Preconditions, Postconditions, and Their Relationship to Assertions: 25-Comp-B11 Advanced Software Design, Undated paper, Q10

Question 10 of 28: Preconditions, Postconditions, and Their Relationship to Assertions

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

17-Comp-B11 Advanced Software Design — National Exams, May 2019. 3 hours, closed book exam with two aid sheets allowed (written on both sides), no calculator permitted. The paper is organized into five parts, and candidates were instructed to answer any five (5) questions in Part I, any three (3) in Part II, any four (4) in Part III, any two (2) in Part IV, and any five (5) in Part V — only the first questions answered, in each part, as they appear in the answer book are marked. All questions carry equal weight, so the 19 questions actually marked (5+3+4+2+5 of 28) each count for 100/19 ≈ 5.26% of the paper. All 28 questions are answered below for completeness.

Reference texts: Sommerville, Software Engineering (10th ed., Pearson) — software processes, requirements engineering, design principles, testing, dependability, reuse; Pressman, Software Engineering: A Practitioner's Approach (9th ed.) — supplementary process/metrics/quality coverage; Gamma, Helm, Johnson & Vlissides (GoF), Design Patterns: Elements of Reusable Object-Oriented Software — pattern-language structure, the GoF pattern catalogue, and the "favor object composition over class inheritance" / "program to an interface, not an implementation" principles; Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, visibility, encapsulation, interfaces; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/class invariants; Barbara Liskov's 1987 substitutability paper for Question 12; Stroustrup, The C++ Programming Language, for friend/access-control semantics (Question 25).

Question 12 prints “Liskpv substitution principle”, a typo in the paper; it is answered as the Liskov substitution principle.

PART I — General Principles (answer any 5 of 7)

Question 10: Preconditions, Postconditions, and Their Relationship to Assertions (Part II)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Specifying a precondition (what the caller must guarantee before calling) and a postcondition (what the module guarantees after returning, provided the precondition held) turns an ordinary function signature into a genuine bilateral CONTRACT: the module promises correct behaviour ONLY in exchange for the caller upholding its side.

Example. sqrt(x): precondition — x ≥ 0; postcondition — the returned value r satisfies r*r approximately equals x (within a stated tolerance) and r ≥ 0. If a caller passes a negative x, the precondition is violated and the module's behaviour is explicitly UNDEFINED (the caller's fault); if x ≥ 0 was passed and the returned value does not satisfy the postcondition, the module itself has a bug.

Relationship to assertions (Question 9). Preconditions and postconditions are the SPECIFICATION of the contract; assertions are the MECHANISM used to check, at runtime, that the specification actually holds — a precondition is realized as an assertion at the top of the method body, a postcondition as an assertion just before it returns. The specification could in principle exist only in documentation, but writing it as an assertion makes the contract self-verifying every time the module executes.