はじめに（Introduction）
実際に使われているトークン（ERC20）のコントラクトコードを読んでみようと思います。
有名なUSDTを見てみようと思います。
2014 年に開始された Tether トークン (USD₮) は、ステーブルコイン モデルの先駆者であり、最も広く取引されています。Tether トークンは、法定通貨の安定性とシンプルさ、そしてブロックチェーン テクノロジーの革新性を兼ね備えており、両方の長所を完璧に組み合わせています。
USDT
USDTはEtherscanにverificationされているので以下にSolidityのソースコードが公開されています。
コード全文
/**
*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() public onlyOwner whenNotPaused {
paused = true;
Pause();
}
/**
* @dev called by the owner to unpause, returns to normal state
*/
function unpause() public onlyOwner whenPaused {
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);
}
コードを部分的に見てみましょう。
/**
*Submitted for verification at Etherscan.io on 2017-11-28
*/
pragma solidity ^0.4.17;
Solidityのバージョンは0.4.17、verificationした日付は2017-11-28です。
Solidity、Version 0.4.17のリリース日が 2017-09-22なので当時は最新に近いバージョンを使用しているようです。
/**
* @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;
}
}
Solidityのuint256計算において四則演算が正しく行われるようにラッパーする為のライブラリです。
/**
* @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;
}
}
}
functionに認証を付与するための修飾子onlyOwnerのコードです。
transferOwnershipによって owner を変更することがでます。
/**
* @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);
}
ERC20のインターフェースです。
現在の仕様「ERC-20: Token Standard」とは一部足りない部分や異なる部分があります。（最終的には全ての要件を満たしています。）
/**
* @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];
}
}
基本トークンBasicTokenの実装で、transferとbalanceOfが実装されています。
basisPointsRateとmaximumFeeか設定されている場合、transfer時に手数料をownerに支払われる仕組みです。
/**
* @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];
}
}
標準トークンStandardTokenの実装です。
transferFrom、approve、allowanceが実装されています。
transferFromはtransferと同様に、basisPointsRateとmaximumFeeか設定されている場合、手数料をownerに支払われる仕組みです。
/**
* @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() public onlyOwner whenNotPaused {
paused = true;
Pause();
}
/**
* @dev called by the owner to unpause, returns to normal state
*/
function unpause() public onlyOwner whenPaused {
paused = false;
Unpause();
}
}
一時停止Pausableを実装しています。
whenNotPaused、whenPaused修飾子で操作可能です。
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);
}
ブラックリストBlackListの実装です。
addBlackList、removeBlackList、destroyBlackFundsが実装されています。
ブラックリストの追加、削除、資産の破棄がownerによって実行可能です。
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;
}
アップグレードUpgradedStandardTokenのインターフェース実装です。
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);
}
メイントークンTetherTokenの実装です。
一時停止Pausable、標準トークンStandardToken、ブラックリストBlackListを継承しています。
ERC20以外の機能は、アップグレードdeprecate、発行issue、償還redeem、手数料設定setParamsが実装されており、ownerによって実行可能です。
まとめ（Conclusion）
ERC20の基本的な機能はもちろん、いろいろな工夫がされています。
ERC20以外の機能としては、以下があげられます。
- 認証（
onlyOwner） - 一時停止（
paused） - 手数料（
fee） - アップグレード（
upgrade） - ブラックリスト（
blacklist） - 発行（
issue） - 償還（
redeem）
これらの機能は最近のトークンでも実装されている機能です。
初期のコードなのでシンプルなコードとは言えませんが、かなり考えられて作られたことがわかります。