Boost C++ Libraries

Suppose you want to ask users to choose some number (an int). One of the valid responses is to choose nothing, which is represented by an uninitialized optional<int>. You want to make a histogram showing how many times each choice was made. You can use an std::map:

std::map<boost::optional<int>, int> choices;

for (int i = 0; i < LIMIT; ++i) {
  boost::optional<int> choice = readChoice();
  ++choices[choice];
}

This works because optional<T> is LessThanComparable whenever T is LessThanComparable. In this case the state of being uninitialized is treated as a yet another value of T, which is compared less than any value of T. optional<T> can also be stored as a key in std::unordered_map and std::unordered_set as it provides specializations for std::hash.