std::codecvt_utf8(3)

NAME
std::codecvt_utf8 - std::codecvt_utf8

Synopsis
   Defined in header <codecvt>
   template<
   class Elem,
   unsigned long Maxcode = 0x10ffff, (since C++11)
   std::codecvt_mode Mode = (std::codecvt_mode)0 (deprecated in C++17)
   > 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 UCS2 or UTF-32 character string (depending on the type
   of Elem). This codecvt facet can be used to read and write UTF-8 files, both text
   and binary.

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 type Definition
   intern_type internT
   extern_type externT
   state_type stateT

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

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

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,State>)
                    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,State>)
                    generates the termination character sequence of externT characters
   do_unshift for incomplete conversion
   [virtual] (virtual protected member function of
                    std::codecvt<InternT,ExternT,State>)
                    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,State>)
                    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,State>)
                    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,State>)
                    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,State>) 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 UCS2 when the size of Elem
   is 16 bits, some implementations use UTF-16 instead. The term "UCS2" was deprecated
   and removed from the Unicode standard.

Example
   The following example demonstrates the difference between UCS2/UTF-8 and
   UTF-16/UTF-8 conversions: the third character in the string is not a valid UCS2
   character. // Run this code
 #include <iostream>
 #include <string>
 #include <locale>
 #include <codecvt>
 int main()
 {
     // UTF-8 data. The character U+1d10b, musical sign segno, does not fit in UCS2
     std::string utf8 = u8"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 << "UTF16 conversion produced " << utf16.size() << " code units:\n";
     for (char16_t c : utf16)
         std::cout << std::hex << std::showbase << c << '\n';
     // the UTF-8 / UCS2 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 << "UCS2 failed after producing " << std::dec << ucs2.size()<<" characters:\n";
         for (char16_t c : ucs2)
             std::cout << std::hex << std::showbase << c << '\n';
     }
 }

Output:
 UTF16 conversion produced 4 code units:
 0x7a
 0x6c34
 0xd834
 0xdd0b
 UCS2 failed after producing 2 characters:
 0x7a
 0x6c34

See also
     Character locale-defined
    conversions multibyte UTF-8 UTF-16
                     (UTF-8, GB18030)
                    mbrtoc16 / codecvt<char16_t, char, mbstate_t>
      UTF-16 c16rtomb(with C11's codecvt_utf8_utf16<char16_t> N/A
                    DR488) codecvt_utf8_utf16<char32_t>
                        codecvt_utf8_utf16<wchar_t>
                    c16rtomb(without codecvt_utf8<char16_t> codecvt_utf16<char16_t>
       UCS2 C11's DR488)
                        codecvt_utf8<wchar_t>(Windows) codecvt_utf16<wchar_t>(Windows)
                        codecvt<char32_t, char, mbstate_t> codecvt_utf16<char32_t>
      UTF-32 mbrtoc32 / c32rtomb codecvt_utf8<char32_t> codecvt_utf16<wchar_t>(non-Windows)
                        codecvt_utf8<wchar_t>(non-Windows)
                    mbsrtowcs /
   system wide: wcsrtombs UTF-32(non-Windows) use_facet<codecvt No No
   UCS2(Windows) <wchar_t, char,
                    mbstate_t>>(locale)
                        converts between character encodings, including UTF-8,
   codecvt UTF-16, UTF-32
                        (class template)
   codecvt_mode tags to alter behavior of the standard codecvt facets
   (C++11)(deprecated in C++17) (enum)
   codecvt_utf16 converts between UTF-16 and UCS2/UCS4
   (C++11)(deprecated in C++17) (class template)
   codecvt_utf8_utf16 converts between UTF-8 and UTF-16
   (C++11)(deprecated in C++17) (class template)