在上一章节中完成了一个非常简单的合约，本节中将按照ERC20协议完成一个合约, 本章部分源代码参考于网络开源代码，详细了解：openzeppelin-contracts/contracts/token/ERC20 at master · OpenZeppelin/openzeppelin-contracts · GitHub。
开始之前先介绍一下什么是ERC20：
ERC-20指的是以太坊网络的一种代币合约标准。ERC-20是现在最出名的标准，ERC-20标准里无价值的差别，Token之间是能够进行互换的。意思就是在ERC-20标准下，你的100块“钱”和我的100块“钱”相同，没什么区别。ERC-20标准里规定了Token要有它的名字、符号、总供应量以及包含转账、汇款等其他功能。这个标准的优势就是：只要Token符合ERC-20标准，这样的话它就会兼容以太坊钱包。也就是说，就可以太坊钱包里加入这个Token，还能通过钱包把它发给别人。由于ERC-20标准的存在，发行Token就会更加简单。现在以太坊上ERC-20 Token的数量超过了180000种。
ERC2.0是一套接口定义，定义了合约的基本功能，其定义如下：
// SPDX-License-Identifier: MIT//file IERC20.solpragma solidity ^0.8.0;interface IERC20 {// 总发行量function totalSupply() external view returns (uint256);// 查看地址余额function balanceOf(address account) external view returns (uint256);/// 从自己帐户给指定地址转账function transfer(address account, uint256 amount) external returns (bool);// 查看被授权人还可以使用的代币余额function allowance(address owner, address spender) external view returns (uint256);// 授权指定帐户使用你拥有的代币function approve(address spender, uint256 amount) external returns (bool);// 从一个地址转账至另一个地址，该函数只能是通过approver授权的用户可以调用function transferFrom(address from,address to,uint256 amount) external returns (bool);/// 定义事件，发生代币转移时触发event Transfer(address indexed from, address indexed to, uint256 value);/// 定义事件 授权时触发event Approval(address indexed owner, address indexed spender, uint256 value);}详细说明参考代码注释。
// SPDX-License-Identifier: MIT//file IERC20Metadata.solpragma solidity ^0.8.0;import "./IERC20.sol";interface IERC20Metadata is IERC20 {// 代币名称, 如:BitCoinfunction name() external view returns (string memory);// 代币符号或简称, 如:BTCfunction symbol() external view returns (string memory);// 代币支持的小数点后位数，若无特别需求，我们一般默认采用18位。function decimals() external view returns (uint8);}MetaData数据定义，该部分比较简单，定义三个函数，分别对应代币名称，代币简称和代币小数点位数。
具体代码实现：
// SPDX-License-Identifier: MITpragma solidity ^0.8.0;import "./IERC20.sol";import "./IERC20Metadata.sol";contract ERC20 is IERC20, IERC20Metadata {// 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;// 设定代币名称符号，并初始化铸造了10000000000代币在发布者帐号下。constructor() {_name = "HarryToken";_symbol = "HYT";_mint(msg.sender, 10000000000);}function name() public view virtual override returns (string memory) {return _name;}function symbol() public view virtual override returns (string memory) {return _symbol;}/// 小数点位数一般为 18function decimals() public view virtual override returns (uint8) {return 18;}// 返回当前流通代币的总量function totalSupply() public view virtualoverride returns (uint256) {return _totalSupply;}// 查询指定帐号地址余额function balanceOf(address account) public view virtual override returns (uint256) {return _balances[account];}// 转帐功能function transfer(address to, uint256 amount) public virtual override returns (bool) {address owner = msg.sender;_transfer(owner, to, amount);return true;}// 获取被授权者可使用授权帐号的可使用余额function allowance(address owner, address spender) public view virtual override returns (uint256) {return _allowances[owner][spender];}// 授权指定帐事情可使用自己一定额度的帐户余额。// 授权spender, 可将自己余额。使用可使用的余额的总量为amountfunction approve(address spender, uint256 amount) public virtual override returns (bool) {address owner = msg.sender;_approve(owner, spender, amount);return true;}//approve函数中的spender调用，将授权人 from 帐户中的代币转入to 帐户中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 {_approve(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: approveto the zero address");_allowances[owner][spender] = amount;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 {}}// 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;ERC20合约中定义了5个变量：
