# 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.

### Specializations

The standard library is guaranteed to provide the following specializations:

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

### 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

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

Run this code

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

    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 << static_cast<unsigned>(c) << '\n';
    }

Possible output:

    0x7a
    0xdf
    0x6c34
    0x1d10b

### 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 21 | C++98 | the standard library did not need to provide any `std::codecvt_byname` specializations | two specializations are required

### See also

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

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