NAME¶

std::codecvt_utf8 - std::codecvt_utf8

Synopsis¶

Defined in header <codecvt>
template<

class Elem, (since C++11)
unsigned long Maxcode = 0x10ffff, (deprecated in C++17)
std::codecvt_mode Mode = (std::codecvt_mode)0 > (removed in C++26)
class codecvt_utf8

: public std::codecvt<Elem, char, std::mbstate_t>;

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 (since C++23)
values in the range U+0000-U+FFFF (Basic Multilingual Plane) only.

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 name Definition
intern_type internT
extern_type externT
state_type stateT

Member objects¶

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

Member functions¶

invokes do_out
out (public member function of std::codecvt<InternT,ExternT,StateT>)

invokes do_in
in (public member function of std::codecvt<InternT,ExternT,StateT>)

invokes do_unshift
unshift (public member function of std::codecvt<InternT,ExternT,StateT>)

invokes do_encoding
encoding (public member function of std::codecvt<InternT,ExternT,StateT>)

invokes do_always_noconv
always_noconv (public member function of std::codecvt<InternT,ExternT,StateT>)

invokes do_length
length (public member function of std::codecvt<InternT,ExternT,StateT>)

invokes do_max_length
max_length (public member function of std::codecvt<InternT,ExternT,StateT>)

Protected member functions¶

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

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