_balances变量以keyv=>value方式存储帐号和其对应的余额。
_allowances变量是一个两层mapping，数据值以下结构存储：0x123456=>[0x123457=>1000, 0x123458=>2000]，代表的意思是0x123456帐号授权0x123457和0x123458两个帐号，分别可以使用0x123456帐号1000和2000余额额度。使用余额的函数为transferFrom。
_totalSupply变量是存储当成代币合约发行的代币总量，一般我们每铸造一个新代币，就在其值上+1。
_name变量是代币的名称，如比特币名称：BitCoin
_symbol变量是代币的简称, 如比特币简称：BTC
// 设定代币名称符号，并初始化铸造了10000000000代币在发布者帐号下。constructor() {_name = "HarryToken";_symbol = "HYT";_mint(msg.sender, 10000000000);}构造函数，指令name和symbol。这里我们调用了一个private的函数_mint，给合约创建者新铸造了10000000000个代币。因为本合约实现的时候并没有public的mint函数可以铸造代币，所以直接初始化入创建者帐户，该合约所有的代币都只能用创建都帐户转出。当前也可以将实现一个public的mint函数，关加上权限控制，让有权限的帐户可以随时调mint铸造新代币。
function name() public view virtual override returns (string memory) {return _name;}function symbol() public view virtual override returns (string memory) {return _symbol;}/// 小数点位数一般为 18function decimals() public view virtual override returns (uint8) {return 18;}IERC20Metadata 接口的实现方法，主要用于获取代币名称，简称及支持的小数点位数。
function totalSupply() public view virtualoverride returns (uint256) {return _totalSupply;}totalSupply查询当前代币的发行总量。
function balanceOf(address account) public view virtual override returns (uint256) {return _balances[account];}balanceOf查询指令帐户的代币余额。
// 转帐功能function transfer(address to, uint256 amount) public virtual override returns (bool) {address owner = msg.sender;_transfer(owner, to, amount);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);}transfer转帐函数，一个比较重要的功能，调用者可以将自己的余额转给其它帐户。
_transfer为private的具体实现函数。主要是作了一些必要的检查，然后从发起帐户扣减余额，再将余额加到接收帐户。最后发送了一个转帐事件，方便开发者监听转帐功能。这里的_beforeTokenTransfer和_afterTokenTransfer并没有实现具体功能，开发中可根据实现需要做一些功能实现。
// 获取被授权者可使用授权帐号的可使用余额function allowance(address owner, address spender) public view virtual override returns (uint256) {return _allowances[owner][spender];}// 授权指定帐事情可使用自己一定额度的帐户余额。// 授权spender, 可将自己余额。使用可使用的余额的总量为amountfunction approve(address spender, uint256 amount) public virtual override returns (bool) {address owner = msg.sender;_approve(owner, spender, amount);return true;}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: approveto the zero address");_allowances[owner][spender] = amount;emit Approval(owner, spender, amount);}allowance、approve主要是实现授权其它帐户可以使用自己的余额，并设定使用上限。相关的授权者存储在_allowances变量中。
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 {_approve(owner, spender, currentAllowance - substractedValue);}return true;}increaseAllowance、decreaseAllowance两个函数是对approve函数功能的加强，对授权额度进行增减，这两个函数关不是ERC20协议中的内容。只是作者在参考源码时觉得有用，就加入了这两个函数。
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 _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);}}}transferFrom，_spendAllowance是在授权额度下，进行转帐的功能实现。
transferFrom函数的from参数是授权帐户，to是余额接受帐户，amount是转帐余额，该函数的功能是将from帐户的余额转移amount个数据至to用户帐户中，调用者必须是from帐户通过_approve对其进行过授权，并且还有剩余的授权额度。该函数与transfer的区别是，transfer只能转移出调用者自己的帐户余额。
_spendAllowance是在进行授权转帐时首先扣减授权额度，保证被授权都在授权额度范围内使用转帐功能。
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);}_mint和_burn是两个相反的功能，一个是新铸造代币，一个是燃烧（销毁）代币。两个方法都是private,关没有对外开放。
完成上述三个源文件的代码就可能编译部署，选中ERC20.sol文件，完成编译。部署时要特别注意在选中Contract中的实现合约ERC20，不要误选了接口合约。
否则会部署后报如下错误，很多刚开始接触合约的读者都遇到该问题了，所以作者提示一下。
部署成功后，就可以查看并调用合约了。
一个比较健全的ERC20代币合约就完成了。下一章节给大家分享在ERC721协议下实现NTF相关的功能。