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std::codecvt_utf16

From cppreference.com

< cpp‎ | locale

Defined in header `<codecvt>`
template< class Elem, unsigned long Maxcode = 0x10ffff, std::codecvt_mode Mode = (std::codecvt_mode)0 > class codecvt_utf16 : public std::codecvt<Elem, char, std::mbstate_t>;		(since C++11) (deprecated in C++17) (removed in C++26)

std::codecvt_utf16 is a std::codecvt facet which encapsulates conversion between a UTF-16 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-16 files in binary mode.

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.

Contents

1 Template Parameters
2 Member functions
3 std::codecvt_utf16::codecvt_utf16
- 3.1 Parameters
4 std::codecvt_utf16::~codecvt_utf16
5 Inherited from std::codecvt
6 Inherited from std::codecvt_base

[edit] 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

[edit] Member functions

(constructor)	constructs a new `codecvt_utf16` facet (public member function)
(destructor)	destroys a `codecvt_utf16` facet (public member function)

std::codecvt_utf16::codecvt_utf16

explicit codecvt_utf16( std::size_t refs = 0 );

Constructs a new std::codecvt_utf16 facet, passes the initial reference counter refs to the base class.

Parameters

refs

-

the number of references that link to the facet

std::codecvt_utf16::~codecvt_utf16

~codecvt_utf16();

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

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

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>`) [edit]
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>`) [edit]
do_unshift [virtual]	generates the termination character sequence of `ExternT` characters for incomplete conversion (virtual protected member function of `std::codecvt<InternT,ExternT,StateT>`) [edit]
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>`) [edit]
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>`) [edit]
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>`) [edit]
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>`) [edit]

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

[edit] 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, making this a non-converting locale. The term "UCS-2" was deprecated and removed from ISO 10646.

[edit] Example

The following example demonstrates decoding of UTF-16le file on a system with 32-bit wchar_t. On a system with 16-bit wchar_t, decoding of the third character will fail because std::codecvt_utf16<char16_t> produces UCS-2, not UTF-16.

Run this code

#include <codecvt>
#include <cwchar>
#include <fstream>
#include <iostream>
#include <locale>
#include <string>
 
void prepare_file()
{
    // UTF-16le data (if host system is little-endian)
    char16_t utf16le[4] = {0x007a,          // latin small letter 'z' U+007a
                           0x6c34,          // CJK ideograph "water"  U+6c34
                           0xd834, 0xdd0b}; // musical sign segno U+1d10b    
 
    // store in a file
    std::ofstream fout("text.txt");
    fout.write(reinterpret_cast<char*>(utf16le), sizeof utf16le);
}
 
int main()
{
    prepare_file();
    // open as a byte stream
    std::wifstream fin("text.txt", std::ios::binary);
    // apply facet
    fin.imbue(std::locale(fin.getloc(),
        new std::codecvt_utf16<wchar_t, 0x10ffff, std::little_endian>));
 
    wchar_t c = 0;
    for (std::cout << std::showbase << std::hex; fin.get(c);
         std::cout << static_cast<std::wint_t>(c) << '\n');
}

Output:

0x7a
0x6c34
0x1d10b

[edit] 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) [edit]
codecvt_mode (C++11)(deprecated in C++17)(removed in C++26)	tags to alter behavior of the standard codecvt facets (enum) [edit]
codecvt_utf8 (C++11)(deprecated in C++17)(removed in C++26)	converts between UTF-8 and UCS-2/UCS-4 (class template) [edit]
codecvt_utf8_utf16 (C++11)(deprecated in C++17)(removed in C++26)	converts between UTF-8 and UTF-16 (class template) [edit]

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