std::codecvt

 

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Localizations library

Locales and facets
locale
use_facet	has_facet
Character classification
isspace isblank (C++11) iscntrl isupper islower isalpha	isdigit ispunct isxdigit isalnum isprint isgraph
Conversions
toupper tolower	wstring_convert (C++11) wbuffer_convert (C++11)
Facet category base classes
ctype_base codecvt_base messages_base	time_base money_base
Facet categories
ctype ctype<char> codecvt collate messages time_get time_put	num_get num_put numpunct money_get money_put moneypunct
Locale-specific facets
ctype_byname codecvt_byname messages_byname collate_byname	time_get_byname time_put_byname numpunct_byname moneypunct_byname
Code conversion facets
codecvt_utf8 (C++11) codecvt_utf16 (C++11)	codecvt_utf8_utf16 (C++11) codecvt_mode (C++11)
C locale
setlocale localeconv lconv	LC_ALLLC_COLLATELC_CTYPELC_MONETARYLC_NUMERICLC_TIME

 

std::codecvt

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

 

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 multibyte encodings

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 <string>
#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 willl 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

See also

Character conversions	narrow multibyte (char)	UTF-8 (char)	UTF-16 (char16_t)
UTF-16	mbrtoc16 / c16rtomb	codecvt<char16_t, char, mbstate_t> codecvt_utf8_utf16<char16_t> codecvt_utf8_utf16<char32_t> codecvt_utf8_utf16<wchar_t>	N/A
UCS2	No	codecvt_utf8<char16_t>	codecvt_utf16<char16_t>
UTF-32/UCS4 (char32_t)	mbrtoc32 / c32rtomb	codecvt<char32_t, char, mbstate_t> codecvt_utf8<char32_t>	codecvt_utf16<char32_t>
UCS2/UCS4 (wchar_t)	No	codecvt_utf8<wchar_t>	codecvt_utf16<wchar_t>
wide (wchar_t)	codecvt<wchar_t, char, mbstate_t> mbsrtowcs / wcsrtombs	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)	converts between UTF-8 and UCS2/UCS4 (class template)
codecvt_utf16 (C++11)	converts between UTF-16 and UCS2/UCS4 (class template)
codecvt_utf8_utf16 (C++11)	converts between UTF-8 and UTF-16 (class template)