Casting Help

[Former Member]

Hi,

i learn OO and maybe someone can explain me

what is the action in bold is doing ?

r_cargo ?= r_plane.

Regards

DATA: r_cargo TYPE REF TO lcl_cargo_plane.

DATA: r_plane TYPE REF TO lcl_airplane.

LOOP AT airplane_list INTO r_plane.

TRY.

r_cargo ?= r_plane.

cargo = r_cargo->get_cargo( ).

IF re_cargo < cargo.

re_cargo = cargo.

ENDIF.

CATCH cx_sy_move_cast_error INTO r_exc.

ENDTRY

[narin_nandivada3]

Hi,

Its nothing but Wide casting Concept in OOABAP.

In short you are increasing the scope of accessibility of that object.

*Lets say there are two classes CLASS A and CLASS B.*

*Class B inherits CLASS A..*

*So you had created object for CLASS A and you had just delcared object for CLASS B.*
*So now to increase the scope of the CLASS B object we can use Wide-casting concept..*

*DATA:*
  *W_ref_A type REF TO CLASS A,*
  *W_REF_B type REF TO CLASS B.*

*Create object CLASS A.*
*-------------------------*
*After some logic (optional)*
*-------------------------*

*W_REF_B ?= W_REF_A.  " Wide casting..*

*By Wide-casting the scope increases like with W_REF_A you cannot access the other methods define* 
*in class B other than inherited methods but using W_REF_B you can access as it is reference to Class B.*

Check these threads for more details..

Hope this would solve your issue.

Good luck

Narin

Edited by: Narin Nandivada on Sep 8, 2008 3:26 PM

[Former Member]

Hi Narin,

Thanks but maybe u can explain me this code:

cl_abap_typedescr is class interface

DATA: gx_struc TYPE REF TO cl_abap_structdescr.
  DATA: gx_table TYPE REF TO cl_abap_tabledescr.

  gx_table ?= cl_abap_typedescr=>describe_by_data( <col_actual> )."--Get Table Colman Numbers--*
  gx_struc ?= gx_table->get_table_line_type( ).
  DESCRIBE TABLE gx_struc->components LINES t_lines.

Regards

[narin_nandivada3]

Hi,

As CL_ABAP_TYPEDESCR is an Abstract class having the static method as DESCRIBE_BY_DATA.

and its RETURNING parameter is typed as a reference to this superclass. since the actual parameter

gx_table is typed on the subclass CL_ABAP_STRUCTDESCR, so the object has to be assigned using

Widening Cast with operator ?= .

Please check this document which would give your more clear idea...

https://www.sdn.sap.com/irj/sdn/go/portal/prtroot/docs/library/uuid/f2dac69e-0e01-0010-e2b6-81c1e8e5...

Hope this would help you.

Good luck

Narin

[Former Member]

?= is Wide casting or down cast

[narin_nandivada3]

Hi,

for additional information please check this thread

Check this link

http://www.abapprogramming.net/2007/10/oops-abap-8.html

Hope this would help you.

Good luck

Narin

[Former Member]

Hi,

'?=' This is widening cast operator. in your example you have assigned instance of superclass to instance of subclass.

After this assignment, you can access the specific components of subclass lcl_cargo_plane using instance r_cargo.

But before doing this, we do narrow casting.

In narrow casting you assign the reference variable of subclass to that of superclass.

I would like to add one more point that

during widening cast there might be a possibility that

at runtime your superclass variable has the reference of any other its subclasses. But at the same time you are assigning a reference variable to its particular subclass. So there can be a assignment which is incompatible.

thats why

cast error occurs only in

Widening cast.

which is handled by

CX_SY_MOVE_CAST_ERROR

Regards

Abhijeet