ERC20代币合约规则简介
ERC20 是各个代币的标准接口。ERC20 代币仅仅是以太坊代币的子集，为了充分兼容 ERC20，开发者需要将一组特定的函数（接口）集成到他们的智能合约中，以便在高层面能够执行这些操作：获得代币总供应量、获得账户余额、转让代币、批准花费代币。
ERC20 让以太坊区块链上的其他智能合约和去中心化应用之间无缝交互。一些具有部分但非所有ERC20标准功能的代币被认为是部分ERC20兼容，这还要视其具体缺失的功能而定，但总体是它们仍然很容易与外部交互。
简单的来说：ERC20接口就是一套在智能合约上开发Token(代币)的规范协议。
需要说明的是：代币是运行在智能合约上的币，其没有自己的私链，交易代币需要调用函数而不是在以太坊上真正的发起一个交易，可以看做以太坊账户的附属品。每个账户可能在多个合约代币中有贮藏代币。
ERC20协议的详细讲解可见尚硅谷视频：https://www.bilibili.com/video/BV1NJ411D7rf?p=34
USDT代币
现在市值较大的代币都基本支持ERC20接口规范，ERC20接口规范构造简单。
本文主要通过研究目前在以太坊上运行的市值最高的USDT代币，以此来了解ERC20这一代币规范的使用。
USDT代币的合约架构
USDT代币功能
- ERC20要求下的转账、余额查询、授权消费功能
- 代币紧急暂停与重起
- 代币用户黑名单
- 合约便捷升级，可适应非ERC20协议代币
- 代币管理者权限移交
USDT代币合约源码
对着源码撸了一遍，做了略微改动（适应编译器版本）以及详细的内容说明。
适应版本： 0.4.22版本编译器无warning无error
pragma solidity >0.4.17;
/**
* @title SafeMath 数学安全函数
* @dev Math operations with safety checks that throw on error.
*/
library SafeMath {
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
if (a == 0) {
return 0;
}
uint256 c = a * b;
assert(c / a == b);
return c;
}
function div(uint256 a, uint256 b) internal pure returns (uint256) {
// 分母大于0在solidity合约中已经会自动判定了
// assert(b > 0); // Solidity automatically throws when dividing by 0
uint256 c = a / b;
// assert(a == b * c + a % b); // There is no case in which this doesn't hold
return c;
}
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
assert(b <= a);
return a - b;
}
function add(uint256 a, uint256 b) internal pure returns (uint256) {
uint256 c = a + b;
assert(c >= a);
return c;
}
}
/**
* @title Ownable 代币的拥有者
* @dev The Ownable contract has an owner address, and provides basic authorization control.
* @dev functions, this simplifies the implementation of "user permissions".
* @dev 这个合约主要是指明合约创建人为代币的创建者，还包括授权控制功能，简化“用户权限”.
*/
contract Ownable{
//"拥有者"
address public owner;
/**
* @dev The Ownable constructor sets the original `owner` of the contract to the sender
* account.
* @dev 把创建合约的人作为初始的“拥有者”.
*/
constructor() public{
owner = msg.sender;
}
/**
* @dev Throws if called by any account other than the owner.
* @dev 暂时未知, 应该是只能拥有者进行的操作.
*/
modifier onlyOwner(){
require(msg.sender == owner, "仅owner调用！");
//这一行表示继承此合约中使用
_;
}
/**
* @dev Allows the current owner to transfer control of the contract to a newOwner.
* @dev 权力转移给新的拥有者
* @param newOwner The address to transfer ownership to.
