pragma solidity ^0.4.16;  
contract Token{  
    uint256 public totalSupply;  
  
    function balanceOf(address _owner) public constant returns (uint256 balance);  
    function transfer(address _to, uint256 _value) public returns (bool success);  
    function transferFrom(address _from, address _to, uint256 _value) public returns     
    (bool success);  
  
    function approve(address _spender, uint256 _value) public returns (bool success);  
  
    function allowance(address _owner, address _spender) public constant returns   
    (uint256 remaining);  
  
    event Transfer(address indexed _from, address indexed _to, uint256 _value);  
    event Approval(address indexed _owner, address indexed _spender, uint256   
    _value);  
}  
  
contract TokenERC20 is Token {  
  
    string public name;                   //名称，例如"My test token"  
    uint8 public decimals;               //返回token使用的小数点后几位。比如如果设置为3，就是支持0.001表示.  
    string public symbol;               //token简称,like MTT  
  
    function TokenERC20(uint256 _initialAmount, string _tokenName, uint8 _decimalUnits, string _tokenSymbol) public {  
        totalSupply = _initialAmount * 10 ** uint256(_decimalUnits);         // 设置初始总量  
        balances[msg.sender] = totalSupply; // 初始token数量给予消息发送者，因为是构造函数，所以这里也是合约的创建者  
  
        name = _tokenName;                     
        decimals = _decimalUnits;            
        symbol = _tokenSymbol;  
    }  
  
    function transfer(address _to, uint256 _value) public returns (bool success) {  
        //默认totalSupply 不会超过最大值 (2^256 - 1).  
        //如果随着时间的推移将会有新的token生成，则可以用下面这句避免溢出的异常  
        require(balances[msg.sender] >= _value && balances[_to] + _value > balances[_to]);  
        require(_to != 0x0);  
        balances[msg.sender] -= _value;//从消息发送者账户中减去token数量_value  
        balances[_to] += _value;//往接收账户增加token数量_value  
        Transfer(msg.sender, _to, _value);//触发转币交易事件  
        return true;  
    }  
  
  
    function transferFrom(address _from, address _to, uint256 _value) public returns   
    (bool success) {  
        require(balances[_from] >= _value && allowed[_from][msg.sender] >= _value);  
        balances[_to] += _value;//接收账户增加token数量_value  
        balances[_from] -= _value; //支出账户_from减去token数量_value  
        allowed[_from][msg.sender] -= _value;//消息发送者可以从账户_from中转出的数量减少_value  
        Transfer(_from, _to, _value);//触发转币交易事件  
        return true;  
    }  
    function balanceOf(address _owner) public constant returns (uint256 balance) {  
        return balances[_owner];  
    }  
  
  
    function approve(address _spender, uint256 _value) public returns (bool success)     
    {   
        allowed[msg.sender][_spender] = _value;  
        Approval(msg.sender, _spender, _value);  
        return true;  
    }  
  
    function allowance(address _owner, address _spender) public constant returns (uint256 remaining) {  
        return allowed[_owner][_spender];//允许_spender从_owner中转出的token数  
    }  
    mapping (address => uint256) balances;  
    mapping (address => mapping (address => uint256)) allowed;  
}