Boost.Hana：将值的元组转换为 constexpr 上下文中相应类型的元组

Question

Recently I had to write a transformation from a tuple of values to the (constexpr) tuple of the types of the values.

namespace hana = boost::hana;

// we have this (not neccessarily constexpr)
auto value_tuple = hana::make_tuple(1, 'a');

// we want this (must be constexpr)
constexpr auto type_tuple = hana::make_tuple(hana::type_c<int>, hana::type_c<char>);

Answer 1

The solution I came up with uses hana::transform and hana::typeid_ in an unevaluated context to allow for the result to be constexpr. It relies on hana::type being default-constructible (as documented in hana::type_c ) and hana::tuple being default-constructible if all of the element types are.

constexpr auto to_types = [](auto values) {
  namespace hana = boost::hana;
  return decltype(hana::transform(
      hana::to_tuple(std::declval<decltype(values)>()), hana::typeid_)){};
};

If you know that values can be used with hana::transform directly then hana::to_tuple can be dropped (eg. std::array cannot be transformed directly).

This can be used to implement finding the common element type of a hana::tuple :

constexpr auto to_common_type = [](auto types) {
  namespace hana = boost::hana;
  return hana::unpack(types, hana::template_<std::common_type_t>);
};

using value_type =
    typename decltype(to_common_type(to_types(values)))::type;