When we get error "incomplete type " ?

[Qt embedded developer]

Hi can anybody tell me why i am getting below error when i write

private:
   QSharedPointer<MessageParser> _messageParserPtr;
   QSharedPointer<MessageSender> _messageSenderPtr;

/home/MessageHandler.h:20: error: field ‘_messageParserPtr’ has incomplete type ‘QSharedPointer<MessageParser>’
In file included from /home/MessageParser.h:5,
from /home/MessageParser.cpp:1:
/home/MessageHandler.h:20:34: error: field ‘_messageParserPtr’ has incomplete type ‘QSharedPointer<MessageParser>’
20 | QSharedPointer<MessageParser> _messageParserPtr;
| ^~~~~~~~~~~~~~~~~

[JoeCFD]

@Qt-embedded-developer
I guess you need to include header for QSharedPointer and definition for class MessageParser