Chapter 44. Specialization for std::hash<void>

#include <x/hash.H>

std::unordered_map<std::string, int,
  std::hash<void>,
  std::equal_to<void>> table;

bool answer(const std::string_view &question)
{
   auto iter=table.find(question);

   return iter != table.end() && iter->second == 42;
}

Including <x/hash.H> defines a transparent specialization for std::hash.