Is it possible to convert integral_constant to hana::string

Question

I would like to be able to generate a compile-time string that contains a compile-time integer, like this. Is it possible?

I know I can insert a single elements to the boost::string using fold_left, but I would like to add a decimal representation of the integer. Is there any library that allows that?

#include <boost/hana/string.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/plus.hpp>
namespace hana = boost::hana;
using namespace hana::literals;

int main() {
    auto a = "test_nr_"_s;
    auto b = 42_c;
    auto c = a + b; 
};

I expect to get a way to make c equal "test_nr_42"_s.

Answer 1

Unfortunately, Boost.Hana does not support converting an integer to hana::string, and implicit conversions are right out.

However, with a little constexpr and a just sprinkle of EuclideanRing we can make our own conversion function like this:

#define BOOST_HANA_CONFIG_ENABLE_STRING_UDL
#include <boost/hana.hpp>

namespace hana = boost::hana;
using namespace hana::literals;

constexpr size_t get_magnitude(size_t num) {
  unsigned i = 0;
  while (num > 0) {
    num /= 10;
    ++i;
  }
  return i;
}

template <typename X, size_t ...i>
constexpr auto to_string(X x,
                         std::index_sequence<i...>) {
  constexpr size_t mag = get_magnitude(X::value);
  return hana::string<
    (x / hana::power(hana::size_c<10>,
                     hana::size_c<mag - i - 1>) % hana::size_c<10> 
                       + hana::size_c<48>)...>{};
}

template <typename X>
constexpr auto to_string(X x) {
  return to_string(hana::size_c<static_cast<size_t>(X::value)>,
                   std::make_index_sequence<get_magnitude(X::value)>());
}

int main() {
  auto a = "test_nr_"_s;
  auto b = 42_c;
  auto c = a + to_string(b);
  static_assert(c == "test_nr_42"_s, "");
}