Class Invariant - Class Invariants and Inheritance

Class Invariants and Inheritance

The useful effect of class invariants in object-oriented software is enhanced in the presence of inheritance. Class invariants are inherited, that is, "the invariants of all the parents of a class apply to the class itself."

Inheritance can allow descendant classes to alter implementation data of parent classes, so it would be possible for a descendant class to change the state of instances in a way that made them invalid from the viewpoint of the parent class. The concern for this type of misbehaving descendant is one reason object-oriented software designers give for favoring composition over inheritance (i.e., inheritance breaks encapsulation).

However, because class invariants are inherited, the class invariant for any particular class consists of any invariant assertions coded immediately on that class, logically "and-ed" with all the invariant clauses inherited from the class's parents. This means that even though descendant classes may have access to the implementation data of the their parents, the class invariant can prevent them from manipulating those data in any way that produces an invalid instance at runtime.