Why ?= used in wide casting

Question

in wide casting , if we are assigning super class object2 to super class object1 using '?='. having the same effect as '='. so y we need to add '?' in wide casting.
class lcl_super DEFINITION.
   PUBLIC SECTION.
   METHODS method1.
  ENDCLASS.
  CLASS lcl_super IMPLEMENTATION.
    method method1.
      write : 'super class method'.
      ENDMETHOD.
      ENDCLASS.
  class lcl_sub DEFINITION INHERITING FROM lcl_super.
    PUBLIC SECTION.
    methods : method1 REDEFINITION,
              method2.
    ENDCLASS.
    CLASS lcl_sub IMPLEMENTATION.
      method method1.
        write ' subclass method1'.
        ENDMETHOD.
        method method2.
          write 'subclass method2'.
          ENDMETHOD.
      ENDCLASS.
data: obj1 type REF TO lcl_super,
       obj2 type REF TO lcl_sub.
START-OF-SELECTION.
create OBJECT obj1.
CREATE OBJECT obj2.
obj1 = obj2.                                     "Narrrowing casting
CALL METHOD obj1->method1.
data obj3 type REF TO lcl_super.
create OBJECT obj3.
obj1 ?= obj3.   or obj1 = obj3             "   is having same effect  in Widecasting,  so why need '?'
obj1->method1( ).

Answer 1

Hi vijay,
in addition to what Rudiger said, to know what ?= means, refer to this link: http://help.sap.com/abapdocu_70/en/ABAPMOVE.htm
In the next example, ?= is needed:
DATA:  obj_struct  TYPE REF TO cl_abap_structdescr.
obj_struct ?= cl_abap_structdescr=>describe_by_name( 'DDTAB_NAME' ).
Regards,
Angelo.