NAME¶

std::codecvt - std::codecvt

Synopsis¶

Defined in header <locale>
template<

class InternT,
class ExternT,
class StateT

> 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

Specializations¶

The standard library is guaranteed to provide the following specializations (they
are required to be implemented by any locale object):

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

Member types¶

Member name Definition
intern_type InternT
extern_type ExternT
state_type StateT

Member functions¶

constructor constructs a new codecvt facet
(public 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¶

static std::locale::id id id of the locale
(public member object)

Protected member functions¶

destructor destructs a codecvt facet
(protected member function)
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, std::mbstate_t> and converts a UTF-8 string to
UTF-16 using one of the standard specializations of std::codecvt.

// Run this code

#include <codecvt>
#include <cstdint>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <locale>
#include <string>

// 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, std::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" << std::hex;
for (wchar_t c; fin >> c;)
std::cout << "U+" << 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 << "\n\nThe UTF-8 file contains the following UTF-16 code units:\n"
<< std::hex;
for (char16_t c : str16)
std::cout << "U+" << std::setw(4) << std::setfill('0')
<< static_cast<uint16_t>(c) << ' ';
std::cout << '\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

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 codecvt_utf8_utf16<char16_t> N/A
DR488) codecvt_utf8_utf16<char32_t>
codecvt_utf8_utf16<wchar_t>
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_utf16<char32_t>
codecvt_utf8<char32_t>
system
wchar_t:
mbsrtowcs / wcsrtombs
UTF-32 use_facet<codecvt codecvt_utf8<wchar_t> codecvt_utf16<wchar_t> (non-Windows) <wchar_t,char,mbstate_t>>(locale)
UCS-2
(Windows)

codecvt_base defines character conversion errors
(class)
represents the system-supplied std::codecvt for the named
codecvt_byname locale
(class template)
codecvt_utf8
(C++11) converts between UTF-8 and UCS-2/UCS-4
(deprecated in C++17) (class template)
(removed in C++26)
codecvt_utf16
(C++11) converts between UTF-16 and UCS-2/UCS-4
(deprecated in C++17) (class template)
(removed in C++26)
codecvt_utf8_utf16
(C++11) converts between UTF-8 and UTF-16
(deprecated in C++17) (class template)
(removed in C++26)