Expression problem for ABAP program

Question

Hi everyone,
Once i have saw a expression in ABAP program. as follows.
DATA:drag_object TYPE REF TO lcl_dragdrop_dataobject.
CATCH SYSTEM-EXCEPRIONS move_cast_error = 1.
drag_object ?= dragdrop_object->object.
ENDCATCH.
IF sy-subrc = 1.
CALL METHOD dragdrop_object->abort.
EXIT.
ENDIF.
I don't know what is "?=" .
Please help me.
Thanks Advanced !

Answer 1

DATA: airplane TYPE REF TO lcl_airplane,
            cargo_airplane TYPE REF TO lcl_cargo_airplane,
            cargo_airplane2 TYPE REF TO lcl_cargo_airplane.
      CREATE OBJECT cargo_airplane.
      airplane = cargo_airplane.
      cargo_airplane2 ?= airplane.
             The type of case described above is known as a widening cast because it changes the
             view to one with more details. The instance assigned (a cargo plane in the above
             example) must correspond to the object reference (cargo_airplane in the above example),
             that is, the instance must have the details implied by the reference. This is also known as
             a u201Cdown castu201D. The widening cast in this case does not cause an error because the
             reference airplane actually points to an instance in the subclass lcl_cargo_airplane. The
             dynamic type is therefore u2018REF TO lcl_cargo_airplaneu2019.
The widening cast logically represents the opposite of the narrowing cast. The widening cast cannot be checked statically, only at runtime. The Cast Operator u201C?=u201D (or the equivalent u201CMOVE ... ?TO u2026u201D) must be used to make this visible.