C++20 implementation of FizzBuzz. Compile-time, templates. No loops, ifs and others.

fizzbuzz-template-compile-time.cpp

#include <iostream>

const long FIZZ_NUM = 3;
const long BUZZ_NUM = 5;

constexpr const char *FIZZ = "FIZZ";
constexpr const char *BUZZ = "BUZZ";
constexpr const char *FIZZ_BUZZ = "FIZZBUZZ";

namespace detail
{
    template<long... digits>
    struct to_chars { static const char value[]; };

    template<long... digits>
    constexpr char to_chars<digits...>::value[] = {('0' + digits)..., 0};

    template<long rem, long... digits>
    struct explode : explode<rem / 10, rem % 10, digits...> {};

    template<long... digits>
    struct explode<0, digits...> : to_chars<digits...> {};
}

template<long num>
struct num_to_string : detail::explode<num> {};

template <long D> constexpr const char *get_fizzbuzz() { return FIZZ_BUZZ; }
template <long D> constexpr const char *get_fizz() { return FIZZ; }
template <long D> constexpr const char *get_buzz() { return BUZZ; }
template <long D> constexpr const char *get_none() { return num_to_string<D>::value; }

template <long D> concept IS_FIZZ = D % FIZZ_NUM == 0;
template <long D> concept IS_BUZZ = D % BUZZ_NUM == 0;

template <long D> concept ABOVE_ZERO = D > 0;
template <long S> requires ABOVE_ZERO<S>
class FizzBuzz {
  const char* data[S];

  template <long D> requires (ABOVE_ZERO<D>)
  constexpr void init() {
    data[D - 1] = get<D>();
    init<D - 1>();
  }
  template <long D> requires (!ABOVE_ZERO<D>)
  constexpr void init() { };

  template<long D> requires (IS_FIZZ<D> && IS_BUZZ<D>)
  constexpr const char *get() { return FIZZ_BUZZ; }
  template<long D> requires IS_FIZZ<D>
  constexpr const char *get() { return FIZZ; }
  template<long D> requires IS_BUZZ<D>
  constexpr const char *get() { return BUZZ; }
  template<long D> requires (!IS_FIZZ<D> && !IS_BUZZ<D>)
  constexpr const char *get() { return num_to_string<D>::value; }

 public:
  constexpr FizzBuzz() : data() { init<S>(); }

  const char* get(const long d) const { return data[d]; }
};

int main(int argc, char *argv[]) {
  const long COUNT = 100;
  constexpr FizzBuzz<COUNT> fb = FizzBuzz<COUNT>();

  for (auto i = 0; i < COUNT; ++i) {
    std::cout << fb.get(i) << std::endl;
  }
}