*/
function transferOwnership(address newOwner) public onlyOwner{
//先确保新用户不是0x0地址
require(newOwner != address(0), "不能给地址0转移owner");
owner = newOwner;
}
}
/**
* @title ERC20Basic 基于REC20，不是直接继承，而是类似的代码
* @dev Simpler version of ERC20 interface 对于ERC20标准接口的简化版本
* @dev see https://github.com/ethereum/EIPs/issues/20
* @dev 新版本的编译器0.6.1要求抽象合约前要加abstract，并且抽象函数要加上virtual
*/
contract ERC20Basic{
//定义接口的一系列函数
uint public _totalSupply;//总发行货币量
function totalSupply() public view returns(uint);//查看总货币量函数
function balanceOf(address who) public view returns(uint);//查某人余额
function transfer(address to, uint value) public;//转账交易函数
event Transfer(address indexed from, address indexed to, uint value);//定义转账记录事件
}
/**
* @title ERC20 interface
* @dev see https://github.com/ethereum/EIPs/issues/20
* @dev 继承与上面的接口
*/
contract ERC20 is ERC20Basic{
//拓展了第三方授权功能
//授权给别人用自己的钱，返回钱数？
function allowance(address owner, address spender) public view returns(uint);
//借助谁（from）向谁（to）转币
function transferFrom(address from, address to, uint value) public;
//授权使用额度函数
function approve(address spender, uint value) public;
//记录授权
event Approval(address indexed owner, address indexed spender, uint value);
}
/**
* @title Basic token 基础代币
* @dev Basic version of StandardToken, with no allowances.
* @dev 仅实现代币基本功能（没有第三方授权）
*
*/
contract BasicToken is Ownable, ERC20Basic{
//使用安全数学函数
using SafeMath for uint;
mapping(address => uint) public balances;
// additional variables for use if transaction fees ever became necessary
// 如果有必要收取交易费用，可使用其他变量
uint public basisPointsRate = 0; //基本利率
uint public maximunFee = 0; //最大利息金额
/**
* @dev Fix for the ERC20 short address attack. 防止短地址攻击，具体可看博客ERC20文章
* @dev 凡是涉及转账交易（合约调用）都需要加上这一限制
*/
modifier onlyPayloadSize(uint size){
//msg.data就是data域（calldata）中的内容，一般来说都是4（函数名）+32（转账地址）+32（转账金额）=68字节
//短地址攻击简单来说就是转账地址后面为0但故意缺省，导致金额32字节前面的0被当做地址而后面自动补0导致转账金额激增。
//参数size就是除函数名外的剩下字节数
//解决方法：对后面的的字节数的长度限制要求
require(!(msg.data.length < size+4), "Invalid short address");
_;
}
/**
* @dev transfer token for a specified address 转给一个符合规定（非短地址）的地址
* @param _to The address to transfer to. 转账地址
* @param _value The amount to be transferred. 转账金额
*/
function transfer(address _to, uint _value) public onlyPayloadSize(2 * 32){
//先算利息: （转账金额*基本利率)/10000 (ps:因为浮点会精度缺失，所以这样计算)
uint fee = (_value.mul(basisPointsRate)).div(10000);
//判断是否超最大额
if (fee > maximunFee) fee = maximunFee;
//计算剩下的钱
uint sendAmount = _value.sub(fee);
//转账的钱要够 源码没加这个判断不知为何？
//不需要检查，因为后面balances[msg.sender].sub(sendAmount)其中会检查，不够会报异常。
//require(balances[msg.sender] >= _value);
//有安全数学函数就不用判断溢出了
//扣钱
balances[msg.sender] = balances[msg.sender].sub(sendAmount);
//加钱
balances[_to] = balances[_to].add(sendAmount);
//利息去向->owner
if (fee > 0){
//因为继承于Ownable，所以可以拿到owner
balances[owner] = balances[owner].add(fee);
//继承于ERCBasic接口，其中申明了Transfer记录
//记录利息去向
emit Transfer(msg.sender, owner, fee);
}
//记录转账去向,注意记录的不是总金额而是去除交易费的金额
emit Transfer(msg.sender, _to, sendAmount);
}
/**
* @dev Gets the balance of the specified address. 查余额函数
* @param _owner The address to query the the balance of.
* @return An uint representing the amount owned by the passed address.
