Question

7.2. ERC20 Token Solidity 0.4.24

https://theethereum.wiki/w/index.php/ERC20_Token_Standard

ERC20 “描述了实现代币合约的标准功能”，ERC20 是各个代币的标准接口。ERC20 代币仅仅是以太坊代币的子集。为了充分兼容 ERC20，开发者需要将一组特定的函数集成到他们的智能合约中，以便在高层面能够执行以下操作：

ERC20 提供的方法

• 获得代币总供应量

• 获得账户余额

• 转让代币

• 批准花费代币

7.2.1. 构造方法

原合约

			
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);
  }
}
			
			

新版合约

			
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);
  }
}
			
			

7.2.2. 官方规定 Method 方法

所有的ERC20代币都是按照下面这些方法来定义的。下面我们讲解一下每个方法的作用。

7.2.2.1. name

function name() view public returns (string name)

返回string类型的ERC20代币的名字，例如：Netkiller Reader Coin

7.2.2.2. symbol

function symbol() view public returns (string symbol)

返回string类型的ERC20代币的符号，也就是代币的简称，例如：NRC。

7.2.2.3. decimals

function decimals() view public returns (uint 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。

7.2.2.4. totalSupply

function totalSupply() view public returns (uint256 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。

7.2.2.5. balanceOf

function balanceOf(address _owner) public returns (uint256 balance)

输入地址，可以获取该地址代币的余额。

7.2.2.6. transfer

function transfer(address _to, uint256 _value) public returns (bool success)

调用transfer函数将自己的token转账给_to地址，_value为转账金额

7.2.2.7. approve

function approve(address _spender, uint256 _value) public returns (bool success)

批准_spender账户从自己的账户转移_value个token。可以分多次转移。

7.2.2.8. transferFrom

function transferFrom(address _from, address _to, uint256 _value) public returns (bool success)

与approve搭配使用，approve批准之后，调用transferFrom函数来转移token。

7.2.2.9. allowance

function allowance(address _owner, address _spender) public returns (uint256 remaining)

返回_spender还能提取token的个数。

approve、transferFrom及allowance解释：账户A有1000个代币，想允许B账户随意调用100个代币。A账户按照以下形式调用approve函数approve(B,100)。当B账户想用这100个代币中的10个代币给C账户时，则调用transferFrom(A, C, 10)。这时调用allowance(A, B)可以查看B账户还能够调用A账户多少个token。

7.2.3. 事件

7.2.3.1. Transfer

event Transfer(address indexed _from, address indexed _to, uint256 _value)

当成功转移token时，一定要触发Transfer事件

7.2.3.2. Approval

event Approval(address indexed _owner, address indexed _spender, uint256 _value)

当调用approval函数成功时，一定要触发Approval事件