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1. Which of these is a has-a relationship, the kind that composition models?

A Square is a ShapeA Car is an EngineA Car has four Wheel objectsA std::out_of_range is a std::exception

2. This Stack is meant to be last-in, first-out. What is the design problem? ``

cpp
class Stack : public std::vector<int> {
public:
    void push(int value) { push_back(value); }

    int pop() {
        int top = back();
        pop_back();
        return top;
    }
};

``

Callers can reach every std::vector method, such as insert, and break the last-in, first-out ruleThe class does not compile, because push must be constpop cannot return an int, because a stack returns its items as objectsThe class needs a destructor to free the vector it inherits

3. True or false: a Car should inherit from Engine so it can call the engine's start() method.

True, because a car can reuse the engine's start method through inheritanceFalse, because a car has an engine, so it should hold one as a member and call start() through it

4. What does this program print? ``

cpp
#include <iostream>
#include <optional>
#include <utility>

class Radio {
public:
    explicit Radio(int station) : station_{station} {}
    void play() const { std::cout << "station " << station_ << '\n'; }

private:
    int station_;
};

class Car {
public:
    void install_radio(Radio r) { radio_ = std::move(r); }
    void music() const {
        if (radio_) {
            radio_->play();
        } else {
            std::cout << "silent\n";
        }
    }

private:
    std::optional<Radio> radio_;
};

int main() {
    Car a;
    a.music();
    a.install_radio(Radio{101});
    a.music();
}

``

station 101 silentsilent station 101silentstation 101

5. The Car below lists only three wheels. Wheel has an explicit constructor that takes a position, and no default constructor. What happens? ``

cpp
class Car {
public:
    Car() : wheels_{Wheel{1}, Wheel{2}, Wheel{3}} {}

private:
    std::array<Wheel, 4> wheels_;
};

``

It compiles, and the fourth wheel gets position 0It compiles, and the program prints only three wheelsIt fails at run time with std::out_of_rangeIt fails to compile, because Wheel has no default constructor to build the fourth element

Composition