constexpr_str.cpp

#include <string>
#include <cassert>
#include <array>
#include <numeric>

template<std::size_t N, char Seed = 42>
class constexpr_str {
  std::array<char, N> data_;

  template<std::size_t... Is>
  static constexpr char reduce(char const (&data)[N], std::index_sequence<Is...>) {
    return (data[Is] ^ ... ^ Seed);
  }

  template<class... Seqs>
  constexpr constexpr_str(char const (&data)[N], Seqs... seq) : data_({reduce(data, seq)...}) {
  }

  template<std::size_t... Is>
  constexpr constexpr_str(char const (&data)[N], std::index_sequence<Is...>) : constexpr_str(data, std::make_index_sequence<Is + 1>()...) {
  }

  public:
  constexpr constexpr_str(char const (&data)[N]) : constexpr_str(data, std::make_index_sequence<N>()) {
  }

  operator std::string() const {
    std::string out;
    out.resize(N - 1);
    char acc = Seed;
    for(std::size_t i = 0; i <N - 1; ++i) {
      out[i] = acc ^ data_[i];
      acc = acc ^ out[i];
    }
    assert(acc ^data_[N-1] == 0);
    return out;
  }

};

template<std::size_t N>
constexpr auto obf_(const char (&s)[N]) noexcept {
  return constexpr_str<N>{s};
}
template <typename CharT, CharT ...c>
constexpr auto operator"" _obf() noexcept {
  static_assert(std::is_same<CharT, char>::value, "on char");
  char data[sizeof...(c) + 1] = {c...};
  data[sizeof...(c) + 1] = 0;
  return obf_(data);
}

auto out0(std::size_t i) {
  constexpr auto val = "bonjour"_obf;
  return std::string(val);
}

#include <iostream>
int main() {
  std::cout << out0(1) << std::endl;
}