Challenge 2: Observing a shared_ptr's Reference Count Change — Possible Solution
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#include <iostream>
#include <memory>

int main() {
    std::shared_ptr<int> a = std::make_shared<int>(42);
    std::cout << "After creating a: " << a.use_count() << std::endl;

    std::shared_ptr<int> b = a;
    std::cout << "After creating b: " << a.use_count() << std::endl;

    {
        std::shared_ptr<int> c = a;
        std::cout << "After creating c: " << a.use_count() << std::endl;
    }  // c goes out of scope here

    std::cout << "After c's scope ends: " << a.use_count() << std::endl;

    return 0;
}

Output:

    After creating a: 1
    After creating b: 2
    After creating c: 3
    After c's scope ends: 2

WHY THIS WORKS AS AN ANSWER
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Each new shared_ptr copy that jointly owns the same object increments
the shared reference count by one (1 -> 2 -> 3), and when c goes out of
scope at the closing brace, its own destructor decrements the count
back down to 2 -- confirming shared_ptr's reference-counting mechanism
directly, and showing that the underlying int is only ever actually
deleted once the count would reach zero (which never happens here,
since a and b are both still alive).