# std::codecvt_utf16

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< 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)

`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 codecvt facet can be used to read and write UTF-16 files in binary mode.

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_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 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, making this a non-converting locale. 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 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

### 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_utf8 (C++11)(deprecated in C++17) | converts between UTF-8 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)

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