Class invariants are established during construction and constantly maintained between calls to public methods.
With concurrency, maintaining the invariant in methods typically requires a critical section to be established by locking the state using a mutex.
Defining class invariants can help programmers and testers to catch more bugs during software testing.
Common programming languages like Python,[3] PHP,[4] JavaScript,[citation needed] C++ and Java support assertions by default, which can be used to define class invariants.
The Ada programming language has native support for type invariants (as well as pre- and postconditions, subtype predicates, etc.).
In this case, the invariant ensures that, for a stack of logical depth N, the first N elements of the array are valid values.
The invariant must hold to be true after the constructor is finished and at the entry and exit of all public member functions.