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std::codecvt(3) C++ Standard Libary std::codecvt(3)


std::codecvt - std::codecvt


Defined in header <locale>

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, 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


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)
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
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) << ' ';


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>
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)
mbsrtowcs /
system wide: wcsrtombs UTF-32(non-Windows) use_facet<codecvt No No
UCS2(Windows) <wchar_t, char,

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)