Question

我想在C ++中将wstring转换为u16string。

我可以将wstring转换为字符串，或反转。但我不知道如何转换为u16string。

u16string CTextConverter::convertWstring2U16(wstring str)

{

        int iSize;
        u16string szDest[256] = {};
        memset(szDest, 0, 256);
        iSize = WideCharToMultiByte(CP_UTF8, NULL, str.c_str(), -1, NULL, 0,0,0);

        WideCharToMultiByte(CP_UTF8, NULL, str.c_str(), -1, szDest, iSize,0,0);
        u16string s16 = szDest;
        return s16;
}

WideCharToMultiByte中的错误（CP_UTF8，NULL，str.c_str（）， - 1，szDest，iSize，0,0）;＆＃39; s szDest。 u16string的原因无法与LPSTR一起使用。

如何修复此代码？

Answer 1

对于与平台无关的解决方案，请参阅this answer。

如果您只需要 Windows平台的解决方案，则以下代码就足够了：

std::wstring wstr( L"foo" );
std::u16string u16str( wstr.begin(), wstr.end() );

在Windows平台上，std::wstring可与std::u16string互换，因为sizeof(wstring::value_type) == sizeof(u16string::value_type)并且都是UTF-16（小端）编码。

wstring::value_type = wchar_t
u16string::value_type = char16_t

唯一的区别是wchar_t已签名，而char16_t是无符号的，因此您只需要进行符号转换，这可以使用带有迭代器对的w16string构造函数执行作为参数。此构造函数将隐式地将wchar_t转换为char16_t。

完整示例控制台应用程序：

#include <windows.h>
#include <string>

int main()
{
    static_assert( sizeof(std::wstring::value_type) == sizeof(std::u16string::value_type),
        "std::wstring and std::u16string are expected to have the same character size" );

    std::wstring wstr( L"foo" );
    std::u16string u16str( wstr.begin(), wstr.end() );

    // The u16string constructor performs an implicit conversion like:
    wchar_t wch = L'A';
    char16_t ch16 = wch;

    // Need to reinterpret_cast because char16_t const* is not implicitly convertible
    // to LPCWSTR (aka wchar_t const*).
    ::MessageBoxW( 0, reinterpret_cast<LPCWSTR>( u16str.c_str() ), L"test", 0 );

    return 0;
}

Answer 2

更新

我原本以为标准版本不起作用，但事实上这只是因为Visual C ++和libstdc ++ 3.4.21运行时库中的错误。它适用于clang++ -std=c++14 -stdlib=libc++。这是一个测试标准方法是否适用于您的编译器的版本：

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

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

using std::wcout;

#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
}

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";

  init_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;
}

上

游戏稍晚，但是这里还有一个版本可以另外检查wchar_t是否为32位（就像在Linux上一样），如果是，则执行代理对转换。我建议将此源保存为带有BOM的UTF-8。 Here is a link to it on ideone.

#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;
}

脚注

由于有人问：我建议使用带有BOM的UTF-8的原因是某些编译器（包括MSVC 2015）会假设源文件是根据当前代码页编码的，除非有BOM或您指定了编码命令行。不幸的是，没有编码适用于所有工具链，但是我使用的每个工具都足够现代，可以支持C ++ 14。

如何将wstring转换为u16string？

2 个答案:

更新

上

脚注