postconditions

Postconditions are formal specifications that describe the state of a program after a function or procedure finishes execution. They are part of contract-based programming, defining what must be true for a call to be considered successful from the caller’s perspective.

A postcondition may specify the returned value, the state of objects, and other observable effects. It is typically considered only when the call terminates normally; if an exception or abnormal termination occurs, the postcondition may not hold. Postconditions help ensure that a routine preserves invariants and produces a consistent result.

Postconditions complement preconditions and invariants. Precondition statements define what must be true before a call, while invariants describe properties that must hold throughout a program’s lifetime. Together, they enable reasoned proofs of correctness and serve as living documentation.

Many languages provide dedicated syntax for postconditions; Eiffel uses an ensure clause, while others rely on annotations or contract libraries (for example, JML, Spec#, Code Contracts). In practice, postconditions may reference the function’s result, parameters, and the object’s visible state.

Key design considerations include making postconditions precise, decidable, and feasible for all valid inputs; avoiding reliance on mutable or internal state; and balancing expressiveness with maintainability. Postconditions can support testing, debugging, and formal verification, but should be kept consistent with overall design.