'Convert between std::u8string and std::string
C++20 added char8_t and std::u8string for UTF-8. However, there is no UTF-8 version of std::cout and OS APIs mostly expect char and execution character set. So we still need a way to convert between UTF-8 and execution character set.
I was rereading a char8_t paper and it looks like the only way to convert between UTF-8 and ECS is to use std::c8rtomb and std::mbrtoc8 functions. However, their API is extremely confusing. Can someone provide an example code?
Solution 1:[1]
UTF-8 "support" in C++20 seems to be a bad joke.
The only UTF functionality in the Standard Library is support for strings and string_views (std::u8string, std::u8string_view, std::u16string, ...). That is all. There is no Standard Library support for UTF coding in regular expressions, formatting, file i/o and so on.
In C++17 you can--at least--easily treat any UTF-8 data as 'char' data, which makes usage of std::regex, std::fstream, std::cout, etc. possible without loss of performance.
In C++20 things will change. You cannot longer write for example std::string text = u8"..."; It will be impossible to write something like
std::u8fstream file; std::u8string line; ... file << line;
since there is no std::u8fstream.
Even the new C++20 std::format does not support UTF at all, because all necessary overloads are simply missing. You cannot write
std::u8string text = std::format(u8"...{}...", 42);
To make matters worse, there is no simple casting (or conversion) between std::string and std::u8string (or even between const char* and const char8_t*). So if you want to format (using std::format) or input/output (std::cin, std::cout, std::fstream, ...) UTF-8 data, you have to internally copy all strings. - That will be an unnecessary performance killer.
Finally, what use will UTF have without input, output, and formatting?
Solution 2:[2]
At present, std::c8rtomb and std::mbrtoc8 are the the only interfaces provided by the standard that enable conversion between the execution encoding and UTF-8. The interfaces are awkward. They were designed to match pre-existing interfaces like std::c16rtomb and std::mbrtoc16. The wording added to the C++ standard for these new interfaces intentionally matches the wording in the C standard for the pre-existing related functions (hopefully these new functions will eventually be added to C; I still need to pursue that). The intent in matching the C standard wording, as confusing as it is, is to ensure that anyone familiar with the C wording recognizes that the char8_t interfaces work the same way.
cppreference.com has some examples for the UTF-16 versions of these functions that should be useful for understanding the char8_t variants.
Solution 3:[3]
The common answer given from C++ authorities at the yearly CppCon convention (like in 2018 and 2019) was that should you pick your own UTF8 library to do so. There are all kinds of flavours just pick the one you like. There is still embarrassing little understanding and support for unicode on the C++ side.
Some people hope there will be something in C++23 but we don't even have an official working group so far.
Solution 4:[4]
Update 2022 APR 19
// warnings but prints ok on LINUX
// g++ prog.cc -Wall -Wextra -std=c++2a
//
// clang++ prog.cc -Wall -Wextra -std=c++2a
// lot of warnings but prints OK on LINUX
//
#include <cassert>
#include <clocale>
#include <cstdio>
#include <cstdlib> // MB_CUR_MAX
#include <cuchar>
#undef P_
#undef P
#define P_(F_, X_) printf("\n%4d : %32s => " F_, __LINE__, #X_, (X_))
#define P(F_, X_) P_(F_, X_)
/*
using mbstate_t = ... see description ...
using size_t = ... see description ...
in the standard but not implemented yet by any of the three
size_t mbrtoc8(char8_t* pc8, const char* s, size_t n, mbstate_t* ps);
size_t c8rtomb(char* s, char8_t c8, mbstate_t* ps);
*/
namespace {
constexpr inline auto bad_size = ((size_t)-1);
// https://en.wikipedia.org/wiki/UTF-8
// a compile time constant not intrinsic function
constexpr inline int UTF8_CHAR_MAX_BYTES = 4;
#ifdef STANDARD_CUCHAR_IMPLEMENTED
template <size_t N>
auto char_star(const char8_t (&in)[N]) noexcept {
mbstate_t state;
constexpr static int out_size = (UTF8_CHAR_MAX_BYTES * N) + 1;
struct {
char data[out_size];
} out = {{0}};
char* one_char = out.data;
for (size_t rc, n = 0; n < N; ++n) {
rc = c8rtomb(one_char, in[n], &state);
if (rc == bad_size) break;
one_char += rc;
}
return out;
}
#endif // STANDARD_CUCHAR_IMPLEMENTED
template <size_t N>
auto char_star(const char16_t (&in)[N]) noexcept {
mbstate_t state;
constexpr static int out_size = (UTF8_CHAR_MAX_BYTES * N) + 1;
struct final {
char data[out_size];
} out = {{0}};
char* one_char = out.data;
for (size_t rc, n = 0; n < N; ++n) {
rc = c16rtomb(one_char, in[n], &state);
if (rc == bad_size) break;
one_char += rc;
}
return out;
}
template <size_t N>
auto char_star(const char32_t (&in)[N]) noexcept {
mbstate_t state;
constexpr static int out_size = (UTF8_CHAR_MAX_BYTES * N) + 1;
struct final {
char data[out_size];
} out = {{0}};
char* one_char = out.data;
for (size_t rc, n = 0; n < N; ++n) {
rc = c32rtomb(one_char, in[n], &state);
if (rc == bad_size) break;
one_char += rc;
}
return out;
}
} // namespace
#define KATAKANA "???"
