Quiz 8 — Abstract Classes and Interfaces

Ten questions on pure virtual functions, contracts, and designing an interface that callers can rely on.

Attempt all of them closed-book, then mark yourself against the answers.

Concepts: pure virtual functions · abstract types · interfaces · contracts · interface segregation · destructors in interfaces.

---

Q1. (MCQ) A class is abstract when it has:

• A. a virtual function
• B. a pure virtual function (= 0)
• C. no constructor
• D. only private members

Q2. (true/false) An abstract class may contain fully implemented member functions.

Q3. (multiple-select) Which are characteristics of a well-designed interface? Select all that apply.

• A. Only pure virtual functions, plus a virtual destructor
• B. A virtual destructor
• C. Data members for shared state
• D. A documented contract for each operation

Q4. (predict) Which line fails to compile, and why?

#include <iostream>
#include <string>

class Report {
public:
    virtual std::string title() const = 0;
    virtual void render() const { std::cout << title() << '\n'; }
};

class Summary : public Report {
public:
    std::string title() const override { return "Summary"; }
};

int main() {
    Report r;          // line A
    Summary s;
    s.render();        // line B
}

Q5. (debug) Fix the program above with the smallest change that still gives a reusable Report interface.

Q6. (predict) Write the output.

#include <iostream>
#include <memory>
#include <vector>

class Shape {
public:
    virtual ~Shape() = default;
    virtual double area() const = 0;
};

class Square : public Shape {
public:
    explicit Square(double s) : s_(s) {}
    double area() const override { return s_ * s_; }
private:
    double s_;
};

class Rect : public Shape {
public:
    Rect(double w, double h) : w_(w), h_(h) {}
    double area() const override { return w_ * h_; }
private:
    double w_, h_;
};

int main() {
    std::vector<std::unique_ptr<Shape>> shapes;
    shapes.push_back(std::make_unique<Square>(3.0));
    shapes.push_back(std::make_unique<Rect>(2.0, 5.0));
    double total = 0.0;
    for (const auto& s : shapes) total += s->area();
    std::cout << total << '\n';
}

Q7. (analysis) What is a contract in the sense this module uses the word? Give one precondition and one postcondition for a withdraw(amount) operation.

Q8. (short answer) Why can an abstract class be a function parameter's type but not a variable's type?

Q9. (design) A kiosk plays media from files, streams and playlists through four operations: open, play, pause, stop. Give the interface's name and signatures, and one rule about failure that every implementation must respect.

Q10. (analysis) Why should a source that can also seek() implement a second interface rather than having seek added to the first?

---

Scoring: mark yourself with the answers, then read the self-assessment guide.