ERC-20 标准以及代码解读
什么是ERC20
据相关的数据显示，现在主流的代币基本上80%以上都是基于ERC20发行的。究竟什么是ERC20呢？为什么要基于ERC20发行代币呢？
可以把ERC20简单理解成以太坊上的一个代币协议，对于开发的同学来说，可以把协议理解为一种接口或者模板。遵守这些协议的代币我们可以认为是标准化的代币，而标准化带来的好处是兼容性好。这些标准化的代币可以被各种以太坊钱包支持，用于不同的平台和项目。比如现在很多的中心化以及去中心化交易所，他们接入的token都要求是ERC20的代币。 非标准化的代币无法接入。以太坊上运用最广泛的钱包metamask上面， 如果需要添加展示某个代币的话也要求这个token是标准代币。 比如下面是以太坊浏览器etherscan，在上面就可以列出排名靠前的erc20代币。
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所以如果你要是想在以太坊上发行代币融资流通，必须要遵守ERC20标准。
ERC20的标准接口
contract ERC20 {
function name() constant returns (string name)
function symbol() constant returns (string symbol)
function decimals() constant returns (uint8 decimals)
function totalSupply() constant returns (uint totalSupply);
function balanceOf(address _owner) constant returns (uint balance);
function transfer(address _to, uint _value) returns (bool success);
function transferFrom(address _from, address _to, uint _value) returns (bool success);
function approve(address _spender, uint _value) returns (bool success);
function allowance(address _owner, address _spender) constant returns (uint remaining);
event Transfer(address indexed _from, address indexed _to, uint _value);
event Approval(address indexed _owner, address indexed _spender, uint _value);
}
1. name
返回string类型的ERC20代币的名字，例如：Mytrade。
2. symbol
返回string类型的ERC20代币的符号，也就是代币的简称，例如：MYT。
3. decimals
支持几位小数点后几位。如果设置为3。也就是支持0.001表示。在智能合约里面是没有小数点的数据类型的，代币的数量是通过uint256来存储，这个decimal代表的是uint256其中有几位是代表小数。
4. totalSupply
发行代币的总量，可以通过这个函数来获取。所有智能合约发行的代币总量是一定的，totalSupply必须设置初始值。如果不设置初始值，这个代币发行就说明有问题。
5. balanceOf
输入地址，可以获取该地址代币的余额。
6. transfer
调用transfer函数将自己的token转账给_to地址，_value为转账个数。
7. approve
批准_spender账户从自己的账户转移_value个token。可以分多次转移。
8. transferFrom
与approve搭配使用，approve批准之后，调用transferFrom函数来转移token。
9. allowance
返回_spender还能提取token的个数。
approve、transferFrom及allowance解释：
approve是授权第三方（比如某个服务合约）从发送者账户转移代币，然后通过 transferFrom() 函数来执行具体的转移操作。
账户A有1000个ETH，想允许B账户随意调用他的100个ETH，过程如下：
- A账户按照以下形式调用approve
- B账户想用这100个ETH中的10个ETH给C账户，调用transferFrom(A, C, 10)；
- 调用allowance(A, B)可以查看B账户还能够调用A账户多少个token。
SafeMath类库
以太坊虚拟机（EVM）为整数指定固定大小的数据类型。这意味着一个整型变量只能有一定范围的数字表示。例如，一个 uint8 ，只能存储在范围 [0,255] 的数字。试图存储 256 到一个 uint8 将变成 0。不加注意的话，只要没有检查用户输入又执行计算，导致数字超出存储它们的数据类型允许的范围，Solidity 中的变量就可以被用来组织攻击。整数溢出的类型包括乘法溢出，加法溢出，减法溢出三种。所以SafeMath可以用来防止溢出。
Ownable基类
这个基类主要定义了Token是否有个所有者， 并且定义了所有者转移的方法。
/**
*Submitted for verification at Etherscan.io on 2017-11-28
*/pragma solidity ^0.4.17;
/**
* @title SafeMath
* @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) {
// 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
* @dev The Ownable contract has an owner address, and provides basic authorization control
* functions, this simplifies the implementation of "user permissions".
*/
contract Ownable {
address public owner;
/**
* @dev The Ownable constructor sets the original `owner` of the contract to the sender
* account.
*/
function Ownable() public {
owner = msg.sender;
}/**
* @dev Throws if called by any account other than the owner.
*/
modifier onlyOwner() {
require(msg.sender == owner);
_;
}/**
* @dev Allows the current owner to transfer control of the contract to a newOwner.