#define KATAKANA8 u8"???"
#define KATAKANA16 u"???"
#define KATAKANA32 U"???"
int main(void) {
P("%s", KATAKANA); // const char *
// lot of warnings but ok output
P("%s", KATAKANA8); // const char8_t *
/*
garbled or no output
P( "%s", KATAKANA16 ); // const char16_t *
P( "%s" , KATAKANA32 ); // const char32_t *
*/
setlocale(LC_ALL, "en_US.utf8");
// no can do as there is no standard <cuchar> yet
// P( "%s", char_star(KATAKANA8).data ); // const char8_t *
P("%s", char_star(KATAKANA16).data); // const char16_t *
P("%s", char_star(KATAKANA32).data); // const char32_t *
}
Update 2021 MAR 19
Few things have (not) happened. __STDC_UTF_8__ is no more and <cuchar> is still not implemented by any of "the Three".
Probably much better code matching this thread is HERE.
Update 2020 MAR 17
std::c8rtomb and std::mbrtoc8 are not yet provided.
2019 NOV
std::c8rtomb and std::mbrtoc8 are not yet provided, by the future C++20 ready compilers made by "The 3", to enable the conversion between the execution encoding and UTF-8. They are described in the C++20 standard.
It might be subjective, but c8rtomb() is not an "awkward" interface, to me.
// g++ prog.cc -std=gnu++2a
// clang++ prog.cc -std=c++2a
#include <stdio.h>
#include <clocale>
#ifndef __clang__
#include <cuchar>
#else
// clang has no <cuchar>
#include <uchar.h>
#endif
#include <climits>
template<size_t N>
void u32sample( const char32_t (&str32)[N] )
{
#ifndef __clang__
std::mbstate_t state{};
#else
mbstate_t state{};
#endif
char out[MB_LEN_MAX]{};
for(char32_t const & c : str32)
{
#ifndef __clang__
/*std::size_t rc =*/ std::c32rtomb(out, c, &state);
#else
/* std::size_t rc =*/ ::c32rtomb(out, c, &state);
#endif
printf("%s", out ) ;
}
}
#ifdef __STDC_UTF_8__
template<size_t N>
void u8sample( const char8_t (& str8)[N])
{
std::mbstate_t state{};
char out[MB_LEN_MAX]{};
for(char8_t const & c : str8)
{
/* std::size_t rc = */ std::c8rtomb(out, c, &state);
printf("%s", out ) ;
}
}
#endif // __STDC_UTF_8__
int main () {
std::setlocale(LC_ALL, "en_US.utf8");
#ifdef __linux__
printf("\nLinux like OS, ") ;
#endif
printf(" Compiler %s\n", __VERSION__ ) ;
printf("\nchar32_t *, Converting to 'char *', and then printing --> " ) ;
u32sample( U"????" ) ;
#ifdef __STDC_UTF_8__
printf("\nchar8_t *, Converting to 'char *', and then printing --> " ) ;
u8sample( u8"????" ) ;
#else
printf("\n\n__STDC_UTF_8__ is not defined, can not use char8_t");
#endif
printf("\n\nDone ..." ) ;
return 42;
}
I have commented out and documented, lines which do not compile as of today.
Solution 5:[5]
VS 2019
ostream& operator<<(ostream& os, const u8string& str)
{
os << reinterpret_cast<const char*>(str.data());
return os;
}
To set console to UTF-8 use https://github.com/MicrosoftDocs/cpp-docs/issues/1915#issuecomment-589644386
Solution 6:[6]
AFAIK C++ doesn't yet provide facilities for such conversion. However, I would recommend using std::u8string in the first place because it is poorly supported in the standard and not supported by any system APIs at all (and will likely never be because of compatibility reasons). On most platforms normal char strings are already UTF-8 and on Windows with MSVC you can compile with /utf-8 which will give you portable Unicode support on major operating systems.
