# 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) (removed in C++26)

`std::codecvt_utf8` is a std::codecvt facet which encapsulates conversion between a UTF-8 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-8 files, both text and binary.

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.

### 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

### 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

### 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. The term "UCS-2" was deprecated and removed from ISO 10646.

### Example

The following example demonstrates the difference between UCS-2/UTF-8 and UTF-16/UTF-8 conversions: the third character in the string is not a valid UCS-2 character.

Run this code

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

    int main()
    {
        // UTF-8 data. The character U+1d10b, musical sign segno, does not fit in UCS-2
        std::string utf8 = "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 << "UTF-16 conversion produced " << utf16.size() << " code units:\n"
                  << std::showbase << std::hex;
        for (char16_t c : utf16)
            std::cout << static_cast<std::uint16_t>(c) << ' ';

        // the UTF-8 / UCS-2 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 << "\nUCS-2 failed after producing " << std::dec << ucs2.size()
                      << " characters:\n" << std::showbase << std::hex;
            for (char16_t c : ucs2)
                std::cout << static_cast<std::uint16_t>(c) << ' ';
            std::cout << '\n';
        }
    }

Output:

    UTF-16 conversion produced 4 code units:
    0x7a 0x6c34 0xd834 0xdd0b
    UCS-2 failed after producing 2 characters:
    0x7a 0x6c34

### 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_utf16 (C++11)(deprecated in C++17)(removed in C++26) | converts between UTF-16 and UCS-2/UCS-4 (class template)
codecvt_utf8_utf16 (C++11)(deprecated in C++17)(removed in C++26) | converts between UTF-8 and UTF-16 (class template)

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