std::codecvt_utf8

 

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Localizations library

Locales and facets
Locales
has_facet use_facet	locale
Facet category base classes
ctype_base codecvt_base messages_base	time_base money_base
ctype facets
ctype ctype_byname ctype<char>	codecvt codecvt_byname
numeric facets
num_get num_put	numpunct numpunct_byname
collate facets
collate	collate_byname
time facets
time_get time_get_byname	time_put time_put_byname
monetary facets
money_get money_put	moneypunct moneypunct_byname
messages facets
messages	messages_byname
Character classification and conversion
Character classification
isspace isblank (C++11) iscntrl isupper islower isalpha	isdigit ispunct isxdigit isalnum isprint isgraph
Conversions
toupper tolower	wstring_convert (C++11/17*) wbuffer_convert (C++11/17*)
Code conversion facets
codecvt_utf8 (C++11/17*) codecvt_utf16 (C++11/17*)	codecvt_utf8_utf16 (C++11/17*) codecvt_mode (C++11/17*)
C locale
LC_ALLLC_COLLATELC_CTYPELC_MONETARYLC_NUMERICLC_TIME	setlocale localeconv lconv

 

Defined in header <codecvt>
template<     class Elem,     unsigned long Maxcode = 0x10ffff,     std::codecvt_mode Mode = (std::codecvt_mode)0 > class codecvt_utf8 : public std::codecvt<Elem, char, std::mbstate_t>;		(since C++11) (deprecated in C++17)

std::codecvt_utf8 is a std::codecvt facet which encapsulates conversion between a UTF-8 encoded byte string and UCS-2 or UTF-32 character string (depending on the type of Elem). This std::codecvt facet can be used to read and write UTF-8 files, both text and binary.

UCS-2 is the same encoding as UTF-16, except that it encodes scalar values in the range U+0000-U+FFFF (Basic Multilingual Plane) only.

(since C++23)

Template Parameters

Elem	-	either char16_t, char32_t, or wchar_t
Maxcode	-	the largest value of Elem that this facet will read or write without error
Mode	-	a constant of type std::codecvt_mode

Member functions

(constructor)	constructs a new codecvt_utf8 facet (public member function)
(destructor)	destroys a codecvt_utf8 facet (public member function)

std::codecvt_utf8::codecvt_utf8

explicit codecvt_utf8( std::size_t refs = 0 );

Constructs a new std::codecvt_utf8 facet, passes the initial reference counter refs to the base class.

Parameters

refs

-

the number of references that link to the facet

std::codecvt_utf8::~codecvt_utf8

~codecvt_utf8();

Destroys the facet. Unlike the locale-managed facets, this facet's destructor is public.

Inherited from std::codecvt

Member types

Member type	Definition
intern_type	internT
extern_type	externT
state_type	stateT

Member objects

Member name	Type
id (static)	std::locale::id

Member functions

out	invokes do_out (public member function of std::codecvt<InternT,ExternT,StateT>)
in	invokes do_in (public member function of std::codecvt<InternT,ExternT,StateT>)
unshift	invokes do_unshift (public member function of std::codecvt<InternT,ExternT,StateT>)
encoding	invokes do_encoding (public member function of std::codecvt<InternT,ExternT,StateT>)
always_noconv	invokes do_always_noconv (public member function of std::codecvt<InternT,ExternT,StateT>)
length	invokes do_length (public member function of std::codecvt<InternT,ExternT,StateT>)
max_length	invokes do_max_length (public member function of std::codecvt<InternT,ExternT,StateT>)

Protected member functions

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

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

Notes

Although the standard requires that this facet works with UCS-2 when the size of Elem is 16 bits, some implementations use UTF-16 instead. The term "UCS-2" was deprecated and removed from ISO 10646.

Since C++23, UCS-2 does not refer to its original definition anymore. It becomes a synonym of UTF-16, but with a smaller encoding range.

Example

The following example demonstrates the difference between UCS-2/UTF-8 and UTF-16/UTF-8 conversions: the third character in the string is not a valid UCS-2 character.

Run this code

#include <codecvt>
#include <cstdint>
#include <iostream>
#include <locale>
#include <string>
 
int main()
{
    // UTF-8 data. The character U+1d10b, musical sign segno, does not fit in UCS-2
    std::string utf8 = "z\u6c34\U0001d10b";
 
    // the UTF-8 / UTF-16 standard conversion facet
    std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> utf16conv;
    std::u16string utf16 = utf16conv.from_bytes(utf8);
    std::cout << "UTF-16 conversion produced " << utf16.size() << " code units:\n"
              << std::showbase << std::hex;
    for (char16_t c : utf16)
        std::cout << static_cast<std::uint16_t>(c) << ' ';
 
    // the UTF-8 / UCS-2 standard conversion facet
    std::wstring_convert<std::codecvt_utf8<char16_t>, char16_t> ucs2conv;
    try
    {
        std::u16string ucs2 = ucs2conv.from_bytes(utf8);
    }
    catch(const std::range_error& e)
    {
        std::u16string ucs2 = ucs2conv.from_bytes(utf8.substr(0, ucs2conv.converted()));
        std::cout << "\nUCS-2 failed after producing " << std::dec << ucs2.size()
                  << " characters:\n" << std::showbase << std::hex;
        for (char16_t c : ucs2)
            std::cout << static_cast<std::uint16_t>(c) << ' ';
        std::cout << '\n';
    }
}

Output:

UTF-16 conversion produced 4 code units:
0x7a 0x6c34 0xd834 0xdd0b
UCS-2 failed after producing 2 characters:
0x7a 0x6c34

See also

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	converts between character encodings, including UTF-8, UTF-16, UTF-32 (class template)
codecvt_mode (C++11)(deprecated in C++17)	tags to alter behavior of the standard codecvt facets (enum)
codecvt_utf16 (C++11)(deprecated in C++17)	converts between UTF-16 and UCS-2/UCS-4 (class template)
codecvt_utf8_utf16 (C++11)(deprecated in C++17)	converts between UTF-8 and UTF-16 (class template)