Solution 7:[7]
Here's the code that should be conforming to C++20. Since no compiler currently (March 2020) implements conversion functions defined in the paper, I decided not to constrain myself with what is currently implemented and use full spec of C++20. So instead of taking std::basic_string or std::basic_string_view I take ranges of code units. The return value is less general but it is trivial to change it to take output range instead. This is left as an exercise to the reader.
/// \brief Converts the range of UTF-8 code units to execution encoding.
/// \tparam R Type of the input range.
/// \param[in] input Input range.
/// \return std::string in the execution encoding.
/// \throw std::invalid_argument If input sequence is ill-formed.
/// \note This function depends on the global locale.
template <std::ranges::input_range R>
requires std::same_as<std::ranges::range_value_t<R>, char8_t>
std::string ToECSString(R&& input)
{
std::string output;
char temp_buffer[MB_CUR_MAX];
std::mbstate_t mbstate{};
auto i = std::ranges::begin(input);
auto end = std::ranges::end(input);
for (; i != end; ++i)
{
std::size_t result = std::c8rtomb(temp_buffer, *i, &mbstate);
if (result == -1)
{
throw std::invalid_argument{"Ill-formed UTF-8 sequence."};
}
output.append(temp_buffer, temp_buffer + result);
}
return output;
}
/// \brief Converts the input range of code units in execution encoding to
/// UTF-8.
/// \tparam R Type of the input range.
/// \param[in] input Input range.
/// \return std::u8string containing UTF-8 code units.
/// \throw std::invalid_argument If input sequence is ill-formed or does not end
/// at the scalar value boundary.
/// \note This function depends on the global C locale.
template <std::ranges::input_range R>
requires std::same_as<std::ranges::range_value_t<R>, char>
std::u8string ToUTF8String(R&& input)
{
std::u8string output;
char8_t temp_buffer;
std::mbstate_t mbstate{};
std::size_t result;
auto i = std::ranges::begin(input);
auto end = std::ranges::end(input);
while (i != end)
{
result = std::mbrtoc8(&temp_buffer, std::to_address(i), 1, &mbstate);
switch (result)
{
case 0:
{
++i;
break;
}
case std::size_t(-3):
{
break;
}
case std::size_t(-2):
{
++i;
break;
}
case std::size_t(-1):
{
throw std::invalid_argument{"Invalid input sequence."};
}
default:
{
std::ranges::advance(i, result);
break;
}
}
if (result != std::size_t(-2))
{
output.append(1, temp_buffer);
}
}
if (result == -2)
{
throw std::invalid_argument{
"Code unit sequence does not end at the scalar value "
"boundary."};
}
return output;
}
/// \brief Converts the contiguous range of code units in execution encoding to
/// UTF-8.
/// \tparam R Type of the contiguous range.
/// \param[in] input Input range.
/// \return std::u8string containing UTF-8 code units.
/// \throw std::invalid_argument If input sequence is ill-formed or does not end
/// at the scalar value boundary.
/// \note This function depends on the global C locale.
template <std::ranges::contiguous_range R>
requires std::same_as<std::ranges::range_value_t<R>, char>
std::u8string ToUTF8String(R&& input)
{
std::u8string output;
char8_t temp_buffer;
std::mbstate_t mbstate{};
std::size_t offset = 0;
std::size_t size = std::ranges::size(input);
while (offset != size)
{
std::size_t result = std::mbrtoc8(&temp_buffer,
std::ranges::data(input) + offset, size - offset, &mbstate);
switch (result)
{
case 0:
{
++offset;
break;
}
case std::size_t(-3):
{
break;
}
case std::size_t(-2):
{
throw std::invalid_argument{
"Input sequence does not end at the scalar value "
"boundary."};
}
case std::size_t(-1):
{
throw std::invalid_argument{"Invalid input sequence."};
}
default:
{
offset += result;
break;
}
}
output.append(1, temp_buffer);
}
return output;
}
Sources
This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.
Source: Stack Overflow
| Solution | Source |
|---|---|
| Solution 1 | Galik |
| Solution 2 | Tom Honermann |
| Solution 3 | |
| Solution 4 | |
| Solution 5 | YShmidt |
| Solution 6 | vitaut |
| Solution 7 |