*/
function balanceOf(address _owner) public view returns(uint balance){
return balances[_owner];
}
}
/**
* @title Standard ERC20 token ERC20标准代币
*
* @dev Implementation of the basic standard token. 依据基本代币准则
* @dev https://github.com/ethereum/EIPs/issues/20
* @dev Based oncode by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
* @dev 借鉴了firstblood代币
* @dev 对代币基础功能的拓展-> 添加了第三方授权功能
*/
contract StandardToken is BasicToken, ERC20{
//授权金额映射：某人对其他所有人授权的金额的映射
mapping(address => mapping(address => uint)) public allowed;
//uint最大值
uint public constant MAX_UINT = 2**256-1;
/**
* @dev Transfer tokens from one address to another 授权转账：从一个账户转到另一个账户
* @param _from address The address which you want to send tokens from 已得到授权的账户
* @param _to address The address which you want to transfer to 转向的账户
* @param _value uint the amount of tokens to be transferred 转账金额
*/
function transferFrom(address _from, address _to, uint _value) public onlyPayloadSize(2 * 32){
//授权金额：授权者对于当前调用者授权其可使用的金额量
uint _allowance = allowed[_from][msg.sender];
//在这里同样不需要检查授权金额是否足够,后面的sub函数这种情况会检测
// require(_allowance >= _value);
//1.先算利息
uint fee = (_value.mul(basisPointsRate)).div(10000);
if (fee > maximunFee) fee = maximunFee;
//2.扣钱
// 这里为什么要判断？
if (_allowance < MAX_UINT){
//注意这里扣去的是总金额，包括了利息都要从授权方的授权金额去除
allowed[_from][msg.sender] = _allowance.sub(_value);
}
balances[_from] = balances[_from].sub(_value);
//3.加钱
uint sendAmount = _value.sub(fee);
balances[_to] = balances[_to].add(sendAmount);
//4.利息去向
if (fee > 0){
balances[owner] = balances[owner].add(fee);
emit Transfer(_from, owner, fee);
}
//5.记录
emit Transfer(_from, _to, sendAmount);
}
/**
* @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
* @dev 调用者授权给他人可使用金额
* @param _spender The address which will spend the funds. 被授权者
* @param _value The amount of tokens to be spent. 金额
*/
function approve(address _spender, uint _value) public onlyPayloadSize(2 * 32){
// To change the approve amount you first have to reduce the addresses`
// allowance to zero by calling `approve(_spender, 0)` if it is not
// already 0 to mitigate the race condition described here:
// https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
//这里的限定条件是：不能将已设置过的授权金额改动，除非改为0。
//也就是说对他人的授权金额只能是从0改为value,这一次机会，再改就只能改回到0
require(!(_value != 0 && allowed[msg.sender][_spender] != 0), "You have only one chance to approve , you can only change it to 0 later");
//1.改allowed
allowed[msg.sender][_spender] = _value;
//2. 记录
emit Approval(msg.sender, _spender, _value);
}
/**
* @dev Function to check the amount of tokens than an owner allowed to a spender.
* @param _owner address The address which owns the funds. 地址拥有资金的地址。
* @param _spender address The address which will spend the funds. 查看授权了多少钱
* @return A uint specifying the amount of tokens still available for the spender.
*/
function allowance(address _owner, address _spender) public view returns(uint remaining){
return allowed[_owner][_spender];
}
}
/**
* @title Pausable 中断
* @dev Base contract which allows children to implement an emergency stop mechanism.
* @dev 实现紧急停止机制
*/
contract Pausable is Ownable{
event Pause();
event Unpause();
bool public paused = false;
/**
* @dev Modifier to make a function callable only when the contract is not paused.
* @dev 限制条件：函数只能是在合约未停止情况下执行.
*/
modifier whenNotPaused(){
require(!paused, "Must be used without pausing");
_;
}
/**
* @dev Modifier to make a function callable only when the contract is paused.
