余额 function allowance(address owner, address spender) external view returns (uint256); /// @dev 授权别人花费你拥有的代币 function approve(address spender, uint256 amount) external returns (bool); /// @dev 双地址转账 function transferFrom( address from, address to, uint256 amount ) external returns (bool); /// @dev 发生代币转移时触发 event Transfer(address indexed from, address indexed to, uint256 value); /// @dev 授权时触发 event Approval(address indexed owner, address indexed spender, uint256 value); } 2.加上Metadata// SPDX-License-Identifier: MIT pragma solidity ^0.8.0; import "IERC20.sol"; interface IERC20Metadata is IERC20 { /// @dev 代币名称 function name() external view returns (string memory); /// @dev 代币符号 function symbol() external view returns (string memory); /// @dev 小数点后位数 function decimals() external view returns (uint8); } 3.编写ERC20// SPDX-License-Identifier: MIT pragma solidity ^0.8.0; import "./IERC20.sol"; import "./IERC20Metadata.sol"; contract ERC20 is IERC20, IERC30Metadata { // 地址余额 mapping(address => uint256) private _balances; // 授权地址余额 mapping(address => mapping(address => uint256)) private _allowances; uint256 private _totalSupply; string private _name; string private _symbol; /// @dev 设定代币名称符号 constructor(string memory name_, string memory symbol_) { _name = name_; _symbol = symbol_; } function name() public view virtual override returns (string memory) { return _name; } function symbol() public view virtual override returns (string memory) { return _symbol; } /// @dev 小数点位数一般为 18 function decimals() public view virtual override returns (uint8) { return 18; } function totalSupply() public view virtual override returns (uint256) { return _totalSupply; } function balanceOf(address account) public view virtual override returns (uint256) { return _balances[account]; } function transfer(address to, uint256 amount) public virtual override returns (bool) { address owner = msg.sender; _transfer(owner, to, amount); return true; } function allowance(address owner, address spender) public view virtual override returns (uint256) { return _allowances[owner][spender]; } function approve(address spender, uint256 amount) public virtual override returns (bool) { address owner = msg.sender; _approve(owner, spender, amount); return true; } function transferFrom( address from, address to, uint256 amount ) public virtual override returns (bool) { address spender = msg.sender; _spendAllowance(from, spender, amount); _transfer(from, to, amount); return true; } function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) { address owner = msg.sender; _approve(owner, spender, _allowances[owner][spender] + addedValue); return true; } function decreaseAllowance(address spender, uint256 substractedValue) public virtual returns (bool) { address owner = msg.sender; uint256 currentAllowance = _allowances[owner][spender]; require(currentAllowance >= substractedValue, "ERC20: decreased allowance below zero"); unchecked { _approval(owner, spender, currentAllowance - substractedValue); } return true; } function _transfer( address from, address to, uint256 amount ) internal virtual { require(from != address(0), "ERC20: transfer from the zero address"); require(to != address(0), "ERC20: transfer to the zero address"); _beforeTokenTransfer(from, to, amount); uint256 fromBalance = _balances[from]; require(fromBalance >= amount, "ERC20: transfer amount exceeds balance"); unchecked { _balances[from] = fromBalance - amount; } _balances[to] += amount; emit Transfer(from, to, amount); _afterTokenTransfer(from, to, amount); } function _mint(address account, uint256 amount) internal virtual { require(account != address(0), "ERC20: mint to the zero address"); _beforeTokenTransfer(address(0), account, amount); _totalSupply += amount; _balances[account] += amount; emit Transfer(address(0), account, amount); _afterTokenTransfer(address(0), account, amount); } function _burn(address account, uint256 amount) internal virtual { require(account != address(0), "ERC20: burn from the zero address"); _beforeTokenTransfer(account, address(0), amount); uint256 accountBalance = _balances[account]; require(accountBalance >= amount, "ERC20: burn amount exceeds balance"); unchecked { _balances[account] = accountBalance - amount; } _totalSupply -= amount; emit Transfer(account, address(0), amount); _afterTokenTransfer(account, address(0), amount); } function _approve( address owner, address spender, uint256 amount ) internal virtual { require(owner != address(0), "ERC20: approve from the zero address"); require(spender != address(0), "ERC20: approve to the zero address"); _allowances[owner][spender]; emit Approval(owner, spender, amount); } function _spendAllowance( address owner, address spender, uint256 amount ) internal virtual { uint256 currentAllowance = allowance(owner, spender); if (currentAllowance != type(uint256).max) { require(currentAllowance >= amount, "ERC20: insufficient allowance"); unchecked { _approve(owner, spender, currentAllowance - amount); } } } function _beforeTokenTransfer( address from, address to, uint256 amount ) internal virtual {} function _afterTokenTransfer( address from, address to, uint256 amount ) internal virtual {} } 总结ERC20 其实就是一种最常见的代币标准，它明确了同质化代币的经典功能并规范了开发者编写 token 时的代码，从而方便各种应用适配。

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