将 Linux 中的 UTF-32 宽字符转换为 UTF-16 宽字符以获得补充平面字符

Question

我们使用 ICU 在 RHEL 上部署了一个 C++ 应用程序。

我们有一种情况需要在 linux 上将 UChar* 转换为 wchar_t*。我们使用 u_strToWCS 来执行转换。

#include <iostream>
#include <wchar.h>

#include "unicode/ustring.h"

void convertUnicodeStringtoWideChar(const UChar* cuniszSource,
                                    const int32_t cunii32SourceLength,
                                    wchar_t*& rpwcharDestination,
                                    int32_t& destCapacity)
{
  UErrorCode uniUErrorCode = U_ZERO_ERROR;

  int32_t pDestLength = 0;

  rpwcharDestination     = 0;
  destCapacity = 0;

  u_strToWCS(rpwcharDestination,
             destCapacity,
             &pDestLength,
             cuniszSource,
             cunii32SourceLength,
             &uniUErrorCode);

  uniUErrorCode = U_ZERO_ERROR;
  rpwcharDestination = new wchar_t[pDestLength+1];
  if(rpwcharDestination)
  {
    destCapacity = pDestLength+1;

    u_strToWCS(rpwcharDestination,
               destCapacity,
               &pDestLength,
               cuniszSource,
               cunii32SourceLength,
               &uniUErrorCode);

    destCapacity = wcslen(rpwcharDestination);
  }
} //function ends

int main()
{
    //                     a       ä       Š       €    (          )
    UChar input[20] = { 0x0061, 0x00e4, 0x0160, 0x20ac, 0xd87e, 0xdd29, 0x0000 };
    wchar_t * output;
    int32_t outlen = 0;
    convertUnicodeStringtoWideChar( input, 6, output, outlen );
    for ( int i = 0; i < outlen; ++i )
    {
        std::cout << std::hex << output[i] << "\n";
    }
    return 0;
}

~~这适用于输入最多 65535 个字符（因为 UChar 在 linux 上在内部实现为 uint16_t）。它无法转换基本多语言平面之外的字符（例如 CJK Unified Ideographs Extension B）~~

~~关于如何执行转换有什么想法吗？~~

更新 1：好的。我看错方向了。 u_strToWCS 工作正常。出现问题是因为我需要使用 CORBA 将该宽字符串传递给 windows 上的 java 应用程序。由于linux中的wchar_t是32位的，我需要找到一种方法将32位wchar_t转换为16位wchar_t

更新2：我用过的代码可以找到

Answer 1

在 C++11 及更高版本中，此转换在标准库中，在 <codecvt> header 中。下面是一些在 UTF-16、UCS-4 和 wchar_t 之间转换的示例代码。（由于开发树中已修复的错误，它在 libstdc++ 6.4.9 上中断。）

#include <codecvt>
#include <cstdlib>
#include <cstring>
#include <cwctype>
#include <iostream>
#include <locale>
#include <vector>

using std::cout;
using std::endl;
using std::exit;
using std::memcmp;
using std::size_t;

using std::wcout;

int main(void)
{
  constexpr char16_t msg_utf16[] = u"¡Hola, mundo! \U0001F600"; // Shouldn't assume endianness.
  constexpr wchar_t msg_w[] = L"¡Hola, mundo! \U0001F600";
  constexpr char32_t msg_utf32[] = U"¡Hola, mundo! \U0001F600";
  constexpr char msg_utf8[] = u8"¡Hola, mundo! \U0001F600";

  // May vary from OS to OS>  "" is the most standard, but might require, e.g. "en_US.utf8".
  constexpr char locale_name[] = "";
  std::locale::global(std::locale(locale_name)); //
  wcout.imbue(std::locale());

  const std::codecvt_utf16<wchar_t, 0x1FFFF, std::little_endian> converter_w;
  const size_t max_len = sizeof(msg_utf16);
  std::vector<char> out(max_len);
  std::mbstate_t state;
  const wchar_t* from_w = nullptr;
  char* to_next = nullptr;

  converter_w.out( state, msg_w, msg_w+sizeof(msg_w)/sizeof(wchar_t), from_w, out.data(), out.data() + out.size(), to_next );

  
  if (memcmp( msg_utf8, out.data(), sizeof(msg_utf8) ) == 0 ) {
    wcout << L"std::codecvt_utf16<wchar_t> converts to UTF-8, not UTF-16!" << endl;
  } else if ( memcmp( msg_utf16, out.data(), max_len ) != 0 ) {
    wcout << L"std::codecvt_utf16<wchar_t> conversion not equal!" << endl;
  } else {
    wcout << L"std::codecvt_utf16<wchar_t> conversion is correct." << endl;
  }
  out.clear();
  out.resize(max_len);

