# std::codecvt_byname

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< cpp‎ | locale

Defined in header `<locale>` |  |
template< class InternT, class ExternT, class State > class codecvt_byname : public std::codecvt <InternT, ExternT, State>; |  |

std::codecvt_byname is a std::codecvt facet which encapsulates multibyte/wide character conversion rules of a locale specified at its construction.

Four specializations are provided by the standard library

Defined in header `<locale>`
std::codecvt_byname<char, char, std::mbstate_t > | identity conversion
std::codecvt_byname<char16_t, char, std::mbstate_t > | conversion between UTF-16 and UTF-8 (since C++11)(deprecated in C++20)
std::codecvt_byname<char16_t, char8_t, std::mbstate_t > | conversion between UTF-16 and UTF-8 (since C++20)
std::codecvt_byname<char32_t, char, std::mbstate_t > | conversion between UTF-32 and UTF-8 (since C++11)(deprecated in C++20)
std::codecvt_byname<char32_t, char8_t, std::mbstate_t > | conversion between UTF-32 and UTF-8 (since C++20)
std::codecvt_byname<wchar_t, char, std::mbstate_t > | locale-specific conversion between wide string and narrow character sets

### Member functions

(constructor) | constructs a new `codecvt_byname` facet (public member function)
(destructor) | destroys a `codecvt_byname` facet (protected member function)

## std::codecvt_byname::codecvt_byname

explicit codecvt_byname( const char* name, std::size_t refs = 0 ); |  |
explicit codecvt_byname( const std::string & name, std::size_t refs = 0 ); |  | (since C++11)

Constructs a new `std::codecvt_byname` facet for a locale with `name`.

`refs` is used for resource management: if refs == 0, the implementation destroys the facet, when the last std::locale object holding it is destroyed. Otherwise, the object is not destroyed.

### Parameters

name | - | the name of the locale
refs | - | the number of references that link to the facet

## std::codecvt_byname::~codecvt_byname

protected: ~codecvt_byname(); |  |

Destroys the facet.

## 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

### Example

This example demonstrates reading a GB18030-encoded file using the codecvt facet from a GB18030-aware locale

Run this code

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

    int main()
    {
        // GB18030 narrow multibyte encoding
        std::ofstream("text.txt") << "\x7a"              // letter 'z', U+007a
                                     "\x81\x30\x89\x38"  // letter 'ß', U+00df
                                     "\xcb\xae"          // CJK ideogram '水' (water), U+6c34
                                     "\x94\x32\xbc\x35"; // musical sign '𝄋' (segno), U+1d10b
        std::wifstream fin("text.txt");
        fin.imbue(std::locale(fin.getloc(),
                  new std::codecvt_byname<wchar_t, char, std::mbstate_t>("zh_CN.gb18030")));
        for (wchar_t c; fin.get(c); )
            std::cout << std::hex << std::showbase << c << '\n';
    }

Output:

    0x7a
    0xdf
    0x6c34
    0x1d10b

### See also

codecvt | converts between character encodings, including UTF-8, UTF-16, UTF-32 (class template)

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