std::codecvt
From cppreference.com
| Defined in header <locale>
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| template< class InternT, |
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Class 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
Four standalone (locale-independent) specializations are provided by the standard library:
| Defined in header
<locale> | |
| std::codecvt<char, char, std::mbstate_t> | identity conversion |
| std::codecvt<char16_t, char, std::mbstate_t> | conversion between UTF-16 and UTF-8 (since C++11) |
| std::codecvt<char32_t, char, std::mbstate_t> | conversion between UTF-32 and UTF-8 (since C++11) |
| std::codecvt<wchar_t, char, std::mbstate_t> | conversion between the system's native wide and narrow character sets |
In addition, every locale object constructed in a C++ program implements its own (locale-specific) versions of these four specializations.
Contents |
Member types
| Member type | Definition |
intern_type
|
InternT
|
extern_type
|
ExternT
|
state_type
|
State
|
Member functions
| constructs a new codecvt facet (public member function) | |
| destructs a codecvt facet (protected 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) |
Member objects
| Member name | Type |
id (static)
|
std::locale::id |
Protected member functions
| [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
| 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
The following examples reads a UTF-8 file using a locale which implements UTF-8 conversion in codecvt<wchar_t, char, mbstate_t> and converts a UTF-8 string to UTF-16 using one of the standard specializations of std::codecvt
Run this code
#include <iostream> #include <fstream> #include <string> #include <locale> #include <iomanip> #include <codecvt> // 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 = u8"z\u00df\u6c34\U0001f34c"; // or 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, 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 points: \n"; for (wchar_t c; fin >> c; ) std::cout << "U+" << std::hex << std::setw(4) << std::setfill('0') << c << '\n'; // 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 << "The UTF-8 file contains the following UTF-16 code points: \n"; for (char16_t c : str16) std::cout << "U+" << std::hex << std::setw(4) << std::setfill('0') << c << '\n'; }
Output:
The UTF-8 file contains the following UCS4 code points: U+007a U+00df U+6c34 U+1f34c The UTF-8 file contains the following UTF-16 code points: U+007a U+00df U+6c34 U+d83c U+df4c
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 template) | |
| creates a codecvt facet 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) |