  const std::codecvt_utf16<char32_t, 0x1FFFF, std::little_endian> converter_u32;
  const char32_t* from_u32 = nullptr;
  converter_u32.out( state, msg_utf32, msg_utf32+sizeof(msg_utf32)/sizeof(char32_t), from_u32, out.data(), out.data() + out.size(), to_next );

  if ( memcmp( msg_utf16, out.data(), max_len ) != 0 ) {
    wcout << L"std::codecvt_utf16<char32_t> conversion not equal!" << endl;
  } else {
    wcout << L"std::codecvt_utf16<char32_t> conversion is correct." << endl;
  }

  wcout << msg_w << endl;
  return EXIT_SUCCESS;
}

这两个方面将在 C++17 中弃用，但并非 <codecvt> 中的所有方面都是。特别是，标准库将支持 std::codecvt<char, char, std::mbstate_t>、std::codecvt<char16_t, char, std::mbstate_t>、std::codecvt<char32_t, char, std::mbstate_t> 和 std::codecvt<wchar_t, char, std::mbstate_t>.

您不会在 Linux 上深入了解此 UTF-16 数据的来源，但这可能会建议一种方法。如果要处理文件，您可以在流上使用 imbue() 来转换读取和写入的数据，如果要处理 Qt 框架，QString 和 QTextCodex提供转换功能。不过，ICU 应该支持整个范围的 UTF-16。

更新 1

真正的问题是如何从宽字符串转换为 UTF-16。我的示例就是这样做的，但是如果您想使用 ICU，它有 u_strFromWCS()、u_strFromUTF32() 和 UnicodeString::fromUTF32().

如果您更喜欢 ICU 而不是 STL 的原因是 STL 的转换器方面声称是 locale-independent，请注意这些 ICU 转换器功能也都声称是 locale-independent。这是因为不同 UTF 编码之间的转换是完全算法化的并且独立于语言环境！（排序顺序和大小写映射之类的其他东西不是，但就是这样。）事实上，STL 确实允许您从特定语言环境请求转换器方面，如果您愿意，可以使用 locale::use_facet<codecvt<...>>()，并且这在C++17。然而，只有与 UTF-8 之间的转换需要以这种方式实现。 “In addition, every locale object constructed in a C++ program implements its own (locale-specific) versions of these four specializations.” 在我的测试中，库的现有实现不支持 locale().use_facet<std_codecvt<wchar_t,char16_t,mbstate_t>>()。

更新 2

我正在重新发布手册 wchar_t 到 utf_16 转换器 from my answer here。它需要一个 std::wstring 和 returns 一个 std::u16string，但该算法可以很容易地适应任何其他容器。不过，u16string 至少与任何其他需要动态内存的数据结构一样高效。

你可能想做的一个改变是，我为最坏的可能情况分配了足够的内存，给定输入字符串的长度，然后是 shrink_to_fit()。这应该不会比最初像 UTF-32 那样对字符串进行编码浪费更多的内存。但是，none 的数据极不可能出现在 BMP 中，因此您可以进行初始传递以计算转换需要多少内存，或者假设 [= 中的代理项对很少55=] 使用并接受必须调整大小和复制目标数组的不太可能的可能性。

#include <cassert>
#include <cwctype>
#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <locale>
#include <string>

#if _WIN32 || _WIN64
// Windows needs a little non-standard magic for this to work.
#include <io.h>
#include <fcntl.h>
#include <locale.h>
#endif

using std::size_t;

void init_locale(void)
// Does magic so that wcout can work.
{
#if _WIN32 || _WIN64
  // Windows needs a little non-standard magic.
  constexpr char cp_utf16le[] = ".1200";
  setlocale( LC_ALL, cp_utf16le );
  _setmode( _fileno(stdout), _O_U16TEXT );
#else
  // The correct locale name may vary by OS, e.g., "en_US.utf8".
  constexpr char locale_name[] = "";
  std::locale::global(std::locale(locale_name));
  std::wcout.imbue(std::locale());
#endif
}

std::u16string make_u16string( const std::wstring& ws )
/* Creates a UTF-16 string from a wide-character string.  Any wide characters
 * outside the allowed range of UTF-16 are mapped to the sentinel value U+FFFD,
 * per the Unicode documentation. (http://www.unicode.org/faq/private_use.html
 * retrieved 12 March 2017.) Unpaired surrogates in ws are also converted to
 * sentinel values.  Noncharacters, however, are left intact.  As a fallback,
 * if wide characters are the same size as char16_t, this does a more trivial
 * construction using that implicit conversion.
 */
{
  /* We assume that, if this test passes, a wide-character string is already
   * UTF-16, or at least converts to it implicitly without needing surrogate
   * pairs.
   */
  if ( sizeof(wchar_t) == sizeof(char16_t) ) {
    return std::u16string( ws.begin(), ws.end() );
  } else {
    /* The conversion from UTF-32 to UTF-16 might possibly require surrogates.
     * A surrogate pair suffices to represent all wide characters, because all
     * characters outside the range will be mapped to the sentinel value
     * U+FFFD.  Add one character for the terminating NUL.
     */
    const size_t max_len = 2 * ws.length() + 1;
    // Our temporary UTF-16 string.
    std::u16string result;

