c – 如何在不破坏Decorator模式的情况下减少胖界面？

// All-purpose interface for any kind of object that can do I/O
class IDataIO
{
public:
   // basic I/O calls
   virtual ssize_t Read(void * buffer,size_t size) = 0;
   virtual ssize_t Write(const void * buffer,size_t size) = 0;

   // Seeking calls (implemented to return error codes
   // for I/O objects that can't actually seek)
   virtual result_t Seek(ssize_t offset,int whence) = 0;
   virtual ssize_t GetCurrentSeekPosition() const = 0;
   virtual ssize_t GetStreamLength() const = 0;

   // Packet-specific calls (implemented to do nothing
   // for I/O objects that aren't packet-oriented)
   virtual const IPAddressAndPort & GetSourceOfLastReadPacket() const = 0;
   virtual result_t SetPacketSendDestination(const IPAddressAndPort & iap) = 0;
};

这很好用 – 我有TCP,UDP,文件,内存缓冲区,SSL,RS232,stdin / stdout等各种具体的子类,我可以编写可以使用的I / O不可知的例程与他们中的任何一个一起.

我还有各种decorator类,它们拥有现有IDataIO对象的所有权,并充当该对象的行为修改前端.这些装饰器类很有用,因为可以使用单个装饰器类来修改/增强任何类型的IDataIO对象的行为.这是一个简单的(玩具)示例：

/** Example decorator class:  This object wraps any given
  * child IDataIO object,such that all data going out is
  * obfuscated by applying an XOR transformation to the bytes,* and any data coming in is de-obfuscated the same way.
  */
class XorDataIO : public IDataIO
{
public:
   XorDataIO(IDataIO * child) : _child(child) {/* empty */}
   virtual ~XorDataIO() {delete _child;}

   virtual ssize_t Read(void * buffer,size_t size)
   {
      ssize_t ret = _child->Read(buffer,size);
      if (ret > 0) XorData(buffer,ret);
      return ret;
   }

   virtual ssize_t Write(const void * buffer,size_t size)
   {
      XorData(buffer,size);   // const-violation here,but you get the idea
      return _child->Write(buffer,size);
   }

   virtual result_t Seek(ssize_t offset,int whence) {return _child->Seek(offset,whence);}
   virtual ssize_t GetCurrentSeekPosition() const    {return _child->GetCurrentSeekPosition();}
   virtual ssize_t GetStreamLength() const           {return _child->GetStreamLength();}

   virtual const IPAddressAndPort & GetSourceOfLastReadPacket() const      {return _child->GetSourceOfLastReadPacket();}
   virtual result_t SetPacketSendDestination(const IPAddressAndPort & iap) {return _child->SetPacketSendDestination(iap);}

private:
   IDataIO * _child;
};

这一切都很好,但令我困扰的是我的IDataIO类看起来像fat interface的一个例子 – 例如,UDPSocketDataIO类永远无法实现Seek(),GetCurrentSeekPosition()和GetStreamLength()方法,而FileDataIO类永远不能实现GetSourceOfLastReadPacket()和SetPacketSendDestination()方法.所以这两个类都被强制实现那些方法作为存根,只是什么也不做,并返回一个错误代码 – 这是有效的,但它很难看.

为了解决这个问题,我想将IDataIO接口划分为单独的块,如下所示：

// The bare-minimum interface for any object that we can
// read bytes from,or write bytes to (e.g. TCP or RS232)
class IDataIO
{
public:
   virtual ssize_t Read(void * buffer,size_t size) = 0;
};

// A slightly extended interface for objects (e.g. files
// or memory-buffers) that also allows us to seek to a
// specified offset within the data-stream.
class ISeekableDataIO : public IDataIO
{
public:
   virtual result_t Seek(ssize_t offset,int whence) = 0;
   virtual ssize_t GetCurrentSeekPosition() const = 0;
   virtual ssize_t GetStreamLength() const = 0;
};

// A slightly extended interface for packet-oriented
// objects (e.g. UDP sockets)
class IPacketDataIO : public IDataIO
{
public:
   virtual const IPAddressAndPort & GetSourceOfLastReadPacket() const = 0;
   virtual result_t SetPacketSendDestination(const IPAddressAndPort & iap) = 0;
};

….所以现在我可以从IPacketDataIO子接口继承UDPSocketDataIO,从ISeekableDataIO接口继承子类FileDataIO,而TCPSocketDataIO仍然可以直接从IDataIO继承,依此类推.这样,每种类型的I / O对象仅向其实际支持的功能提供接口,并且没有人必须实现与其所做的无关的无操作/存根版本的方法.

到目前为止,这么好,但此时出现的问题是装饰器类 – 我的XorDataIO子类应该在这种情况下继承什么接口？我想我可以写一个XorDataIO,一个XorSeekableDataIO和一个XorPacketDataIO,这样所有三种类型的接口都可以完全装饰,但我真的不愿意 – 这似乎是很多冗余/开销,特别是如果我有已经有多个不同的适配器类,我不想将它们的数字进一步倍增三倍.

是否有一些众所周知的聪明/优雅的方法来解决这个问题,这样我就可以吃蛋糕了？