* @param newOwner The address to transfer ownership to.
*/
function transferOwnership(address newOwner) public onlyOwner {
if (newOwner != address(0)) {
owner = newOwner;
}
}}/**
* @title ERC20Basic
* @dev Simpler version of ERC20 interface
* @dev see https://github.com/ethereum/EIPs/issues/20
*/
contract ERC20Basic {
uint public _totalSupply;
function totalSupply() public constant returns (uint);
function balanceOf(address who) public constant returns (uint);
function transfer(address to, uint value) public;
event Transfer(address indexed from, address indexed to, uint value);
}/**
* @title ERC20 interface
* @dev see https://github.com/ethereum/EIPs/issues/20
*/
contract ERC20 is ERC20Basic {
function allowance(address owner, address spender) public constant returns (uint);
function transferFrom(address from, address to, uint value) public;
function approve(address spender, uint value) public;
event Approval(address indexed owner, address indexed spender, uint value);
}/**
* @title Basic token
* @dev Basic version of StandardToken, with no allowances.
*/
contract BasicToken is Ownable, ERC20Basic {
using SafeMath for uint;
mapping(address => uint) public balances;
// additional variables for use if transaction fees ever became necessary
uint public basisPointsRate = 0;
uint public maximumFee = 0;
/**
* @dev Fix for the ERC20 short address attack.
*/
modifier onlyPayloadSize(uint size) {
require(!(msg.data.length < size + 4));
_;
}/**
* @dev transfer token for a specified address
* @param _to The address to transfer to.
* @param _value The amount to be transferred.
*/
function transfer(address _to, uint _value) public onlyPayloadSize(2 * 32) {
uint fee = (_value.mul(basisPointsRate)).div(10000);
if (fee > maximumFee) {
fee = maximumFee;
}
uint sendAmount = _value.sub(fee);
balances[msg.sender] = balances[msg.sender].sub(_value);
balances[_to] = balances[_to].add(sendAmount);
if (fee > 0) {
balances[owner] = balances[owner].add(fee);
Transfer(msg.sender, owner, fee);
}
Transfer(msg.sender, _to, sendAmount);
}/**
* @dev Gets the balance of the specified address.
* @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 constant returns (uint balance) {
return balances[_owner];
}}/**
* @title Standard ERC20 token
*
* @dev Implementation of the basic standard token.
* @dev https://github.com/ethereum/EIPs/issues/20
* @dev Based oncode by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
*/
contract StandardToken is BasicToken, ERC20 {mapping (address => mapping (address => uint)) public allowed;
uint public constant MAX_UINT = 2**256 - 1;
/**
* @dev Transfer tokens from one address to another
* @param _from address The address which you want to send tokens from
* @param _to address The address which you want to transfer to
* @param _value uint the amount of tokens to be transferred
*/
function transferFrom(address _from, address _to, uint _value) public onlyPayloadSize(3 * 32) {
var _allowance = allowed[_from][msg.sender];
// Check is not needed because sub(_allowance, _value) will already throw if this condition is not met
// if (_value > _allowance) throw;
uint fee = (_value.mul(basisPointsRate)).div(10000);
if (fee > maximumFee) {
fee = maximumFee;
}
if (_allowance < MAX_UINT) {
allowed[_from][msg.sender] = _allowance.sub(_value);
}
uint sendAmount = _value.sub(fee);
balances[_from] = balances[_from].sub(_value);
balances[_to] = balances[_to].add(sendAmount);
if (fee > 0) {
balances[owner] = balances[owner].add(fee);
Transfer(_from, owner, fee);
}
Transfer(_from, _to, sendAmount);
}/**
* @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
* @param _spender The address which will spend the funds.
* @param _value The amount of tokens to be spent.
*/
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
require(!((_value != 0) && (allowed[msg.sender][_spender] != 0)));
allowed[msg.sender][_spender] = _value;
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.
* @param _spender address The address which will spend the funds.
* @return A uint specifying the amount of tokens still available for the spender.
*/
function allowance(address _owner, address _spender) public constant returns (uint remaining) {
return allowed[_owner][_spender];
}}/**
* @title Pausable
* @dev Base contract which allows children to implement an emergency stop mechanism.
*/
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.
*/
modifier whenNotPaused() {
require(!paused);
_;
}/**
* @dev Modifier to make a function callable only when the contract is paused.
