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# std::codecvt_utf8_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_utf8_utf16 : public std::codecvt<Elem, char, std::mbstate_t>; |  |

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 codepoint will be stored in each 32-bit character of the output sequence. This codecvt facet can be used to read and write UTF-8 files, both text and binary.

### [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_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

### 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`) [edit]
in | invokes `do_in` (public member function of `std::codecvt`) [edit]
unshift | invokes `do_unshift` (public member function of `std::codecvt`) [edit]
encoding | invokes `do_encoding` (public member function of `std::codecvt`) [edit]
always_noconv | invokes `do_always_noconv` (public member function of `std::codecvt`) [edit]
length | invokes `do_length` (public member function of `std::codecvt`) [edit]
max_length | invokes `do_max_length` (public member function of `std::codecvt`) [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`) [edit]
do_in [virtual] | converts a string from externT to internT, such as when reading from file (virtual protected member function of `std::codecvt`) [edit]
do_unshift [virtual] | generates the termination character sequence of externT characters for incomplete conversion (virtual protected member function of `std::codecvt`) [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`) [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`) [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`) [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`) [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] Example

The following example demonstrates reading a UTF-8 file into a UTF-16 string on a system with 32-bit wchar_t

Run this code

    #include <fstream>
    #include <iostream>
    #include <string>
    #include <locale>
    #include <codecvt>

    int main()
    {
        std::ofstream("text.txt") << u8"z\u6c34\U0001d10b";

        std::wifstream file1("text.txt");
        file1.imbue(std::locale("en_US.UTF8"));
        std::cout << "Normal read from file (using default UTF-8/UTF-32 codecvt)\n";
        for (wchar_t c; file1 >> c; )
            std::cout << std::hex << std::showbase << c << '\n';

        std::wifstream file2("text.txt");
        file2.imbue(std::locale(file2.getloc(), new std::codecvt_utf8_utf16<wchar_t>));
        std::cout << "UTF-16 read from the same file (using codecvt_utf8_utf16)\n";
        for (wchar_t c; file2 >> c; )
            std::cout << std::hex << std::showbase << c << '\n';
    }

Output:

    Normal read from file (using default UTF-8/UTF-32 codecvt)
    0x7a
    0x6c34
    0x1d10b
    UTF-16 read from the same file (using codecvt_utf8_utf16)
    0x7a
    0x6c34
    0xd834
    0xdd0b

### [edit] See also

Character conversions | narrow multibyte (char) | UTF-8 (char) | UTF-16 (char16_t)
--- | --- | --- | ---
UTF-16 | `mbrtoc16`(one way) | `codecvt`<char16_t, char, mbstate_t> `codecvt_utf8_utf16`<char16_t> `codecvt_utf8_utf16`<char32_t> `codecvt_utf8_utf16`<wchar_t> | N/A
UCS2 | `c16rtomb`(one way) | `codecvt_utf8`<char16_t> | `codecvt_utf16`<char16_t>
UTF-32/UCS4 (char32_t) | `mbrtoc32` / `c32rtomb` | `codecvt`<char32_t, char, mbstate_t> `codecvt_utf8`<char32_t> | `codecvt_utf16`<char32_t>
UCS2/UCS4 (wchar_t) | No | `codecvt_utf8`<wchar_t> | `codecvt_utf16`<wchar_t>
wide (wchar_t) | `codecvt`<wchar_t, char, mbstate_t> `mbsrtowcs` / `wcsrtombs` | No | No

codecvt | converts between character encodings, including UTF-8, UTF-16, UTF-32 (class template) [edit]
codecvt_mode (C++11) | tags to alter behavior of the standard codecvt facets (enum) [edit]
codecvt_utf8 (C++11) | converts between UTF-8 and UCS2/UCS4 (class template) [edit]
codecvt_utf16 (C++11) | converts between UTF-16 and UCS2/UCS4 (class template) [edit]

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