Challenge 2: A Pair<T> Class Template With sum() — Possible Solution
====================================================================

#include <iostream>

template<typename T>
class Pair {
public:
    T first;
    T second;

    Pair(T a, T b) : first(a), second(b) {}

    T sum() {
        return first + second;
    }
};

int main() {
    Pair<int> intPair(3, 4);
    Pair<double> doublePair(2.5, 1.5);

    std::cout << intPair.sum() << std::endl;
    std::cout << doublePair.sum() << std::endl;

    return 0;
}

Output:

    7
    4

WHY THIS WORKS AS AN ANSWER
------------------------------
Pair<int> and Pair<double> are two distinct instantiations of the same
class template -- exactly the chapter's own Box<T> pattern, here
extended with a real method. Each instantiation genuinely gets its own
compiled class with first/second of the correct concrete type, and
sum() correctly adds either two ints or two doubles depending on which
instantiation is being used.