std::codecvt_utf16(3)

NAME
std::codecvt_utf16 - std::codecvt_utf16

Synopsis
   Defined in header <codecvt>
   template<
       class Elem, (since C++11)
       unsigned long Maxcode = 0x10ffff, (deprecated in C++17)
       std::codecvt_mode Mode = (std::codecvt_mode)0 > (removed in C++26)
   class codecvt_utf16
       : public std::codecvt<Elem, char, std::mbstate_t>;
   std::codecvt_utf16 is a std::codecvt facet which encapsulates conversion between a
   UTF-16 encoded byte string and UCS-2 or UTF-32 character string (depending on the
   type of Elem). This std::codecvt facet can be used to read and write UTF-16 files in
   binary mode.
   UCS-2 is the same encoding as UTF-16, except that it encodes scalar (since C++23)
   values in the range U+0000-U+FFFF (Basic Multilingual Plane) only.

Template Parameters
   Elem - either char16_t, char32_t, or wchar_t
   Maxcode - the largest value of Elem that this facet will read or write without error
   Mode - a constant of type std::codecvt_mode

Member functions
   constructor constructs a new codecvt_utf16 facet
                 (public member function)
   destructor destroys a codecvt_utf16 facet
                 (public member function) std::codecvt_utf16::codecvt_utf16
   explicit codecvt_utf16( std::size_t refs = 0 );
   Constructs a new std::codecvt_utf16 facet, passes the initial reference counter refs
   to the base class.

Parameters
   refs - the number of references that link to the facet std::codecvt_utf16::~codecvt_utf16
   ~codecvt_utf16();
   Destroys the facet. Unlike the locale-managed facets, this facet's destructor is
   public. Inherited from std::codecvt

Member types
   Member name Definition
   intern_type internT
   extern_type externT
   state_type stateT

Member objects
   Member name Type
   id (static) std::locale::id

Member functions
                 invokes do_out
   out (public member function of std::codecvt<InternT,ExternT,StateT>)
                 invokes do_in
   in (public member function of std::codecvt<InternT,ExternT,StateT>)
                 invokes do_unshift
   unshift (public member function of std::codecvt<InternT,ExternT,StateT>)
                 invokes do_encoding
   encoding (public member function of std::codecvt<InternT,ExternT,StateT>)
                 invokes do_always_noconv
   always_noconv (public member function of std::codecvt<InternT,ExternT,StateT>)
                 invokes do_length
   length (public member function of std::codecvt<InternT,ExternT,StateT>)
                 invokes do_max_length
   max_length (public member function of std::codecvt<InternT,ExternT,StateT>)

Protected member functions
                    converts a string from InternT to ExternT, such as when writing to
   do_out file
   [virtual] (virtual protected member function of
                    std::codecvt<InternT,ExternT,StateT>)
                    converts a string from ExternT to InternT, such as when reading
   do_in from file
   [virtual] (virtual protected member function of
                    std::codecvt<InternT,ExternT,StateT>)
                    generates the termination character sequence of ExternT characters
   do_unshift for incomplete conversion
   [virtual] (virtual protected member function of
                    std::codecvt<InternT,ExternT,StateT>)
                    returns the number of ExternT characters necessary to produce one
   do_encoding InternT character, if constant
   [virtual] (virtual protected member function of
                    std::codecvt<InternT,ExternT,StateT>)
                    tests if the facet encodes an identity conversion for all valid
   do_always_noconv argument values
   [virtual] (virtual protected member function of
                    std::codecvt<InternT,ExternT,StateT>)
                    calculates the length of the ExternT string that would be consumed
   do_length by conversion into given InternT buffer
   [virtual] (virtual protected member function of
                    std::codecvt<InternT,ExternT,StateT>)
                    returns the maximum number of ExternT characters that could be
   do_max_length converted into a single InternT character
   [virtual] (virtual protected member function of
                    std::codecvt<InternT,ExternT,StateT>) Inherited from std::codecvt_base
   Member type Definition
   enum result { ok, partial, error, noconv }; Unscoped enumeration type
   Enumeration constant Definition
   ok conversion was completed with no error
   partial not all source characters were converted
   error encountered an invalid character
   noconv no conversion required, input and output types are the same

Notes
   Although the standard requires that this facet works with UCS-2 when the size of
   Elem is 16 bits, some implementations use UTF-16 instead, making this a
   non-converting locale. The term "UCS-2" was deprecated and removed from ISO 10646.
   Since C++23, UCS-2 does not refer to its original definition anymore. It becomes a
   synonym of UTF-16, but with a smaller encoding range.

Example
   The following example demonstrates decoding of UTF-16le file on a system with 32-bit
   wchar_t. On a system with 16-bit wchar_t, decoding of the third character will fail
   because std::codecvt_utf16<char16_t> produces UCS-2, not UTF-16. // Run this code
 #include <codecvt>
 #include <cwchar>
 #include <fstream>
 #include <iostream>
 #include <locale>
 #include <string>
 void prepare_file()
 {
     // UTF-16le data (if host system is little-endian)
     char16_t utf16le[4] = {0x007a, // latin small letter 'z' U+007a
                        0x6c34, // CJK ideograph "water" U+6c34
                        0xd834, 0xdd0b}; // musical sign segno U+1d10b
     // store in a file
     std::ofstream fout("text.txt");
     fout.write(reinterpret_cast<char*>(utf16le), sizeof utf16le);
 }
 int main()
 {
     prepare_file();
     // open as a byte stream
     std::wifstream fin("text.txt", std::ios::binary);
     // apply facet
     fin.imbue(std::locale(fin.getloc(),
         new std::codecvt_utf16<wchar_t, 0x10ffff, std::little_endian>));
     wchar_t c = 0;
     for (std::cout << std::showbase << std::hex; fin.get(c);
          std::cout << static_cast<std::wint_t>(c) << '\n');
 }

Output:
 0x7a
 0x6c34
 0x1d10b

See also
  Character locale-defined multibyte UTF-8 UTF-16
 conversions (UTF-8, GB18030)
                        codecvt<char16_t,char,mbstate_t>
   UTF-16 mbrtoc16 / c16rtomb (with C11's codecvt_utf8_utf16<char16_t> N/A
              DR488) codecvt_utf8_utf16<char32_t>
                        codecvt_utf8_utf16<wchar_t>
    UCS-2 c16rtomb (without C11's DR488) codecvt_utf8<char16_t> codecvt_utf16<char16_t>
   UTF-32 mbrtoc32 / c32rtomb codecvt<char32_t,char,mbstate_t> codecvt_utf16<char32_t>
                        codecvt_utf8<char32_t>
   system
  wchar_t:
              mbsrtowcs / wcsrtombs
   UTF-32 use_facet<codecvt codecvt_utf8<wchar_t> codecvt_utf16<wchar_t> (non-Windows) <wchar_t,char,mbstate_t>>(locale)
    UCS-2
  (Windows)
                        converts between character encodings, including UTF-8, UTF-16,
   codecvt UTF-32
                        (class template)
   codecvt_mode
   (C++11) tags to alter behavior of the standard codecvt facets
   (deprecated in C++17) (enum)
   (removed in C++26)
   codecvt_utf8
   (C++11) converts between UTF-8 and UCS-2/UCS-4
   (deprecated in C++17) (class template)
   (removed in C++26)
   codecvt_utf8_utf16
   (C++11) converts between UTF-8 and UTF-16
   (deprecated in C++17) (class template)
   (removed in C++26)