The predicate condition that can be used to ensure that the CAST will work is IS INSTANCE OF. The IS INSTANCE OF predicate condition checks whether the operand is an instance of the specified class or interface. This is useful when you want to perform a downcast, which is a conversion from a more general type to a more specific type. A downcast can fail if the operand is not an instance of the target type, and this can cause a runtime error. Therefore, you can use the IS INSTANCE OF predicate condition to check whether the downcast is possible before using the CAST operator12. For example:
The following code snippet uses the IS INSTANCE OF predicate condition to check whether the variable g_super is an instance of the class lcl_super. If it is, the CAST will work and the variable g_sub1 will be assigned the value of g_super.
DATA: g_super TYPE REF TO lcl_super, g_sub1 TYPE REF TO lcl_sub1. IF g_super IS INSTANCE OF lcl_super. g_sub1 = CAST #( g_super ). g_sub1->method( ... ). ENDIF.
You cannot do any of the following:
IS SUPPLIED: The IS SUPPLIED predicate condition checks whether an optional parameter of a method or a function module has been supplied by the caller. This is useful when you want to handle different cases depending on whether the parameter has a value or not. However, this predicate condition has nothing to do with the CAST operator or the type of the operand12.
IS NOT INITIAL: The IS NOT INITIAL predicate condition checks whether the operand has a non-initial value. This is useful when you want to check whether the operand has been assigned a value or not. However, this predicate condition does not guarantee that the CAST will work, because the operand may have a value but not be an instance of the target type12.
IS BOUND: The IS BOUND predicate condition checks whether the operand is a bound reference variable. This is useful when you want to check whether the operand points to an existing object or not. However, this predicate condition does not guarantee that the CAST will work, because the operand may point to an object but not be an instance of the target type12.