# std::codecvt_utf8_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_utf8_utf16 : public std::codecvt <Elem, char, std::mbstate_t >; |  | (since C++11) (deprecated in C++17) (removed in C++26)

`std::codecvt_utf8_utf16` is a std::codecvt facet which encapsulates conversion between a UTF-8 encoded byte string and UTF-16 encoded character string. If `Elem` is a 32-bit type, one UTF-16 code unit will be stored in each 32-bit character of the output sequence.

This is an N:M conversion facet, and cannot be used with std::basic_filebuf (which only permits 1:N conversions, such as UTF-32/UTF-8, between the internal and the external encodings). This facet can be used with std::wstring_convert.

### 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_utf16` facet (public member function)
(destructor) | destroys a `codecvt_utf8_utf16` facet (public member function)

## std::codecvt_utf8_utf16::codecvt_utf8_utf16

explicit codecvt_utf8_utf16( std::size_t refs = 0 ); |  |

Constructs a new `std::codecvt_utf8_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_utf8_utf16::~codecvt_utf8_utf16

~codecvt_utf8_utf16(); |  |

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

### Example

Run this code

    #include <cassert>
    #include <codecvt>
    #include <cstdint>
    #include <iostream>
    #include <locale>
    #include <string>

    int main()
    {
        std::string u8 = "z\u00df\u6c34\U0001f34c";
        std::u16string u16 = u"z\u00df\u6c34\U0001f34c";

        // UTF-8 to UTF-16/char16_t
        std::u16string u16_conv = std::wstring_convert<
            std::codecvt_utf8_utf16<char16_t>, char16_t>{}.from_bytes(u8);
        assert(u16 == u16_conv);
        std::cout << "UTF-8 to UTF-16 conversion produced " << u16_conv.size()
                  << " code units:\n" << std::showbase << std::hex;
        for (char16_t c : u16_conv)
            std::cout << static_cast<std::uint16_t>(c) << ' ';

        // UTF-16/char16_t to UTF-8
        std::string u8_conv = std::wstring_convert<
            std::codecvt_utf8_utf16<char16_t>, char16_t>{}.to_bytes(u16);
        assert(u8 == u8_conv);
        std::cout << "\nUTF-16 to UTF-8 conversion produced "
                  << std::dec << u8_conv.size() << " bytes:\n" << std::hex;
        for (char c : u8_conv)
            std::cout << +static_cast<unsigned char>(c) << ' ';
        std::cout << '\n';
    }

Output:

    UTF-8 to UTF-16 conversion produced 5 code units:
    0x7a 0xdf 0x6c34 0xd83c 0xdf4c
    UTF-16 to UTF-8 conversion produced 10 bytes:
    0x7a 0xc3 0x9f 0xe6 0xb0 0xb4 0xf0 0x9f 0x8d 0x8c

### 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

### 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)(removed in C++26) | tags to alter behavior of the standard codecvt facets (enum)
codecvt_utf8 (C++11)(deprecated in C++17)(removed in C++26) | converts between UTF-8 and UCS-2/UCS-4 (class template)
codecvt_utf16 (C++11)(deprecated in C++17)(removed in C++26) | converts between UTF-16 and UCS-2/UCS-4 (class template)

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