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Widening and narrowing cast



 
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PostPosted: Sat Sep 15, 2007 8:00 pm    Post subject: Widening and narrowing cast Reply with quote

In fact, narrowing and widening casts only change object view to less or more detailed, switching the reference variable that points to the object. Real object is not changed by both casts. It's forbidden to assign object reference to subclass object reference even using widening cast, for example, initializing the extra components with their default values. System-exception move_cast_error will be raised.

Code:
REPORT ZVV0003.
*----------------------------------------------------------------------*
class c_base definition.
  public section.
    methods: constructor importing value(p_name) type string,
             m_show.
    data     a_name1 type string.
endclass.
*----------------------------------------------------------------------*
class c_derived definition inheriting from c_base.
  public section.
    methods: constructor importing value(p_name1) type string
                                   value(p_name2) type string,
             m_show redefinition.
    data     a_name2 type string.
endclass.
*----------------------------------------------------------------------*
class c_base implementation.
  method constructor.
    a_name1 = p_name.
  endmethod.
  method m_show.
    write: / 'Base:', a_name1, '.'.
  endmethod.
endclass.
*----------------------------------------------------------------------*
class c_derived implementation.
  method constructor.
    call method: super->constructor exporting p_name = p_name1.
    a_name2 = p_name2.
  endmethod.
  method m_show.
    write: / 'Derived:', a_name1, ',', a_name2, '.'.
  endmethod.
endclass.
*----------------------------------------------------------------------*
data: o_1a type ref to c_base,
      o_1b type ref to c_base,
      o_2a type ref to c_derived,
      o_2b type ref to c_derived.
*----------------------------------------------------------------------*
END-OF-SELECTION.
*----------------------------------------------------------------------*
create object: o_2a exporting p_name1 = 'First' p_name2 = 'Second',
               o_1a exporting p_name  = '=1st='.
o_1b = o_2a.                                      "no problem
call method: o_1a->m_show,
             o_2a->m_show,
             o_1b->m_show.
catch system-exceptions move_cast_error = 4.
  o_2b ?= o_1a.                                   "impossible
endcatch.
if sy-subrc = 4.   write / 'Cannot cast o_2b ?= o_1a.'.
else.              call method o_2b->m_show.
endif.
catch system-exceptions move_cast_error = 4.
  o_2b ?= o_1b.                                   "only casting possible
endcatch.
if sy-subrc = 4.   write / 'Cannot cast o_2b ?= o_1b.'.
else.              call method o_2b->m_show.
endif.
*----------------------------------------------------------------------*
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