title: How Protocol buffer interact with legacy protocol code
body: <p>every one, I have a question about how protocol buffer interact with existed protocol mechanism,Say code below:</p>
  
  <pre><code>class PacketBase
  {
  public:
     PacketBase();
  private:
     int msgType;
     int msgLen;
  private:
     MessageBuilder* m_pMsgBuilder;   /// do Write and Read From msg stream
  
  };
  
  class LoginRequest : public PacketBase
  {
   /// here we can use proto replace the msg field defined here
   /// invoke write or read method in packetbase to serialze or de-serialize
  };
  </code></pre>
  
  <p>Can protocol buffer do the job while maintain the class hierarchy unchanged?</p>