std::codecvt
Defined in header <locale>
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template< class InternT, |
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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
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InternT
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extern_type
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ExternT
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state_type
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StateT
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[edit] Data members
Member | Description |
std::locale::id id [static]
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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
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conversion was completed with no error |
partial
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not all source characters were converted |
error
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encountered an invalid character |
noconv
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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
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#include <codecvt> #include <cstdint> #include <fstream> #include <iomanip> #include <iostream> #include <locale> #include <string> // utility wrapper to adapt locale-bound facets for wstring/wbuffer convert template<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 encoding std::string data = reinterpret_cast<const char*>(+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 facet std::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 facet std::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 |
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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 |
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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) |