std::codecvt

Defined in header `<locale>`
template< 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

Four 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)
std::codecvt<char32_t, char, std::mbstate_t>	conversion between UTF-32 and UTF-8 (since C++11)
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

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

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>

// 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"z"
                       // 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 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
    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 << "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';
}

Output:

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

Language
Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Member functions
codecvt::codecvt
codecvt::~codecvt
codecvt::outcodecvt::do_out
codecvt::incodecvt::do_in
codecvt::unshiftcodecvt::do_unshift
codecvt::encodingcodecvt::do_encoding
codecvt::always_noconvcodecvt::do_always_noconv
codecvt::lengthcodecvt::do_length
codecvt::max_lengthcodecvt::do_max_length

Character conversions	locale-defined multibyte (UTF-8, GB18030)	UTF-8	UTF-16
UTF-16	`mbrtoc16` / `c16rtomb`(with C11's DR488)	`codecvt`<char16_t, char, mbstate_t> `codecvt_utf8_utf16`<char16_t> `codecvt_utf8_utf16`<char32_t> `codecvt_utf8_utf16`<wchar_t>	N/A
UCS2	`c16rtomb`(without C11's DR488)	`codecvt_utf8`<char16_t> `codecvt_utf8`<wchar_t>(Windows)	`codecvt_utf16`<char16_t> `codecvt_utf16`<wchar_t>(Windows)
UTF-32	`mbrtoc32` / `c32rtomb`	`codecvt`<char32_t, char, mbstate_t> `codecvt_utf8`<char32_t> `codecvt_utf8`<wchar_t>(non-Windows)	`codecvt_utf16`<char32_t> `codecvt_utf16`<wchar_t>(non-Windows)
system wide: UTF-32(non-Windows) UCS2(Windows)	`mbsrtowcs` / `wcsrtombs` `use_facet`<`codecvt` <wchar_t, char, mbstate_t>>(`locale`)	No	No

codecvt_base	defines character conversion errors (class template)
codecvt_byname	creates a codecvt facet for the named locale (class template)
codecvt_utf8 (C++11)(deprecated in C++17)	converts between UTF-8 and UCS2/UCS4 (class template)
codecvt_utf16 (C++11)(deprecated in C++17)	converts between UTF-16 and UCS2/UCS4 (class template)
codecvt_utf8_utf16 (C++11)(deprecated in C++17)	converts between UTF-8 and UTF-16 (class template)