*/
modifier whenPaused() {
require(paused);
_;
}/**
* @dev called by the owner to pause, triggers stopped state
*/
function pause() onlyOwner whenNotPaused public {
paused = true;
Pause();
}/**
* @dev called by the owner to unpause, returns to normal state
*/
function unpause() onlyOwner whenPaused public {
paused = false;
Unpause();
}
}contract BlackList is Ownable, BasicToken {/////// Getters to allow the same blacklist to be used also by other contracts (including upgraded Tether) ///////
function getBlackListStatus(address _maker) external constant returns (bool) {
return isBlackListed[_maker];
}function getOwner() external constant returns (address) {
return owner;
}mapping (address => bool) public isBlackListed;
function addBlackList (address _evilUser) public onlyOwner {
isBlackListed[_evilUser] = true;
AddedBlackList(_evilUser);
}function removeBlackList (address _clearedUser) public onlyOwner {
isBlackListed[_clearedUser] = false;
RemovedBlackList(_clearedUser);
}function destroyBlackFunds (address _blackListedUser) public onlyOwner {
require(isBlackListed[_blackListedUser]);
uint dirtyFunds = balanceOf(_blackListedUser);
balances[_blackListedUser] = 0;
_totalSupply -= dirtyFunds;
DestroyedBlackFunds(_blackListedUser, dirtyFunds);
}event DestroyedBlackFunds(address _blackListedUser, uint _balance);
event AddedBlackList(address _user);
event RemovedBlackList(address _user);
}contract UpgradedStandardToken is StandardToken{
// those methods are called by the legacy contract
// and they must ensure msg.sender to be the contract address
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;
}contract TetherToken is Pausable, StandardToken, BlackList {string public name;
string public symbol;
uint public decimals;
address public upgradedAddress;
bool public deprecated;
//The contract can be initialized with a number of tokens
//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
function TetherToken(uint _initialSupply, string _name, string _symbol, uint _decimals) public {
_totalSupply = _initialSupply;
name = _name;
symbol = _symbol;
decimals = _decimals;
balances[owner] = _initialSupply;
deprecated = false;
}// Forward ERC20 methods to upgraded contract if this one is deprecated
function transfer(address _to, uint _value) public whenNotPaused {
require(!isBlackListed[msg.sender]);
if (deprecated) {
return UpgradedStandardToken(upgradedAddress).transferByLegacy(msg.sender, _to, _value);
} else {
return super.transfer(_to, _value);
}
}// Forward ERC20 methods to upgraded contract if this one is deprecated
function transferFrom(address _from, address _to, uint _value) public whenNotPaused {
require(!isBlackListed[_from]);
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
function balanceOf(address who) public constant 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 onlyPayloadSize(2 * 32) {
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 constant returns (uint remaining) {
if (deprecated) {
return StandardToken(upgradedAddress).allowance(_owner, _spender);
} else {
return super.allowance(_owner, _spender);
}
}// deprecate current contract in favour of a new one
function deprecate(address _upgradedAddress) public onlyOwner {
deprecated = true;
upgradedAddress = _upgradedAddress;
Deprecate(_upgradedAddress);
}// deprecate current contract if favour of a new one
function totalSupply() public constant returns (uint) {
if (deprecated) {
return StandardToken(upgradedAddress).totalSupply();
} else {
return _totalSupply;
}
}// Issue a new amount of tokens
// these tokens are deposited into the owner address
//
// @param _amount Number of tokens to be issued
function issue(uint amount) public onlyOwner {
require(_totalSupply + amount > _totalSupply);
require(balances[owner] + amount > balances[owner]);
balances[owner] += amount;
_totalSupply += amount;
Issue(amount);
}// Redeem tokens.
// These tokens are withdrawn from the owner address
// if the balance must be enough to cover the redeem
// or the call will fail.
// @param _amount Number of tokens to be issued
function redeem(uint amount) public onlyOwner {
require(_totalSupply >= amount);
require(balances[owner] >= amount);
_totalSupply -= amount;
balances[owner] -= amount;
Redeem(amount);
}function setParams(uint newBasisPoints, uint newMaxFee) public onlyOwner {
// Ensure transparency by hardcoding limit beyond which fees can never be added
require(newBasisPoints < 20);
require(newMaxFee < 50);
basisPointsRate = newBasisPoints;
maximumFee = newMaxFee.mul(10**decimals);
Params(basisPointsRate, maximumFee);
}// 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);
}作者：Bernie G、Time
原文链接：http://blockgeek.com/b748/82f2
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【ERC-20 标准以及代码解读】
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