contract BanliangCoin is ERC20Interface {
      string public constant symbol = "BLC"; //单位
      string public constant name = "BanliangCoin Token"; //名称
      uint8 public constant decimals = 18; //小数点后的位数
      uint256 _totalSupply = 10000000000 * 10 ** uint256(decimals); //发行总量

      // 智能合约的所有者
      address public owner;

      // 每个账户的余额
      mapping(address => uint256) balances;

      // 帐户的所有者批准将金额转入另一个帐户。从上面的说明我们可以得知allowed[被转移的账户][转移钱的账户]
      mapping(address => mapping (address => uint256)) allowed;

      // 只能通过智能合约的所有者才能调用的方法
      modifier onlyOwner() {
          if (msg.sender != owner) {
              throw;
          }
          _;
      }

      // 构造函数
      function BanliangCoin() {
          owner = msg.sender;
          balances[owner] = _totalSupply;
      }

      function totalSupply() constant returns (uint256 totalSupply) {
          totalSupply = _totalSupply;
      }

      // 特定账户的余额
      function balanceOf(address _owner) constant returns (uint256 balance) {
          return balances[_owner];
      }

      // 转移余额到其他账户
      function transfer(address _to, uint256 _amount) returns (bool success) {
          if (balances[msg.sender] >= _amount 
              && _amount > 0
              && balances[_to] + _amount > balances[_to]) {
              balances[msg.sender] -= _amount;
              balances[_to] += _amount;
              Transfer(msg.sender, _to, _amount);
              return true;
          } else {
              return false;
          }
      }

      //从一个账户转移到另一个账户，前提是需要有允许转移的余额
      function transferFrom(
          address _from,
          address _to,
          uint256 _amount
      ) returns (bool success) {
          if (balances[_from] >= _amount
              && allowed[_from][msg.sender] >= _amount
              && _amount > 0
              && balances[_to] + _amount > balances[_to]) {
              balances[_from] -= _amount;
              allowed[_from][msg.sender] -= _amount;
              balances[_to] += _amount;
              Transfer(_from, _to, _amount);
              return true;
          } else {
              return false;
          }
      }

      //允许账户从当前用户转移余额到那个账户，多次调用会覆盖
      function approve(address _spender, uint256 _amount) returns (bool success) {
          allowed[msg.sender][_spender] = _amount;
          Approval(msg.sender, _spender, _amount);
          return true;
      }

      //返回被允许转移的余额数量
      function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
          return allowed[_owner][_spender];
      }
  }
//继承接口后的实例
function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
    return allowed[_owner][_spender];
}

//返回被允许转移的余额数量