std::codecvt
From cppreference.com
Defined in header <locale> | ||
template< class InternT, | ||
Class template std::codecvt encapsulates conversion of character strings, including wide and multibyte, from one encoding to another. All file I/O operations performed through std::basic_fstream<CharT> use the std::codecvt<CharT, char, std::mbstate_t> facet of the locale imbued in the stream.
Inheritance diagram
Contents |
[edit]Specializations
The standard library is guaranteed to provide the following specializations (they are required to be implemented by any locale object):
Defined in header <locale> | |
| std::codecvt<char, char, std::mbstate_t> | identity conversion |
| std::codecvt<char16_t, char, std::mbstate_t> (since C++11)(deprecated in C++20) | conversion between UTF-16 and UTF-8 |
| std::codecvt<char16_t, char8_t, std::mbstate_t> (since C++20)(deprecated) | conversion between UTF-16 and UTF-8 |
| std::codecvt<char32_t, char, std::mbstate_t> (since C++11)(deprecated in C++20) | conversion between UTF-32 and UTF-8 |
| std::codecvt<char32_t, char8_t, std::mbstate_t> (since C++20)(deprecated) | conversion between UTF-32 and UTF-8 |
| std::codecvt<wchar_t, char, std::mbstate_t> | conversion between the system's native wide and the single-byte narrow character sets |
[edit]Nested types
| Type | Definition |
intern_type | InternT |
extern_type | ExternT |
state_type | StateT |
[edit]Data members
| Member | Description |
std::locale::idid[static] | the identifier of the facet |
[edit]Member functions
constructs a new codecvt facet (public member function) | |
invokes do_out(public member function) | |
invokes do_in(public member function) | |
invokes do_unshift(public member function) | |
invokes do_encoding(public member function) | |
invokes do_always_noconv(public member function) | |
invokes do_length(public member function) | |
invokes do_max_length(public member function) |
[edit]Protected member functions
destructs a codecvt facet (protected member function) | |
[virtual] | converts a string from InternT to ExternT, such as when writing to file (virtual protected member function) |
[virtual] | converts a string from ExternT to InternT, such as when reading from file (virtual protected member function) |
[virtual] | generates the termination character sequence of ExternT characters for incomplete conversion (virtual protected member function) |
[virtual] | returns the number of ExternT characters necessary to produce one InternT character, if constant (virtual protected member function) |
[virtual] | tests if the facet encodes an identity conversion for all valid argument values (virtual protected member function) |
[virtual] | calculates the length of the ExternT string that would be consumed by conversion into given InternT buffer (virtual protected member function) |
[virtual] | returns the maximum number of ExternT characters that could be converted into a single InternT character (virtual protected member function) |
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 |
[edit]Example
The following examples reads a UTF-8 file using a locale which implements UTF-8 conversion in codecvt<wchar_t, char, std::mbstate_t> and converts a UTF-8 string to UTF-16 using one of the standard specializations of std::codecvt.
Run this code
#include <codecvt>#include <cstdint>#include <fstream>#include <iomanip>#include <iostream>#include <locale>#include <string> // utility wrapper to adapt locale-bound facets for wstring/wbuffer converttemplate<class Facet>struct deletable_facet : Facet {template<class... Args> deletable_facet(Args&&... args): Facet(std::forward<Args>(args)...){} ~deletable_facet(){}}; int main(){// UTF-8 narrow multibyte encodingstd::string data =reinterpret_cast<constchar*>(+u8"z\u00df\u6c34\U0001f34c");// or reinterpret_cast<const char*>(+u8"zß水🍌")// or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9f\x8d\x8c" std::ofstream("text.txt")<< data; // using system-supplied locale's codecvt facetstd::wifstream fin("text.txt");// reading from wifstream will use codecvt<wchar_t, char, std::mbstate_t>// this locale's codecvt converts UTF-8 to UCS4 (on systems such as Linux) fin.imbue(std::locale("en_US.UTF-8"));std::cout<<"The UTF-8 file contains the following UCS4 code units:\n"<<std::hex;for(wchar_t c; fin >> c;)std::cout<<"U+"<<std::setw(4)<<std::setfill('0')<<static_cast<uint32_t>(c)<<' '; // using standard (locale-independent) codecvt facetstd::wstring_convert< deletable_facet<std::codecvt<char16_t, char, std::mbstate_t>>, char16_t> conv16;std::u16string str16 = conv16.from_bytes(data); std::cout<<"\n\nThe UTF-8 file contains the following UTF-16 code units:\n"<<std::hex;for(char16_t c : str16)std::cout<<"U+"<<std::setw(4)<<std::setfill('0')<<static_cast<uint16_t>(c)<<' ';std::cout<<'\n';}
Output:
The UTF-8 file contains the following UCS4 code units: U+007a U+00df U+6c34 U+1f34c The UTF-8 file contains the following UTF-16 code units: U+007a U+00df U+6c34 U+d83c U+df4c
[edit]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 3767 | C++20 | std::codecvt<char16_t, char8_t, std::mbstate_t> and std::codecvt<char32_t, char8_t, std::mbstate_t> are locale-independent | deprecated them |
[edit]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> | N/A |
| UCS-2 | c16rtomb(without C11's DR488) | codecvt_utf8<char16_t> | codecvt_utf16<char16_t> |
| UTF-32 | codecvt<char32_t,char,mbstate_t> | codecvt_utf16<char32_t> | |
| system wchar_t: UTF-32 (non-Windows) | mbsrtowcs / wcsrtombs | codecvt_utf8<wchar_t> | codecvt_utf16<wchar_t> |
| defines character conversion errors (class) | |
| represents the system-supplied std::codecvt for the named locale (class template) | |
(C++11)(deprecated in C++17)(removed in C++26) | converts between UTF-8 and UCS-2/UCS-4 (class template) |
(C++11)(deprecated in C++17)(removed in C++26) | converts between UTF-16 and UCS-2/UCS-4 (class template) |
(C++11)(deprecated in C++17)(removed in C++26) | converts between UTF-8 and UTF-16 (class template) |
