Preparing your code
The C++ and C Standard Libraries have a wide range of functions that allow you to apply tracing and reporting functions so that you can test if code is handling data in expected ways. Much of these facilities use conditional compilation so that the reporting only occurs in debug builds, but if you provide the traces with meaningful messages they will form part of the documentation of your code. Before you can report on the behavior of your code, you first have to know what to expect from it.
Invariants and conditions
Class invariants are conditions, the object state, that you know remain true. During a method call the object state will change, possibly to something that invalidates the object, but once a public method has completed, the object state must be left in a consistent state. There is no guarantee what order the user will call methods on a class, or even if they call methods at all, so an object must be usable whatever methods the user calls. The invariant aspects...
