Quiz 2 — Classes and Objects

Ten questions on defining types, access control, this, and member functions. Attempt all of them closed-book, then mark yourself against the answers.

Concepts: class definition · access specifiers · this · member functions · separating interface from implementation.

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Q1. (MCQ) In a class, what is the default access for members?

• A. public
• B. private
• C. protected
• D. there is no default

Q2. (true/false) Two objects of the same class share the class's member functions but each has its own data members.

Q3. (multiple-select) Which of these make a good reason to make a member private? Select all that apply.

• A. Keeping two fields consistent with each other
• B. Preventing a caller from setting an impossible value
• C. Hiding a helper the caller should not depend on
• D. Making the code shorter to type

Q4. (predict) Write the line this prints.

#include <iostream>

class Counter {
public:
    Counter() : value_(0) {}
    void increment() { ++value_; }
    int  value() const { return value_; }

private:
    int value_;
};

int main() {
    Counter a;
    Counter b;
    a.increment();
    a.increment();
    b.increment();
    std::cout << a.value() << ' ' << b.value() << '\n';
}

Q5. (short answer) What does the this pointer refer to inside a member function?

Q6. (analysis) Why does a member function that only reads a data member need to be declared const? Give one concrete situation where omitting it causes a compile error.

Q7. (MCQ) Which line is the class's interface rather than its implementation?

class Rectangle {
public:
    double area() const { return width_ * height_; }   // (1)
private:
    double width_;
    double height_;
};

• A. line (1), because it is public
• B. the private members
• C. both, taken together
• D. neither

Q8. (debug) The accessor cannot be called on a const object. Fix it.

#include <iostream>

class Sensor {
public:
    explicit Sensor(double celsius) : celsius_(celsius) {}
    double celsius() { return celsius_; }        // the problem
private:
    double celsius_;
};

void report(const Sensor& s) {
    std::cout << s.celsius() << '\n';            // error here
}

int main() {
    Sensor s{21.5};
    report(s);
}

Q9. (predict) Write what this prints.

#include <iostream>

class Box {
public:
    Box() : open_(false) {}
    void open()  { open_ = true; }
    void close() { open_ = false; }
    bool isOpen() const { return open_; }
private:
    bool open_;
};

int main() {
    Box b;
    std::cout << b.isOpen() << '\n';
    b.open();
    std::cout << b.isOpen() << '\n';
}

Q10. (design) A shopping-cart item needs a name, a unit price and a quantity. Give the class's public operations (signatures only) and one invariant you would enforce, and say which operation enforces it.

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