# std::codecvt_utf8

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

std::codecvt_utf8 is a std::codecvt facet which encapsulates conversion between a UTF-8 encoded byte string and UCS2 or UTF-32 character string (depending on the type of `Elem`). This codecvt facet can be used to read and write UTF-8 files, both text and binary.

### 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

### 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,State>`)
in | invokes `do_in` (public member function of `std::codecvt<InternT,ExternT,State>`)
unshift | invokes `do_unshift` (public member function of `std::codecvt<InternT,ExternT,State>`)
encoding | invokes `do_encoding` (public member function of `std::codecvt<InternT,ExternT,State>`)
always_noconv | invokes `do_always_noconv` (public member function of `std::codecvt<InternT,ExternT,State>`)
length | invokes `do_length` (public member function of `std::codecvt<InternT,ExternT,State>`)
max_length | invokes `do_max_length` (public member function of `std::codecvt<InternT,ExternT,State>`)

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

## 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 UCS2 when the size of `Elem` is 16 bits, some implementations use UTF-16 instead. The term "UCS2" was deprecated and removed from the Unicode standard.

### Example

The following example demonstrates the difference between UCS2/UTF-8 and UTF-16/UTF-8 conversions: the third character in the string is not a valid UCS2 character.

Run this code

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

    int main()
    {
        // UTF-8 data. The character U+1d10b, musical sign segno, does not fit in UCS2
        std::string utf8 = u8"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 << "UTF16 conversion produced " << utf16.size() << " code units:\n";
        for (char16_t c : utf16)
            std::cout << std::hex << std::showbase << c << '\n';

        // the UTF-8 / UCS2 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 << "UCS2 failed after producing " << std::dec << ucs2.size()<<" characters:\n";
            for (char16_t c : ucs2)
                std::cout << std::hex << std::showbase << c << '\n';
        }
    }

Output:

    UTF16 conversion produced 4 code units:
    0x7a
    0x6c34
    0xd834
    0xdd0b
    UCS2 failed after producing 2 characters:
    0x7a
    0x6c34

### 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
UCS2 | `c16rtomb`(without C11's DR488) | `codecvt_utf8`<char16_t> `codecvt_utf8`<wchar_t>(Windows) | `codecvt_utf16`<char16_t> `codecvt_utf16`<wchar_t>(Windows)
UTF-32 | `mbrtoc32` / `c32rtomb` | `codecvt`<char32_t, char, mbstate_t> `codecvt_utf8`<char32_t> `codecvt_utf8`<wchar_t>(non-Windows) | `codecvt_utf16`<char32_t> `codecvt_utf16`<wchar_t>(non-Windows)
system wide: UTF-32(non-Windows) UCS2(Windows) | `mbsrtowcs` / `wcsrtombs` `use_facet`<`codecvt` <wchar_t, char, mbstate_t>>(`locale`) | No | No

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_utf16 (C++11)(deprecated in C++17) | converts between UTF-16 and UCS2/UCS4 (class template)
codecvt_utf8_utf16 (C++11)(deprecated in C++17) | converts between UTF-8 and UTF-16 (class template)

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