std::codecvt_byname

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< cpp‎ | locale
 
C++
 
Localization library
 
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> 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 name Definition
intern_type internT
extern_type externT
state_type stateT

Member objects

Member name Type
id (static) std::locale::id

Member functions

 invokes do_out
(public member function of std::codecvt<InternT,ExternT,StateT>)
invokes do_in
(public member function of std::codecvt<InternT,ExternT,StateT>)
invokes do_unshift
(public member function of std::codecvt<InternT,ExternT,StateT>)
invokes do_encoding
(public member function of std::codecvt<InternT,ExternT,StateT>)
invokes do_always_noconv
(public member function of std::codecvt<InternT,ExternT,StateT>)
invokes do_length
(public member function of std::codecvt<InternT,ExternT,StateT>)
invokes do_max_length
(public member function of std::codecvt<InternT,ExternT,StateT>)

Protected member functions

[virtual]
 converts a string from InternT to ExternT, such as when writing to file
(virtual protected member function of std::codecvt<InternT,ExternT,StateT>)
[virtual]
 converts a string from ExternT to InternT, such as when reading from file
(virtual protected member function of std::codecvt<InternT,ExternT,StateT>)
[virtual]
 generates the termination character sequence of ExternT characters for incomplete conversion
(virtual protected member function of std::codecvt<InternT,ExternT,StateT>)
[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>)
[virtual]
 tests if the facet encodes an identity conversion for all valid argument values
(virtual protected member function of std::codecvt<InternT,ExternT,StateT>)
[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>)
[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

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 <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

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