The DDX_Radio function manages the transfer of int data between a radio control group in a dialog box, form view, or control view object and a int data member of the dialog box, form view, or control view object. The value of the int data member is determined according to which radio button within the group is selected.
When DDX_Radio is called, value is set to the current state of the radio control group. The value is set as a 0-based index of the radio control that is currently checked, or -1 if no radio controls are checked.
For example, in case that the first radio button in the group is checked (the button with WS_GROUP style) the value of the int member is 0 and so on.
For more information about DDX, see Dialog Data Exchange and Validation.