std::codecvt(3)

NAME
std::codecvt - std::codecvt

Synopsis
   Defined in header <locale>
   template<
       class InternT,
       class ExternT,
       class StateT
   > class codecvt;
   Class template std::codecvt encapsulates conversion of character strings, including
   wide and multibyte, from one encoding to another. All file I/O operations performed
   through std::basic_fstream<CharT> use the std::codecvt<CharT, char, std::mbstate_t>
   facet of the locale imbued in the stream.
   std-codecvt-inheritance.svg
                        Inheritance diagram

Specializations
   The standard library is guaranteed to provide the following specializations (they
   are required to be implemented by any locale object):
   Defined in header <locale>
   std::codecvt<char, char, std::mbstate_t> identity conversion
   std::codecvt<char16_t, char,
   std::mbstate_t> conversion between UTF-16 and UTF-8
   (since C++11)(deprecated in C++20)
   std::codecvt<char16_t, char8_t,
   std::mbstate_t> conversion between UTF-16 and UTF-8
   (since C++20)
   std::codecvt<char32_t, char,
   std::mbstate_t> conversion between UTF-32 and UTF-8
   (since C++11)(deprecated in C++20)
   std::codecvt<char32_t, char8_t,
   std::mbstate_t> conversion between UTF-32 and UTF-8
   (since C++20)
   std::codecvt<wchar_t, char, conversion between the system's native
   std::mbstate_t> wide and the single-byte narrow character
                        sets

Member types
   Member name Definition
   intern_type InternT
   extern_type ExternT
   state_type StateT

Member functions
   constructor constructs a new codecvt facet
                 (public member function)
   out invokes do_out
                 (public member function)
   in invokes do_in
                 (public member function)
   unshift invokes do_unshift
                 (public member function)
   encoding invokes do_encoding
                 (public member function)
   always_noconv invokes do_always_noconv
                 (public member function)
   length invokes do_length
                 (public member function)
   max_length invokes do_max_length
                 (public member function)

Member objects
   static std::locale::id id id of the locale
                        (public member object)

Protected member functions
   destructor destructs a codecvt facet
                    (protected member function)
   do_out converts a string from InternT to ExternT, such as when writing to
   [virtual] file
                    (virtual protected member function)
   do_in converts a string from ExternT to InternT, such as when reading
   [virtual] from file
                    (virtual protected member function)
   do_unshift generates the termination character sequence of ExternT characters
   [virtual] for incomplete conversion
                    (virtual protected member function)
   do_encoding returns the number of ExternT characters necessary to produce one
   [virtual] InternT character, if constant
                    (virtual protected member function)
   do_always_noconv tests if the facet encodes an identity conversion for all valid
   [virtual] argument values
                    (virtual protected member function)
   do_length calculates the length of the ExternT string that would be consumed
   [virtual] by conversion into given InternT buffer
                    (virtual protected member function)
   do_max_length returns the maximum number of ExternT characters that could be
   [virtual] converted into a single InternT character
                    (virtual protected member function) 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

Example
   The following examples reads a UTF-8 file using a locale which implements UTF-8
   conversion in codecvt<wchar_t, char, std::mbstate_t> and converts a UTF-8 string to
   UTF-16 using one of the standard specializations of std::codecvt. // Run this code
 #include <codecvt>
 #include <cstdint>
 #include <fstream>
 #include <iomanip>
 #include <iostream>
 #include <locale>
 #include <string>
 // utility wrapper to adapt locale-bound facets for wstring/wbuffer convert
 template<class Facet>
 struct deletable_facet : Facet
 {
     template<class... Args>
     deletable_facet(Args&&... args) : Facet(std::forward<Args>(args)...) {}
     ~deletable_facet() {}
 };
 int main()
 {
     // UTF-8 narrow multibyte encoding
     std::string data = reinterpret_cast<const char*>(+u8"z\u00df\u6c34\U0001f34c");
                        // or reinterpret_cast<const char*>(+u8"zß水🍌")
                        // or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9f\x8d\x8c"
     std::ofstream("text.txt") << data;
     // using system-supplied locale's codecvt facet
     std::wifstream fin("text.txt");
     // reading from wifstream will use codecvt<wchar_t, char, std::mbstate_t>
     // this locale's codecvt converts UTF-8 to UCS4 (on systems such as Linux)
     fin.imbue(std::locale("en_US.UTF-8"));
     std::cout << "The UTF-8 file contains the following UCS4 code units:\n" << std::hex;
     for (wchar_t c; fin >> c;)
         std::cout << "U+" << std::setw(4) << std::setfill('0')
                   << static_cast<uint32_t>(c) << ' ';
     // using standard (locale-independent) codecvt facet
     std::wstring_convert<
         deletable_facet<std::codecvt<char16_t, char, std::mbstate_t>>, char16_t> conv16;
     std::u16string str16 = conv16.from_bytes(data);
     std::cout << "\n\nThe UTF-8 file contains the following UTF-16 code units:\n"
               << std::hex;
     for (char16_t c : str16)
         std::cout << "U+" << std::setw(4) << std::setfill('0')
                   << static_cast<uint16_t>(c) << ' ';
     std::cout << '\n';
 }

Output:
 The UTF-8 file contains the following UCS4 code units:
 U+007a U+00df U+6c34 U+1f34c
 The UTF-8 file contains the following UTF-16 code units:
 U+007a U+00df U+6c34 U+d83c U+df4c

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)
   codecvt_base defines character conversion errors
                        (class)
                        represents the system-supplied std::codecvt for the named
   codecvt_byname locale
                        (class template)
   codecvt_utf8
   (C++11) converts between UTF-8 and UCS-2/UCS-4
   (deprecated in C++17) (class template)
   (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)