std::codecvt_byname

 

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Localizations library

Locales and facets
Locales
has_facet use_facet	locale
Facet category base classes
ctype_base codecvt_base messages_base	time_base money_base
ctype facets
ctype ctype_byname ctype<char>	codecvt codecvt_byname
numeric facets
num_get num_put	numpunct numpunct_byname
collate facets
collate	collate_byname
time facets
time_get time_get_byname	time_put time_put_byname
monetary facets
money_get money_put	moneypunct moneypunct_byname
messages facets
messages	messages_byname
Character classification and conversion
Character classification
isspace isblank (C++11) iscntrl isupper islower isalpha	isdigit ispunct isxdigit isalnum isprint isgraph
Conversions
toupper tolower	wstring_convert (C++11/17*) wbuffer_convert (C++11/17*)
Code conversion facets
codecvt_utf8 (C++11/17*) codecvt_utf16 (C++11/17*)	codecvt_utf8_utf16 (C++11/17*) codecvt_mode (C++11/17*)
C locale
LC_ALLLC_COLLATELC_CTYPELC_MONETARYLC_NUMERICLC_TIME	setlocale localeconv lconv

 

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 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,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

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 << 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)