std::codecvt (3) - Linux Manuals

std::codecvt: std::codecvt


std::codecvt - std::codecvt


Defined in header <locale>
class InternT,
class ExternT,
class State
> class codecvt;

Class 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.
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, 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, std::mbstate_t> conversion between the system's native wide and the single-byte narrow character sets

In addition, every locale object constructed in a C++ program implements its own (locale-specific) versions of these four specializations.

Member types

Member type Definition
intern_type InternT
extern_type ExternT
state_type State

Member functions

              constructs a new codecvt facet
constructor (public member function)
              destructs a codecvt facet
destructor (protected member function)
              invokes do_out
out (public member function)
              invokes do_in
in (public member function)
              invokes do_unshift
unshift (public member function)
              invokes do_encoding
encoding (public member function)
              invokes do_always_noconv
always_noconv (public member function)
              invokes do_length
length (public member function)
              invokes do_max_length
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 file
                 (virtual protected member function)

do_in converts a string from externT to internT, such as when reading from file
                 (virtual protected member function)

do_unshift generates the termination character sequence of externT characters for incomplete conversion
                 (virtual protected member function)

do_encoding returns the number of externT characters necessary to produce one internT character, if constant
                 (virtual protected member function)

do_always_noconv tests if the facet encodes an identity conversion for all valid argument values
                 (virtual protected member function)

do_length calculates the length of the externT string that would be consumed by conversion into given internT buffer
                 (virtual protected member function)

do_max_length returns the maximum number of externT characters that could be 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


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>

  // 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 = u8"z\u00df\u6c34\U0001f34c";
                         // or 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)
      std::cout << "The UTF-8 file contains the following UCS4 code points: \n";
      for (wchar_t c; fin >> c; )
          std::cout << "U+" << std::hex << std::setw(4) << std::setfill('0') << c << '\n';

      // using standard (locale-independent) codecvt facet
          deletable_facet<std::codecvt<char16_t, char, std::mbstate_t>>, char16_t> conv16;
      std::u16string str16 = conv16.from_bytes(data);

      std::cout << "The UTF-8 file contains the following UTF-16 code points: \n";
      for (char16_t c : str16)
          std::cout << "U+" << std::hex << std::setw(4) << std::setfill('0') << c << '\n';


  The UTF-8 file contains the following UCS4 code points:
  The UTF-8 file contains the following UTF-16 code points:

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 DR488) codecvt_utf8_utf16<char16_t> N/A
UCS2 c16rtomb(without C11's DR488) codecvt_utf8<char16_t> codecvt_utf16<char16_t>
                                                          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)
system wide: mbsrtowcs / wcsrtombs
UTF-32(non-Windows) use_facet<codecvt No No
UCS2(Windows) <wchar_t, char, mbstate_t>>(locale)

                      defines character conversion errors
codecvt_base (class template)
                      creates a codecvt facet for the named locale
codecvt_byname (class template)

codecvt_utf8 converts between UTF-8 and UCS2/UCS4
                      (class template)
(deprecated in C++17)

codecvt_utf16 converts between UTF-16 and UCS2/UCS4
                      (class template)
(deprecated in C++17)

codecvt_utf8_utf16 converts between UTF-8 and UTF-16
                      (class template)
(deprecated in C++17)