1、ERC20Basic
安全考虑定义抽象函数和发送事件，个人认为隔离设计更安全
function totalSupply()
function balanceOf()
function transfer()
event Transfer();
2、ERC20
安全考虑定义抽象函数和发送事件，这里ERC20符合ERC20的标准
function allowance ()
function transferFrom ()
function approve()
event Transfer();
3、ERC20Basic的实现----------------BasicToken
function totalSupply()
function balanceOf()
function transfer()
4、ERC20 的实现----------------StandardToken
实现了ERC20标准的所有函数，另外增加了对允许转账金额的增加和修改
function allowance ()
function transferFrom ()
function approve()
function increaseApproval()
function decreaseApproval()
具体代码：
pragma solidity ^0.4.18;
/**
* @title ERC20Basic
* @dev Simpler version of ERC20 interface
* @dev see https://github.com/ethereum/EIPs/issues/179
*/
contract ERC20Basic {
function totalSupply() public view returns (uint256); // totalSupply - 总发行量
function balanceOf(address who) public view returns (uint256); // 余额
function transfer(address to, uint256 value) public returns (bool); // 交易
event Transfer(address indexed from, address indexed to, uint256 value); // 交易事件
}
/**
* @title SafeMath
* @dev Math operations with safety checks that throw on error
*/
library SafeMath {
/**
* @dev Multiplies two numbers, throws on overflow.
*/
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;
}
/**
* @dev Integer division of two numbers, truncating the quotient.
*/
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;
}
/**
* @dev Substracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
*/
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
assert(b <= a);
return a - b;
}
/**
* @dev Adds two numbers, throws on overflow.
*/
function add(uint256 a, uint256 b) internal pure returns (uint256) {
uint256 c = a + b;
assert(c >= a);
return c;
}
}
/**
* @title ERC20 interface
* @dev see https://github.com/ethereum/EIPs/issues/20
*/
contract ERC20 is ERC20Basic {
function allowance(address owner, address spender) public view returns (uint256); // 获取被授权令牌余额,获取 _owner 地址授权给 _spender 地址可以转移的令牌的余额
function transferFrom(address from, address to, uint256 value) public returns (bool); // A账户-》B账户的转账
function approve(address spender, uint256 value) public returns (bool); // 授权，允许 _spender 地址从你的账户中转移 _value 个令牌到任何地方
event Approval(address indexed owner, address indexed spender, uint256 value); // 授权事件
}
/**
* @title Basic token
* @dev Basic version of StandardToken, with no allowances.
*/
contract BasicToken is ERC20Basic {
using SafeMath for uint256;
mapping(address => uint256) balances; // 余额
uint256 totalSupply_; // 发行总量
/**
* @dev total number of tokens in existence
*/
function totalSupply() public view returns (uint256) {
return totalSupply_;
}
/**
* @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, uint256 _value) public returns (bool) {
require(_to != address(0)); // 无效地址
require(_value <= balances[msg.sender]); // 转账账户余额大于转账数目
// SafeMath.sub will throw if there is not enough balance.
balances[msg.sender] = balances[msg.sender].sub(_value); // 转账账户余额=账户余额-转账金额
balances[_to] = balances[_to].add(_value); // 接收账户的余额=原先账户余额+账金额
Transfer(msg.sender, _to, _value); // 转账
return true;
}
/**
* @dev Gets the balance of the specified address.
* @param _owner The address to query the the balance of.
* @return An uint256 representing the amount owned by the passed address.
*/
function balanceOf(address _owner) public view returns (uint256 balance) {
return balances[_owner]; // 查询合约调用者的余额
}
}
/**
* @title Standard ERC20 token
*
* @dev Implementation of the basic standard token.
* @dev https://github.com/ethereum/EIPs/issues/20
* @dev Based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
*/
contract StandardToken is ERC20, BasicToken {
mapping (address => mapping (address => uint256)) internal allowed;
/**
* @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 uint256 the amount of tokens to be transferred
*/
function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
require(_to != address(0)); // 到达B账户的地址不能为无效地址
require(_value <= balances[_from]); // 转账账户余额大于转账金额
require(_value <= allowed[_from][msg.sender]); // 允许_from地址转账给 _to地址
balances[_from] = balances[_from].sub(_value);
balances[_to] = balances[_to].add(_value);
allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value); // 允许转账的余额
Transfer(_from, _to, _value);
return true;
}
/**
* 设置帐户允许支付的最大金额
*
* 一般在智能合约的时候，避免支付过多，造成风险
*
* @param _spender 帐户地址
* @param _value 金额
*/
function approve(address _spender, uint256 _value) public returns (bool) {
allowed[msg.sender][_spender] = _value;
Approval(msg.sender, _spender, _value);
return true;
}
/**
* @dev Function to check the amount of tokens that 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 uint256 specifying the amount of tokens still available for the spender.
获取 _owner 地址授权给 _spender 地址可以转移的令牌的余额
*/
function allowance(address _owner, address _spender) public view returns (uint256) {
return allowed[_owner][_spender];
}
/**
* @dev Increase the amount of tokens that an owner allowed to a spender.
*
* approve should be called when allowed[_spender] == 0. To increment
* allowed value is better to use this function to avoid 2 calls (and wait until
* the first transaction is mined)
* From MonolithDAO Token.sol
* @param _spender The address which will spend the funds.
* @param _addedValue The amount of tokens to increase the allowance by.
增加允许支付的最大额度
*/
function increaseApproval(address _spender, uint _addedValue) public returns (bool) {
allowed[msg.sender][_spender] = allowed[msg.sender][_spender].add(_addedValue);
Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
return true;
}
/**
* @dev Decrease the amount of tokens that an owner allowed to a spender.
*
* approve should be called when allowed[_spender] == 0. To decrement
* allowed value is better to use this function to avoid 2 calls (and wait until
* the first transaction is mined)
* From MonolithDAO Token.sol
* @param _spender The address which will spend the funds.
* @param _subtractedValue The amount of tokens to decrease the allowance by.
减少允许支付的最大额度
*/
function decreaseApproval(address _spender, uint _subtractedValue) public returns (bool) {
uint oldValue = allowed[msg.sender][_spender];
if (_subtractedValue > oldValue) {
allowed[msg.sender][_spender] = 0;
} else {
allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
}
Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
return true;
}
}
可以参考：
https://www.jianshu.com/writer#/notebooks/24554485/notes/28544778
