ERC-20 标准以及代码解读

2021-09-06 16

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?

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。
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函数approve(B,100)
  • 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。

ERC-20代码样板解读

USDT是ERC20代币里面使用场景最为广泛稳定币, 下面是其代码样板。 我们会发现这个里面除了上面提到的ERC20的标准之外, 我们看能看到下面几个:

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