    result.reserve(max_len);

    for ( const wchar_t& wc : ws ) {
      const std::wint_t chr = wc;

      if ( chr < 0 || chr > 0x10FFFF || (chr >= 0xD800 && chr <= 0xDFFF) ) {
        // Invalid code point.  Replace with sentinel, per Unicode standard:
        constexpr char16_t sentinel = u'\uFFFD';
        result.push_back(sentinel);
      } else if ( chr < 0x10000UL ) { // In the BMP.
        result.push_back(static_cast<char16_t>(wc));
      } else {
        const char16_t leading = static_cast<char16_t>( 
          ((chr-0x10000UL) / 0x400U) + 0xD800U );
        const char16_t trailing = static_cast<char16_t>( 
          ((chr-0x10000UL) % 0x400U) + 0xDC00U );

        result.append({leading, trailing});
      } // end if
    } // end for

   /* The returned string is shrunken to fit, which might not be the Right
    * Thing if there is more to be added to the string.
    */
    result.shrink_to_fit();

    // We depend here on the compiler to optimize the move constructor.
    return result;
  } // end if
  // Not reached.
}

int main(void)
{
  static const std::wstring wtest(L"☪☮∈✡℩☯✝ \U0001F644");
  static const std::u16string u16test(u"☪☮∈✡℩☯✝ \U0001F644");
  const std::u16string converted = make_u16string(wtest);

  init_locale();

  std::wcout << L"sizeof(wchar_t) == " << sizeof(wchar_t) << L".\n";

  for( size_t i = 0; i <= u16test.length(); ++i ) {
    if ( u16test[i] != converted[i] ) {
      std::wcout << std::hex << std::showbase
                 << std::right << std::setfill(L'0')
                 << std::setw(4) << (unsigned)converted[i] << L" ≠ "
                 << std::setw(4) << (unsigned)u16test[i] << L" at "
                 << i << L'.' << std::endl;
      return EXIT_FAILURE;
    } // end if
  } // end for

  std::wcout << wtest << std::endl;

  return EXIT_SUCCESS;
}

Answer 2

以下是将UTF-32编码的宽字符转为UTF-16的代码

//Function to convert a Unicode string from platform-specific "wide characters" (wchar_t) to UTF-16.
void ConvertUTF32ToUTF16(wchar_t* source,
                         const uint32_t sourceLength,
                         wchar_t*& destination,
                         uint32_t& destinationLength)
{

  wchar_t wcharCharacter;
  uint32_t uniui32Counter = 0;

  wchar_t* pwszDestinationStart = destination;
  wchar_t* sourceStart = source;

  if(0 != destination)
  {
    while(uniui32Counter < sourceLength)
    {
      wcharCharacter = *source++;
      if(wcharCharacter <= 0x0000FFFF)
      {
        /* UTF-16 surrogate values are illegal in UTF-32
           0xFFFF or 0xFFFE are both reserved values */
        if(wcharCharacter >= 0xD800 && 
           wcharCharacter <= 0xDFFF)
        {
          *destination++ = 0x0000FFFD;
          destinationLength += 1;
        }
        else
        {
          /* source is a BMP Character */
          destinationLength += 1;
          *destination++ = wcharCharacter;
        }
      }
      else if(wcharCharacter > 0x0010FFFF)
      {
        /* U+10FFFF is the largest code point of Unicode Character Set */
        *destination++ = 0x0000FFFD;
        destinationLength += 1;
      }
      else
      {
        /* source is a character in range 0xFFFF - 0x10FFFF */
        wcharCharacter -= 0x0010000UL;
        *destination++ = (wchar_t)((wcharCharacter >> 10) + 0xD800);
        *destination++ = (wchar_t)((wcharCharacter & 0x3FFUL) + 0xDC00);
        destinationLength += 2;
      }

      ++uniui32Counter;
    }

    destination = pwszDestinationStart;
    destination[destinationLength] = '[=10=]';
  }

  source = sourceStart;
} //function ends

将 Linux 中的 UTF-32 宽字符转换为 UTF-16 宽字符以获得补充平面字符

Convert UTF-32 wide char to UTF-16 wide char in Linux for Supplementary Plane characters

c++

linux

wchar-t

icu

更新 1

更新 2