std:: codecvt_utf8

From cppreference.com

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) (removed in C++26)

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 an archaic encoding that is a subset of UTF-16, which encodes scalar 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

Nested types

Type	Definition
`intern_type`	`internT`
`extern_type`	`externT`
`state_type`	`stateT`

Data members

Member	Description
std::locale::id `id` [static]	the identifier of the facet

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

Nested 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.

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2229	C++98	the constructor and destructor were not specified	specifies them

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Character conversions
toupper
tolower
String and stream conversions
wstring_convert ( C++11/17/26* )
wbuffer_convert ( C++11/17/26* )

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) (removed in C++26)	tags to alter behavior of the standard codecvt facets (enum)
codecvt_utf16 (C++11) (deprecated in C++17) (removed in C++26)	converts between UTF-16 and UCS-2/UCS-4 (class template)
codecvt_utf8_utf16 (C++11) (deprecated in C++17) (removed in C++26)	converts between UTF-8 and UTF-16 (class template)

std:: codecvt_utf8

Template Parameters

Member functions

std::codecvt_utf8:: codecvt_utf8

Parameters

std::codecvt_utf8:: ~codecvt_utf8

Inherited from std:: codecvt

Nested types

Data members

Member functions

Protected member functions

Inherited from std:: codecvt_base

Notes

Example

Defect reports

See also

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