charconv-ext

charconv-ext is a single-header library which provides to_chars and from_chars functions that operate on 128-bit integers. These have the same interface as std::to_chars and std::from_chars. It also provides support for _BitInt (Clang compiler extension) for up to _BitInt(128).

For example, it can be used like:

#include <array>
#include <print>
#include "charconv_ext/charconv_ext.hpp"

int main() {
  std::array<char, 256> buffer;
  auto value = __int128(1) << 127;
  const auto [p, ec] =
    charconv_ext::to_chars(buffer.data(), buffer.data() + buffer.size(), value);
  std::println(std::string_view(buffer.data(), p));
}

This prints:

-170141183460469231731687303715884105728

The example could have also been written to use _BitInt(128), with the same output produced.

Note

charconv_ext::to_chars and charconv_ext::from_chars also accept any other integer type because they pull in the std:: versions via using-declaration. This means you don't need to special-case __int128 in generic code and always call charconv_ext::to_chars for integers.

Interface

namespace charconv_ext {

template <std::size_t N>
  using bit_int = _BitInt(N); // optional
template <std::size_t N>
  using bit_uint = unsigned _BitInt(N); // optional

}

The bit_int and bit_uint alias templates provide access to the compiler's _BitInt type, without triggering warnings such as -Wbit-int-extension (due to use of compiler extensions). These alias templates are only defined if the compiler supports _BitInt.

std::to_chars_result charconv_ext::to_chars(
  char* first,
  char* last,
  /* integer-type */ value,
  int base = 10
);

Effects: Equivalent to std::to_chars(first, last, value, base), except that __int128 and unsigned __int128 are also supported.

template <std::size_t N>
std::to_chars_result charconv_ext::to_chars( // optional
  char* first,
  char* last,
  bit_int<N> value,
  int base = 10
);

Mandates: N ≤ 128.

Effects: Equivalent to to_chars(first, last, int_type(value), base), where to_chars is std::to_chars or charconv_ext::to_chars, and where int_type is int_least8_t, int_least16_t, int_least32_t, int_least64_t, or int128_t, whichever first is at least N bits wide.

template <std::size_t N>
std::to_chars_result charconv_ext::to_chars( // optional
  char* first,
  char* last,
  bit_uint<N> value,
  int base = 10
);

Mandates: N ≤ 128.

Effects: Equivalent to to_chars(first, last, int_type(value), base), where to_chars is std::to_chars or charconv_ext::to_chars, and where int_type is uint_least8_t, uint_least16_t, uint_least32_t, uint_least64_t, or uint128_t, whichever first is at least N bits wide.

std::from_chars_result charconv_ext::from_chars(
  const char* first,
  const char* last,
  /* integer-type */& value,
  int base = 10
);

Effects: Equivalent to std::from_chars(first, last, value, base), except that __int128 and unsigned __int128 are also supported.

template <std::size_t N>
std::to_chars_result charconv_ext::from_chars( // optional
  char* first,
  char* last,
  bit_int<N>& value,
  int base = 10
);

Mandates: N ≤ 128.

Effects: Equivalent to from_chars(first, last, int_type(value), base), where from_chars is std::from_chars or charconv_ext::from_chars, and where int_type is int_least8_t, int_least16_t, int_least32_t, int_least64_t, or int128_t, whichever first is at least N bits wide.

template <std::size_t N>
std::to_chars_result charconv_ext::from_chars( // optional
  char* first,
  char* last,
  bit_uint<N>& value,
  int base = 10
);

Mandates: N ≤ 128.

Effects: Equivalent to from_chars(first, last, int_type(value), base), where from_chars is std::from_chars or charconv_ext::from_chars, and where int_type is uint_least8_t, uint_least16_t, uint_least32_t, uint_least64_t, or uint128_t, whichever first is at least N bits wide.