constexpr itoa
書きました。
constexpr なので、配列のポインタを渡して文字列を代入する事は出来ません。
処理した結果の文字列は固定長配列として返します。
[ソース]
#include <cstddef> #include <limits> namespace ce{ template<typename Char, std::size_t N> struct c_string{ Char elems[N ? N : 1]; constexpr Char const* c_str() const{ return &elems[0]; } }; template<std::size_t... Indices> struct index_tuple{}; template< std::size_t Start, std::size_t Finish, std::size_t Step = 1, typename Acc = index_tuple<>, bool Break = (Start >= Finish) > struct index_range{ typedef Acc type; }; template< std::size_t Start, std::size_t Finish, std::size_t Step, std::size_t... Indices > struct index_range<Start, Finish, Step, index_tuple<Indices...>, false> : index_range<Start + Step, Finish, Step, index_tuple<Indices..., Start>> {}; constexpr char itoc(int n){ return 0 <= n && n <= 9 ? '0' + n : '\0'; } template<typename Char, std::size_t N, std::size_t ...Indices> constexpr c_string<Char, N> itoa_push_front_impl(Char const (&str)[N], index_tuple<Indices...>, Char c){ return {{ c, str[Indices]..., '\0' }}; } template<typename Char, std::size_t N> constexpr c_string<Char, N> itoa_push_front(Char const (&str)[N], Char c){ return itoa_push_front_impl(str, typename index_range<0, N-2>::type(), c); } template<typename Char, std::size_t N> constexpr c_string<Char, N> itoa_push_front(c_string<Char, N> const& str, Char c){ return itoa_push_front(str.elems, c); } template<typename T, typename String> constexpr String itoa_impl(T n, String const& str){ return n == 0 ? str : itoa_impl(n/10, itoa_push_front(str, itoc(n%10))); } template<typename T, typename String> constexpr auto itoa(T n, String const& str)->decltype(itoa_impl(n, str)){ return n < 0 ? itoa_push_front(itoa_impl(n*-1, str), '-') : itoa_impl(n, str); } template<typename T> constexpr c_string<char, std::numeric_limits<T>::digits10+3> itoa(T n){ return itoa(n, c_string<char, std::numeric_limits<T>::digits10+3>{{}}); } template<typename Char> constexpr bool equal(Char const* a, Char const* b){ return a[0] == Char('\0') && b[0] == Char('\0') ? true : a[0] == b[0] ? equal(a+1, b+1) : false; } } // namespace ce #include <iostream> #include <type_traits> int main(){ static constexpr auto str = ce::itoa(94581048); static_assert(ce::equal(str.c_str(), "94581048"), ""); static constexpr auto str2 = ce::itoa(-142); static_assert(ce::equal(str2.c_str(), "-142"), ""); static constexpr auto str3 = ce::itoa(-142, ce::c_string<char, 10>{{}}); static_assert(ce::equal(str3.c_str(), "-142"), ""); static constexpr auto str4 = ce::itoa(-142, ce::c_string<char, 3>{{}}); static_assert(ce::equal(str4.c_str(), "-1"), ""); static constexpr auto str5 = ce::itoa(42, ce::c_string<char, 3>{{}}); static_assert(ce::equal(str5.c_str(), "42"), ""); static constexpr auto str6 = ce::itoa(2147483647); static_assert(ce::equal(str6.c_str(), "2147483647"), ""); static constexpr auto str7 = ce::itoa(-2147483647); static_assert(ce::equal(str7.c_str(), "-2147483647"), ""); static_assert(std::is_same< decltype(ce::itoa(10)), ce::c_string<char, 12> >::value, ""); return 0; }
んーこれで大丈夫かしら。
constexpr の文字列の扱いはまだまだですね…。
