[C++][C++17] sum type

std::variant    sum types
std::visit       process the values in a std::variant

Sum types are compound types that have a range of values that is the sum of the ranges of their parts.

std::variant as a "safer union"

using ItsOk = std::variant<std::vector<int>, double>;

int main() {
  ItsOk io = std::vector{22, 44, 66}; //set the variant to vector, this constructs the internal vector
  io = 13.7; // reset to double - the internal vector is properly destroyed
  int i = std::get<std::vector<int>>(io)[2]; // There's no vector in the variant - throws an exception
}

std::ostream& operator<< (std::ostream& os, MyVariant const& v) {
  std::visit([&os](auto const& e){ os << e; }, v);
  return os;
}