ABAP_to_the_Future

Design by Contract 7.3
automatically call the clas s invariant and raise an exception if the kitchen had
gone up in flames.
Turning to implementing class invarian ts using ABAP, you cannot achieve the
same functionality that exists in Eiffel (c alling the same test automatically at the
end of every method) directly in ABAP, but you can come close.
Coding a class invariant involves using the standard SAP interface IF_CONSTRAINT
(originally discussed in Chapter 3), which lets you create quite complicated check
routines. To demonstrate this, turn once again to the monster example. You want
to make sure that after every method call to your monster, it still remains a monster,
rather than turning into a fluffy pink bunny or some such (let’s agree, fluffy
pink things are not scary).
The example code is shown in Listing 7.13. First, im port the instance of your
monster class. The constraint interface takes a data object as input, so you need to
cast that into a local variable represen ting the monster object before you can
check the data. Presume failure, and then run a series of tests on the various attributes
of the monster to make sure it has not turned pink or gone all fluffy. If all
the tests are passed, then the monster is judged not pink and not fluffy, and you
return a TRUE value to indicate that the method call has not ruined the essential
nature of the monster.
The get description method also has to be implemented to describe what has
happened in the event of a failure of the various tests (e.g., in this case, the monster
is no longer a monster).
CLASS lcl_monster_constraint DEFINITION.
PUBLIC SECTION.
INTERFACES if_constraint.
ENDCLASS. "Monster Constraint Definition
CLASS lcl_monster_constraint IMPLEMENTATION.
METHOD if_constraint~is_valid.
*-------------------------------------------------------*
* IMPORTING data_object TYPE data
* RETURNING result TYPE abap_bool
*-------------------------------------------------------*
* Local Variables
DATA: lo_monster TYPE REF TO ycl_monster.
279