What is the use of CCmdTarget? Explain Command or  Message routing with SDI & MDI  applications.
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This class is at the second level on MFC class inheritence hierarchy, derived direcly from CObject class. This class provides the Command routing or message routing through the different MFC framework classes.

Command routing or Message routing.

Windows event based model -- Let say an MFC application is running and user clicks on a menu item. Follwing sequence of steps are executed.

1. Any event (message) occured on any application running on window machine, is first received by windows operating system. Windows operating system retrives the thread running that application. OS send that message to message queue of the application for which it is created.

2. From own message queue application retrives the message and sends to the window of which handle is specifed..

This message is passed to the different classess for processing as follows.

In SDI Application

First sent to the Frame window class ( Derived from CFrameWnd). Frame window passes it to View class( Derived from CView or any view classes derived from CView). View class tries to handle it ( if handler is defined ), if not, passes to Document class ( Derived from CDocument). If document class is not able to handle it , passes it to fram class again. Frame class tries to handle it itself, if cant handle passes it to Application class( Derived from CWinApp). it application class is not able to handle it, it will be handled by Windows Default procedure.

in MDI Application

Similar to the SDI except it has saveral child frame windows, Views, documents. thus, before processed by main frame window all the DocTemplates tries to process it by roming through view, docs, child frames.

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