What is "class invariant"

n. (context software design English) an invariant used to constrain objects of a class that is preserved by class methods and that constrains the state stored in the object.

In computer programming, specifically object-oriented programming, a class invariant (or type invariant) is an invariant used to constrain objects of a class. Methods of the class should preserve the invariant. The class invariant constrains the state stored in the object.

Class invariants are established during construction and constantly maintained between calls to public methods. Temporary breaking of class invariance between private method calls is possible, although not encouraged.

An object invariant, or representation invariant, is a computer programming construct consisting of a set of invariant properties that remain uncompromised regardless of the state of the object. This ensures that the object will always meet predefined conditions, and that methods may, therefore, always reference the object without the risk of making inaccurate presumptions. Defining class invariants can help programmers and testers to catch more bugs during software testing.