* @dev 函数只能在停止条件下执行
*/
modifier whenPaused(){
require(paused, "Must be used under pause");
_;
}
/**
* @dev called by the owner to pause, triggers stopped state
* @dev 只能由代币管理者进行停止
*
*/
function pause() public onlyOwner whenNotPaused {
paused = true;
emit Pause();
}
/**
* @dev called by the owner to unpause, returns to normal state
* @dev 只能是代币管理者进行重开
*/
function unpause() public onlyOwner whenPaused{
paused = false;
emit Unpause();
}
}
/**
* @dev 列黑名单
*/
contract BlackList is Ownable, BasicToken{
//黑名单映射
mapping(address => bool) isBlackListed;
//事件
event DestroyedBlackFunds(address _blackListedUser, uint _balance);
event AddedBlackList(address _user);
event RemovedBlackList(address _user);
//Getters to allow the same blacklist to be used also by other contracts (including upgraded Tether)
//允许其他合约调用此黑名单(external)，查看此人是否被列入黑名单
function getBlackListStatus(address _maker) external view returns(bool){
return isBlackListed[_maker];
}
//获取当前代币的Owner
function getOwner() external view returns(address){
return owner;
}
//增加黑名单
function addBlackList(address _evilUser) public onlyOwner{
isBlackListed[_evilUser] = true;
emit AddedBlackList(_evilUser);
}
//去除某人黑名单
function removeBlackList(address _clearUser) public onlyOwner{
isBlackListed[_clearUser] = false;
emit RemovedBlackList(_clearUser);
}
//去除掉黑名单账户的钱
function destroyBlackFunds(address _blackListUser) public onlyOwner{
//1. 检查是否在黑名单
require(isBlackListed[_blackListUser], "You can only clear the money of users in the blacklist");
//2. 查看要清除的钱
uint dirtyFunds = balanceOf(_blackListUser);
//3. 扣除清零
balances[_blackListUser] = 0;
//4. 总代币发行量减少
_totalSupply = _totalSupply.sub(dirtyFunds);
//5. 记录
emit DestroyedBlackFunds(_blackListUser, dirtyFunds);
}
}
//标准代币拓展(为了适应不支持ERC20的情况或者是拓展)
contract UpgradedStandardToken is StandardToken{
// those methods are called by the legacy contract
// and they must ensure msg.sender to be the contract address
// 这些拓展方法都是来自遗留合同
// 并且合约调用者必须是合约地址
function transferByLegacy(address from, address to, uint value) public;
function transferFromByLegacy(address sender, address from, address spender, uint value) public;
function approveByLegacy(address from, address spender, uint value) public;
}
//主体代币
contract TetherToken is Pausable, StandardToken, BlackList{
string public name; //代币名
string public symbol; //标志
uint public decimals; //精度/小数点后几位
address public upgradedAddress; //升级合约的地址（必须是合约地址）
bool public deprecated; //弃用（支持ERC20与否）
// The contract can be initialized with a number of tokens 可初始化多个代币
// All the tokens are deposited to the owner address
//
// @param _balance Initial supply of the contract
// @param _name Token Name
// @param _symbol Token symbol
// @param _decimals Token decimals
constructor(
uint _initialSupply,
string _name,
string _symbol,
uint _decimals
) public {
//总发行币都给owner
_totalSupply = _initialSupply;
balances[owner] = _initialSupply;
name = _name;
symbol = _symbol;
decimals = _decimals;
deprecated = false;
}
// Called when new token are issued
event Issue(uint amount);
// Called when tokens are redeemed
event Redeem(uint amount);
// Called when contract is deprecated
event Deprecate(address newAddress);
// Called if contract ever adds fees
event Params(uint feeBasisPoints, uint maxFee);
// Forward ERC20 methods to upgraded contract if this one is deprecated
//如果不推荐使用ERC20方法，则将其转为升级的合同
function transfer(address _to, uint _value) public whenNotPaused{
//排除黑名单
require(!isBlackListed[msg.sender], "The account you applied for is on the blacklist and cannot be transferred");
//判断是否支持ERC20
if(deprecated){
//不支持的话就调用用upgradedAddress实例化的对象的transferByLegacy函数
//不知道这里为什么要传msg.sender？
//我猜测是重新升级适配函数的话调用此函数的人(msg.sender)也要转过去
return UpgradedStandardToken(upgradedAddress).transferByLegacy(msg.sender, _to, _value);
}else{
//支持的话就直接调用ERC20的转账
//这里没有返回值，不知道为什么还要加return
return super.transfer(_to, _value);
}
}
//同理：
// Forward ERC20 methods to upgraded contract if this one is deprecated
function transferFrom(address _from, address _to, uint _value) public whenNotPaused{
require(!isBlackListed[_from], "The account you applied for is on the blacklist and cannot be transferred");
