Design-by-Contract — Definition, Example, Pros and Cons: 25-Comp-B11 Advanced Software Design, December 2019, Q8

Question 8 of 28: Design-by-Contract — Definition, Example, Pros and Cons

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

Notes on this paper

17-Comp-B11 Advanced Software Design — National Exams, December 2019. 3 hours, closed book exam with one aid sheet 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; Pressman, Software Engineering: A Practitioner's Approach (9th ed.) — supplementary process and quality coverage; Gamma, Helm, Johnson & Vlissides (GoF), Design Patterns: Elements of Reusable Object-Oriented Software — creational/structural/behavioural pattern catalogue and the "program to an interface, not an implementation" / "favor object composition over class inheritance" principles; Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, visibility, and multiple inheritance semantics; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/class invariants; Barbara Liskov's 1987 substitutability paper for Question 12; Karl Wiegers, Software Requirements (3rd ed.); Myers, The Art of Software Testing, for Question 6.

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

PART II — Design by Contract (answer any 3 of 5)

Question 8: Design-by-Contract — Definition, Example, Pros and Cons (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.

Design-by-contract (DbC), introduced by Bertrand Meyer with Eiffel, specifies each module's obligations as a formal contract: a precondition the caller must satisfy, a postcondition the module guarantees if the precondition held, and a class invariant (Question 11) that must hold before and after every public operation.

Example. Stack.pop(): precondition — the stack is not empty; postcondition — the returned value is the element most recently pushed, and the stack's size decreases by exactly one.

Pros. The contract makes responsibility unambiguous: if the precondition was violated, the fault is the caller's; if the postcondition fails despite a valid precondition, the fault is the module's. Contracts double as always-accurate, executable documentation, and (implemented as assertions, Question 9) catch bugs at the exact point and call that caused them rather than downstream.

Cons. Specifying contracts precisely takes real up-front design discipline and can be verbose for simple modules; runtime-checked assertions add overhead if left enabled in production; and a contract is only as correct as the designer who wrote it — a wrong contract still needs conventional testing to catch.