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https://theethereum.wiki/w/index.php/ERC20_Token_Standard
ERC20 “描述了實現代幣合約的標準功能”，ERC20 是各個代幣的標準介面。ERC20 代幣僅僅是以太坊代幣的子集。為了充分兼容 ERC20，開發者需要將一組特定的函數整合到他們的智能合約中，以便在高層面能夠執行以下操作：
ERC20 提供的方法
獲得代幣總供應量
獲得賬戶餘額
轉讓代幣
批准花費代幣
原合約
pragma solidity ^0.4.21;
contract NetkillerToken {
address public owner;
string public name;
string public symbol;
uint public decimals;
uint256 public totalSupply;
event Transfer(address indexed from, address indexed to, uint256 value);
/* This creates an array with all balances */
mapping (address => uint256) public balanceOf;
function NetkillerToken(uint256 initialSupply, string tokenName, string tokenSymbol, uint decimalUnits) public {
owner = msg.sender;
name = tokenName;
symbol = tokenSymbol;
decimals = decimalUnits;
totalSupply = initialSupply * 10 ** uint256(decimals);
balanceOf[msg.sender] = totalSupply;
}
/* Send coins */
function transfer(address _to, uint256 _value) public {
/* Check if the sender has balance and for overflows */
require(balanceOf[msg.sender] >= _value && balanceOf[_to] + _value >= balanceOf[_to]);
/* Add and subtract new balances */
balanceOf[msg.sender] -= _value;
balanceOf[_to] += _value;
/* Notify anyone listening that this transfer took place */
emit Transfer(msg.sender, _to, _value);
}
}
新版合約
pragma solidity ^0.4.24;
contract NetkillerToken {
address public owner;
string public name;
string public symbol;
uint public decimals;
uint256 public totalSupply;
event Transfer(address indexed from, address indexed to, uint256 value);
/* This creates an array with all balances */
mapping (address => uint256) public balanceOf;
constructor(uint256 initialSupply, string tokenName, string tokenSymbol, uint decimalUnits) public {
owner = msg.sender;
name = tokenName;
symbol = tokenSymbol;
decimals = decimalUnits;
totalSupply = initialSupply * 10 ** uint256(decimals);
balanceOf[msg.sender] = totalSupply;
}
/* Send coins */
function transfer(address _to, uint256 _value) public {
/* Check if the sender has balance and for overflows */
require(balanceOf[msg.sender] >= _value && balanceOf[_to] + _value >= balanceOf[_to]);
/* Add and subtract new balances */
balanceOf[msg.sender] -= _value;
balanceOf[_to] += _value;
/* Notify anyone listening that this transfer took place */
emit Transfer(msg.sender, _to, _value);
}
}
所有的ERC20代幣都是按照下面這些方法來定義的。下面我們講解一下每個方法的作用。
function name() view public returns (string name)
返回string類型的ERC20代幣的名字，例如：Netkiller Reader Coin
function symbol() view public returns (string symbol)
返回string類型的ERC20代幣的符號，也就是代幣的簡稱，例如：NRC。
function decimals() view public returns (uint decimals)
支持幾位小數點後幾位。如果設置為3。也就是支持0.001表示。
function totalSupply() view public returns (uint256 totalSupply)
發行代幣的總量，可以通過這個函數來獲取。所有智能合約發行的代幣總量是一定的，totalSupply必須設置初始值。
function balanceOf(address _owner) public returns (uint256 balance)
輸入地址，可以獲取該地址代幣的餘額。
function transfer(address _to, uint256 _value) public returns (bool success)
調用transfer函數將自己的token轉賬給_to地址，_value為轉賬金額
function approve(address _spender, uint256 _value) public returns (bool success)
批准_spender賬戶從自己的賬戶轉移_value個token。可以分多次轉移。
function transferFrom(address _from, address _to, uint256 _value) public returns (bool success)
與approve搭配使用，approve批准之後，調用transferFrom函數來轉移token。
function allowance(address _owner, address _spender) public returns (uint256 remaining)
返回_spender還能提取token的個數。
approve、transferFrom及allowance解釋：賬戶A有1000個代幣，想允許B賬戶隨意調用100個代幣。A賬戶按照以下形式調用approve函數approve(B,100)。當B賬戶想用這100個代幣中的10個代幣給C賬戶時，則調用transferFrom(A, C, 10)。這時調用allowance(A, B)可以查看B賬戶還能夠調用A賬戶多少個token。