A class invariant is a set of rules that define whether an object instance is in a valid state. It is a set of boundaries that data within an object must fall within. Unlike standard error checking, class invariants are used to ensure that a class’s internal implementation works. They are similar to standard error checking, but are mainly used to catch errors that shouldn’t occur unless there is an inherent flaw in the code. When a class invariant check fails, the program should terminate because the object is in a compromised state. In object-oriented programming languages, the invariant is inherited by any subclass to prevent it from breaking the design contract established by the superclass.
In object-oriented computer design and programming, a class invariant is a set of rules that can be used to define whether an object instance exists in a valid state. From a design perspective, it is a set of boundaries that data within an object must fall within to be considered in a proper and functional state. It can be defined in design documentation or source code comments, or, in some programming languages, it can be implemented directly into usable computer code. A program that uses hard-coded invariants and assertions can cause the program to hang or generate various errors when the invariant conditions are not satisfied. Unlike standard error checking, class invariants are typically used only for the purpose of ensuring that a class’s internal implementation works, and are usually not listed in public documentation or programming interfaces.
From a very basic level, a class invariant is essentially a collection of assertions for a class. An assertion, again in simple terms, is a statement that checks some state of the class and must return true for program execution to continue. An example of an assertion is a statement that ensures that a given integer is always between 1 and 10. When a class invariant is used, assertions are evaluated for all relevant parts of the data held by the object, essentially validating that all data in the object is within the defined ranges.
In many cases, using a class invariant is very similar to standard error checking, where variables are measured to make sure they are either within usable limits or not null. The difference between using class invariants and standard error checking, however, is that invariants and assertions are mainly used to catch errors that shouldn’t occur unless there is an inherent flaw in the code. Another difference is that standard error checking tends to involve rollbacks and changes in program control flow, whereas the result of an invariant error should be program termination. The reason why most programs terminate when a class invariant check fails is because the object is in a compromised state and is unable, from a design perspective, to satisfy its preconditions and postconditions necessary for adhere to its design contract.
One of the properties of a class invariant in object-oriented programming languages where they are implicitly defined is that the invariant is a mechanism inherited by any subclass. This prevents a subclass from overriding any invariant checks performed in the parent class. Ultimately, this means that a subclass is unable to break the design contract established by the superclass, which could lead to unpredictable results or hard-to-find program errors.
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