if(deprecated){
return UpgradedStandardToken(upgradedAddress).transferFromByLegacy(msg.sender, _from, _to, _value);
}else{
return super.transferFrom(_from, _to, _value);
}
}
// Forward ERC20 methods to upgraded contract if this one is deprecated
//注意这里查询余额在代币暂停的情况下也是可以使用的
function balanceOf(address who) public view returns(uint){
if(deprecated){
return UpgradedStandardToken(upgradedAddress).balanceOf(who);
}else{
return super.balanceOf(who);
}
}
// Forward ERC20 methods to upgraded contract if this one is deprecated
function approve(address _spender, uint _value) public whenNotPaused{
//这里不用检查了，如果是在黑名单中，那么授权再多也没用，transferFrom的时候就检测出来了
if(deprecated){
return UpgradedStandardToken(upgradedAddress).approveByLegacy(msg.sender, _spender, _value);
}else{
return super.approve(_spender, _value);
}
}
// Forward ERC20 methods to upgraded contract if this one is deprecated
function allowance(address _owner, address _spender) public view returns(uint){
if(deprecated){
return UpgradedStandardToken(upgradedAddress).allowance(_owner, _spender);
}else{
return super.allowance(_owner, _spender);
}
}
// deprecate current contract in favour of a new one
//反对现行合同，改用新合同. upgradedAddress新合同地址
function deprecate(address _upgradedAddress) public onlyOwner{
deprecated = true;
upgradedAddress = _upgradedAddress;
//记录
emit Deprecate(_upgradedAddress);
}
// deprecate current contract if favour of a new one
//反对现行合同，如果想换一个新合约,需要提前知道当前发行量
function totalSupply() public view returns(uint){
if(deprecated){
return UpgradedStandardToken(upgradedAddress).totalSupply();
}else{
return _totalSupply;
}
}
// Issue a new amount of tokens 发行新数量的代币
// these tokens are deposited into the owner address
//
// @param _amount Number of tokens to be issued
function issue(uint _amount) public onlyOwner{
//增加拥有者的量
balances[owner] = balances[owner].add(_amount);
//增加发行的总代币量
_totalSupply = _totalSupply.add(_amount);
//记录
emit Issue(_amount);
}
// 调整利息率和最大利息限制
function setParams(uint newBasisPoints, uint newMaxFee) public onlyOwner{
// Ensure transparency by hardcoding limit beyond which fees can never be added
//通过硬编码限制来确保透明度，超过这个限度就不能再增加费用了
require(newBasisPoints < 20, "The new BasisPoints cannot exceed 20"); //0.002
require(newMaxFee < 50, "The new MaxFee cannot exceed 50"); //5*10**(decimals+1)
basisPointsRate = newBasisPoints;
maximunFee = newMaxFee.mul(10**decimals);
//记录
emit Params(newBasisPoints, newMaxFee);
}
}
代币运行效果（功能测试）
测试账户：
- 0xca35b7d915458ef540ade6068dfe2f44e8fa733c(账户一733c)
- 0x14723a09acff6d2a60dcdf7aa4aff308fddc160c(账户二160c)
-
发布：
发布需要注意的是因为合约内容较多，gaslimit要设置的大一些，默认的值可能不够用。
消耗gas的操作函数：
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view或public参数查询：
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合约发布者就是当前的owner：
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-
transfer 转账
代币发行总量（部署时已设置）：
这些代币都会初始化在owner账户上，由其转发给其他账户。
两种查询余额的方式：
账户一转账给账户二：
注意：此时应该是账户一发起交易调用。
余额变动：
-
transferFrom 授权转账
先查看授权额度：
账户一对账户二的授权默认都是0：
设置授权额度：
再次查询：
授权额度的使用：
登录账户二：
转788给自己，之前的余额是999
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余额变化：
现在的额度还剩：
100
还剩100额度不够花了，能否直接增加授权额度？试试
不能。授权额度只能在第一次设置，设置后只能再设置回0再设置新的额度！
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转移owner
此功能只能是当前的owner调用
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owner是账户二了
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紧急暂停合约（暂停所有交易，但是查询还是可以）
此功能只能是当前的owner调用
无参数：
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交易无法实现：
恢复同理不再演示
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黑名单功能
此功能只能是当前的owner调用
把账户一设置为黑名单
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被列为黑名单后是无法交易的。
删除黑名单中人的钱（销毁）
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总量少了：
去除黑名单人员：
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新发行代币
此功能只能是当前的owner调用
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这些币同样的会到当前owner账户中
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设置交易利息率和最高利息
此功能只能是当前的owner调用
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因为精度设置的是16
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升级代币合约
此功能只能是当前的owner调用
填入新的合约地址，那么这些操作函数被调用时会执行新合约地址中的函数。
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