Explain Customized control development? What is subclassing ?

When we are putting one control on any dialog, we are handling its messages or events in its parent (classes). We use functionalilty and interface provided by that control only. We cannot add our own fuctionality as per our need while handling messages in parent.
Let us take up a scenario when we want an edit control which should accept only letters, digit and specific symbols.

The default functionality will allow on datatypes and minimum maximum values and range validation using DDV(Dynamic Data validation ).
If you are not using Dynamic Data validation then you can validate the edit control values only when you are clicking/hitting return key at list once in control or parent dialog.

To validate the values entered in edit control at runtime we nee to customize the control. This is also called as subclassing, its similar to API Hooking.

Windows Subclassing- usually when and application runs it seeks for messages for its windows. if any messages comes that is handled in the Windows procedure for that application.
While executing the handler for one particular message one can inject some code at some point. this injected code will be executed and control returns to the actual handle code when the code was injected.

To do the subclassing we need to derive a class from control type (which you want to subclass)class. Add the event handlers where you want to do extra processing. And Call the subclass method for that control.

Ex. for an Edit control.

class MyEdit : public CEdit {

// add handlers
};

Call for subclass.

SubClassDlgItem();