std::codecvt

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.

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 above specializations.

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

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

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 = 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: \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 units: \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 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

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Character conversions
toupper
tolower
String and stream conversions
wstring_convert (C++11/17/26*)
wbuffer_convert (C++11/17/26*)

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
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_utf8`<char32_t>	`codecvt_utf16`<char32_t>
system wchar_t: UTF-32 (non-Windows) UCS-2 (Windows)	`mbsrtowcs` / `wcsrtombs` `use_facet`<`codecvt` <wchar_t,char,mbstate_t>>(`locale`)	`codecvt_utf8`<wchar_t>	`codecvt_utf16`<wchar_t>

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)(removed in C++26)	converts between UTF-8 and UCS-2/UCS-4 (class template) [edit]
codecvt_utf16 (C++11)(deprecated in C++17)(removed in C++26)	converts between UTF-16 and UCS-2/UCS-4 (class template) [edit]
codecvt_utf8_utf16 (C++11)(deprecated in C++17)(removed in C++26)	converts between UTF-8 and UTF-16 (class template) [edit]

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std::codecvt

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Inherited from std::codecvt_base

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Member type	Definition
`intern_type`	`InternT`
`extern_type`	`ExternT`
`state_type`	`State`