Editing Assertion (software development) (section)

=== Assertions in design by contract ===

Assertions can function as a form of documentation: they can describe the state the code expects to find before it runs (its [[precondition]]s), and the state the code expects to result in when it is finished running ([[postcondition]]s); they can also specify [[invariant (computer science)|invariants]] of a [[Class (computer science)|class]]. [[Eiffel (programming language)|Eiffel]] integrates such assertions into the language and automatically extracts them to document the class. This forms an important part of the method of [[design by contract]].

This approach is also useful in languages that do not explicitly support it: the advantage of using assertion statements rather than assertions in [[comment (computer programming)|comments]] is that the program can check the assertions every time it runs; if the assertion no longer holds, an error can be reported. This prevents the code from getting out of sync with the assertions.