contract StandardToken is ERC20, BasicToken {
  mapping (address => mapping (address => uint256)) internal allowed;

  /**
   * @dev 从一个地址向另外一个地址转token
   * @param _from 转账的from地址
   * @param _to address 转账的to地址
   * @param _value uint256 转账token数量
   */
  function transferFrom(
    address _from,
    address _to,
    uint256 _value
  )
    public
    returns (bool)
  {
    // 做合法性检查
    require(_to != address(0));
    require(_value <= balances[_from]);
    require(_value <= allowed[_from][msg.sender]);
    balances[_from] = balances[_from].sub(_value);
    balances[_to] = balances[_to].add(_value);
    allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
    emit Transfer(_from, _to, _value);
    return true;
  }

  function approve(address _spender, uint256 _value) public returns (bool) {
    allowed[msg.sender][_spender] = _value;
    emit Approval(msg.sender, _spender, _value);
    return true;
  }

  function allowance(
    address _owner,
    address _spender
   )
    public
    view
    returns (uint256)
  {
    return allowed[_owner][_spender];
  }

}
function transferFrom(
  address _from,
  address _to,
  uint256 _value
)
  public
  returns (bool)
{
  // 做合法性检查
  require(_to != address(0));
  require(_value <= balances[_from]);
  require(_value <= allowed[_from][msg.sender]);
  balances[_from] = balances[_from].sub(_value);
  balances[_to] = balances[_to].add(_value);
  allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
  emit Transfer(_from, _to, _value);
  return true;
}

/**
 * @dev 从一个地址向另外一个地址转token
 * @param _from 转账的from地址
 * @param _to address 转账的to地址
 * @param _value uint256 转账token数量
 */