std::codecvt_utf8(3)

NAME
std::codecvt_utf8 - std::codecvt_utf8

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_utf8
       : public std::codecvt<Elem, char, std::mbstate_t>;
   std::codecvt_utf8 is a std::codecvt facet which encapsulates conversion between a
   UTF-8 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-8 files,
   both text and binary.
   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_utf8 facet
                 (public member function)
   destructor destroys a codecvt_utf8 facet
                 (public member function) std::codecvt_utf8::codecvt_utf8
   explicit codecvt_utf8( std::size_t refs = 0 );
   Constructs a new std::codecvt_utf8 facet, passes the initial reference counter refs
   to the base class.

Parameters
   refs - the number of references that link to the facet std::codecvt_utf8::~codecvt_utf8
   ~codecvt_utf8();
   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. 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 the difference between UCS-2/UTF-8 and
   UTF-16/UTF-8 conversions: the third character in the string is not a valid UCS-2
   character. // Run this code
 #include <codecvt>
 #include <cstdint>
 #include <iostream>
 #include <locale>
 #include <string>
 int main()
 {
     // UTF-8 data. The character U+1d10b, musical sign segno, does not fit in UCS-2
     std::string utf8 = "z\u6c34\U0001d10b";
     // the UTF-8 / UTF-16 standard conversion facet
     std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> utf16conv;
     std::u16string utf16 = utf16conv.from_bytes(utf8);
     std::cout << "UTF-16 conversion produced " << utf16.size() << " code units:\n"
               << std::showbase << std::hex;
     for (char16_t c : utf16)
         std::cout << static_cast<std::uint16_t>(c) << ' ';
     // the UTF-8 / UCS-2 standard conversion facet
     std::wstring_convert<std::codecvt_utf8<char16_t>, char16_t> ucs2conv;
     try
     {
         std::u16string ucs2 = ucs2conv.from_bytes(utf8);
     }
     catch(const std::range_error& e)
     {
         std::u16string ucs2 = ucs2conv.from_bytes(utf8.substr(0, ucs2conv.converted()));
         std::cout << "\nUCS-2 failed after producing " << std::dec << ucs2.size()
                   << " characters:\n" << std::showbase << std::hex;
         for (char16_t c : ucs2)
             std::cout << static_cast<std::uint16_t>(c) << ' ';
         std::cout << '\n';
     }
 }

Output:
 UTF-16 conversion produced 4 code units:
 0x7a 0x6c34 0xd834 0xdd0b
 UCS-2 failed after producing 2 characters:
 0x7a 0x6c34

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_utf16
   (C++11) converts between UTF-16 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)