std::codecvt(3)

NAME
std::codecvt - std::codecvt

Synopsis
   Defined in header <locale>
   template<
   class InternT,
   class ExternT,
   class State
   > 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
   The following standalone (locale-independent) specializations are provided by the
   standard library:
   Defined in header <locale>
   std::codecvt<char, char, std::mbstate_t> identity conversion
   std::codecvt<char16_t, char, conversion between UTF-16 and UTF-8 (since
   std::mbstate_t> C++11)(deprecated in C++20)
   std::codecvt<char16_t, char8_t, conversion between UTF-16 and UTF-8 (since
   std::mbstate_t> C++20)
   std::codecvt<char32_t, char, conversion between UTF-32 and UTF-8 (since
   std::mbstate_t> C++11)(deprecated in C++20)
   std::codecvt<char32_t, char8_t, conversion between UTF-32 and UTF-8 (since
   std::mbstate_t> C++20)
   std::codecvt<wchar_t, char, conversion between the system's native wide
   std::mbstate_t> and the single-byte narrow character sets
   In addition, every locale object constructed in a C++ program implements its own
   (locale-specific) versions of above specializations.

Member types
   Member type Definition
   intern_type InternT
   extern_type ExternT
   state_type State

Member functions
   constructor constructs a new codecvt facet
                 (public member function)
   destructor destructs a codecvt facet
                 (protected 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
   Member name Type
   id [static] std::locale::id

Protected member functions
   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, mbstate_t> and converts a UTF-8 string to
   UTF-16 using one of the standard specializations of std::codecvt. // Run this code
 #include <iostream>
 #include <fstream>
 #include <string>
 #include <locale>
 #include <iomanip>
 #include <codecvt>
 #include <cstdint>
 // 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, 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: ";
     for (wchar_t c; fin >> c; )
         std::cout << "U+" << std::hex << 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 << "\nThe UTF-8 file contains the following UTF-16 code units: ";
     for (char16_t c : str16)
         std::cout << "U+" << std::hex << std::setw(4) << std::setfill('0')
                   << static_cast<uint16_t>(c) << ' ';
 }

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
    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)
   codecvt_base defines character conversion errors
                        (class template)
   codecvt_byname creates a codecvt facet for the named locale
                        (class template)
   codecvt_utf8 converts between UTF-8 and UCS2/UCS4
   (C++11)(deprecated in C++17) (class template)
   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)