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Learn the difference between narrowing (upcast) and widening (downcast) cast in ABAP.
This video will teach you how to implement both, and when to use them in a real case scenario.
Narrowing Cast
Variables of the type “reference to superclass” can also refer to subclass instances at runtime.
For example:
*superclass
DATA lv_vehicle TYPE REF TO zcl_vehicle.
*subclass
DATA lv_truck TYPE REF TO zcl_truck.
*narrowing cast
lv_vehicle = lv_truck.
- The term upcast is also common.
- The subclass always contains at least the same components as the superclass.
- The user can therefore address the subclass instance in the same way as the superclass instance.
- However, it is restricted to using only the inherited components.
- The view is thus generally narrowed. That is why we describe this type of assignment of reference variables as narrowing cast.
- There is a switch from a view of several components to a view of a few components.
Practical Example
- A typical use for narrowing cast assignments is to prepare for generic access.
- A user, who is not at all interested in the finer points of the instances of the subclasses but who simply wants to address the shared components, could use a superclass reference for this access.
- For example, a travel agency needs to manage all imaginable kinds of vehicles in one list.
- This leads to the question of what type should be assigned to the internal table for the references of the different types of vehicles.
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