Assertions and How They Implement Design-by-Contract: 25-Comp-B11 Advanced Software Design, Undated paper, Q9

Question 9 of 28: Assertions and How They Implement Design-by-Contract

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 9: Assertions and How They Implement Design-by-Contract (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.

An assertion is a boolean expression placed at a specific point in a program that the programmer declares must be true at that point; if it evaluates to false at runtime, it signals that a contract has been violated (a bug), and execution is typically halted or an exception raised rather than allowed to continue silently.

Usage in DbC. Assertions are the concrete IMPLEMENTATION MECHANISM for the abstract obligations of Question 8: a precondition becomes an assertion checked at the START of a method body; a postcondition becomes an assertion checked just before RETURN; a class invariant becomes an assertion checked after construction and after every public method.

Example (Stack.pop()):

int pop() {
    assert !isEmpty() : "precondition: stack must not be empty";
    int oldSize = size();
    int top = data[oldSize - 1];
    size--;
    assert size() == oldSize - 1 : "postcondition: size decreases by one";
    return top;
}

The first assertion checks the precondition on entry; the second checks the postcondition just before returning — together they make the contract of Question 8 self-checking at runtime rather than a comment nobody verifies.