Variable assignment inconsistency

Question

Hi expers
I have have a question about variable assigement
Here are my variables declared.
data: a_sofm_document    TYPE REF TO cl_document_bcs,
         an_if_document     TYPE REF TO if_document_bcs,
This portion of code works perpectly
a_sofm_document = cl_document_bcs=>create_from_text( i_text = xt i_subject = subject )
an_if_document = a_sofm_document.  
Try to assign back from  an_if_document
a_sofm_document  = an_if_document .  " Doest not work and generate error when compiling
Somebody understand why ?? and How to obtain a_sofm_document back?
Thank you

Answer 1

Dominik,
but ths is not a good programming practice. From OO perspective you should never need to do something like this.
I must strongly disagree here. The basic usage is in combination with polimorphism . Imagine we have a container type as ref to some superclass. We store there a number of subclasses' references, treating them generically i.e. calling same poliphormical method for each. Now we want some specific operation to objects of only certain type. Downcasting here is ideal for such operation. This would go like
loop at me->ref_container into r_vehicle.
   try.
      r_plane ?= r_vehicle.
       "do addtional calc. only for planes
   catch cx_sy_move_cast_error.
endloop.
Another example would be RTTS classes where from superclass cl_abap_typedescr we derive some specialized one i.e
data r_tabledescr type ref to cl_abap_structdescr.
r_tabledescr ?= cl_abap_typedescr=>describe_by_data( itab ).
I think these proves downcasting is not only very important aspect of OO design but also let us smoothly switch b/w generalization and specialization which often is required to achieve some kind of functionality.
Any comments are welcome:)
Regards
Marcin