Quiz 5 — Operators and Static Members

Ten questions on making a type behave like a built-in one, and on state that belongs to the class rather than to an object.

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

Concepts: operator overloading · member vs free functions · operator<< · prefix and postfix ++ · static data and function members · strict weak ordering.

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Q1. (MCQ) Which operator must be a member function?

• A. operator+
• B. operator==
• C. operator=
• D. operator<<

Q2. (true/false) operator< may use <= inside without any consequence, as long as it is consistent.

Q3. (multiple-select) For which of these is a free function usually the better choice? Select all that apply.

• A. A symmetric operator+ where either operand may need conversion
• B. operator<< for output
• C. operator= for copy assignment
• D. operator[] for element access

Q4. (predict) Write the three output lines.

#include <iostream>

class Money {
public:
    explicit Money(long long cents) : cents_(cents) {}
    Money operator+(const Money& other) const { return Money{cents_ + other.cents_}; }
    bool  operator<(const Money& other)  const { return cents_ <  other.cents_; }
    bool  operator==(const Money& other) const { return cents_ == other.cents_; }
    long long cents() const { return cents_; }
private:
    long long cents_;
};

int main() {
    Money price{1250};
    Money tax{175};
    Money total = price + tax;
    std::cout << total.cents() << '\n';
    std::cout << (price < tax) << '\n';
    std::cout << (price == price) << '\n';
}

Q5. (predict) Write the three output lines.

#include <iostream>

class Ticket {
public:
    Ticket() : id_(++issued_) {}
    int id() const { return id_; }
    static int issuedCount() { return issued_; }
private:
    int id_;
    static int issued_;
};

int Ticket::issued_ = 100;

int main() {
    std::cout << Ticket::issuedCount() << '\n';
    Ticket a;
    Ticket b;
    std::cout << a.id() << ' ' << b.id() << '\n';
    std::cout << Ticket::issuedCount() << '\n';
}

Q6. (debug) The comparator makes std::sort misbehave. State why and fix it.

class Score {
public:
    explicit Score(int points) : points_(points) {}
    bool operator<(const Score& other) const { return points_ <= other.points_; }
    int points() const { return points_; }
private:
    int points_;
};

Q7. (analysis) A member operator+ lets m + 2 compile but blocks 2 + m. Explain why, and say what fixes it.

Q8. (short answer) What must operator<< return, and why does it matter?

Q9. (predict) Write the two output lines.

#include <iostream>

class Counter {
public:
    Counter& operator++() { ++n_; return *this; }
    Counter  operator++(int) { Counter old = *this; ++n_; return old; }
    int n() const { return n_; }
private:
    int n_ = 0;
};

int main() {
    Counter c;
    ++c;
    c++;
    std::cout << c.n() << '\n';
    Counter d = c++;
    std::cout << c.n() << ' ' << d.n() << '\n';
}

Q10. (design) A Money type must never hold a negative amount. Decide how it is stored and which operators it needs, and say which operator you would not provide and why.

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