Challenge 2: A Rule-of-Zero Class With a unique_ptr Member — Possible Solution
====================================================================

#include <iostream>
#include <memory>

class Owner {
public:
    std::unique_ptr<int> value;

    Owner(int v) : value(std::make_unique<int>(v)) {}
    // no destructor, no copy/move constructor, no copy/move assignment
    // written by hand anywhere -- the compiler generates all of them
};

int main() {
    Owner a(42);
    Owner b = std::move(a);  // moves the unique_ptr member correctly

    if (a.value == nullptr) {
        std::cout << "a's value is now null after the move" << std::endl;
    }
    std::cout << "b's value: " << *b.value << std::endl;

    return 0;
}

Output:

    a's value is now null after the move
    b's value: 42

WHY THIS WORKS AS AN ANSWER
------------------------------
Owner never defines a destructor, copy constructor, or move
constructor by hand -- per the Rule of Zero, its ONLY member is a
unique_ptr, an RAII type that already correctly implements move
semantics itself. The COMPILER-GENERATED move constructor for Owner
simply moves each member in turn, which for a unique_ptr member means
correctly transferring ownership (nulling out a's own copy) -- exactly
matching unique_ptr's own move behavior from cpp2-4, with zero
hand-written resource-management code in Owner itself.