From cppreference.com
template<classInternT,classExternT,classStateT>classcodecvt;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
Specializations
The standard library is guaranteed to provide the following specializations (they are
required to be implemented by any locale object
):
Defined in header
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 Nested types
Type Definition intern_typeInternTextern_typeExternTstate_typeStateTData members
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)
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
Nested type Definition enumresult{ok,partial,error,noconv};Unscoped enumeration type Enumeration constant Definition okconversion was completed with no error partialnot all source characters were converted errorencountered an invalid character noconvno 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,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<classFacet>structdeletable_facet:Facet{template<class...Args>deletable_facet(Args&&...args):Facet(std::forward<Args>(args)...){}~deletable_facet(){}};intmain(){// UTF-8 narrow multibyte encodingstd::stringdata=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::wifstreamfin("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_tc;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::u16stringstr16=conv16.from_bytes(data);std::cout<<"\n\nThe UTF-8 file contains the following UTF-16 code units:\n"<<std::hex;for(char16_tc: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 Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++20 std::codecvt<char16_t,char8_t,std::mbstate_t> and
std::codecvt<char32_t,char8_t,std::mbstate_t> are locale-independent deprecate them See also
Character
conversions locale-defined multibyte
(UTF-8, GB18030) UTF-8
UTF-16
UTF-16
/
(with C11's DR488)
<char16_t,char,mbstate_t>
<char16_t>
<char32_t>
<wchar_t>
N/AUCS-2
(without C11's DR488)
<char16_t>
<char16_t>UTF-32
/
<char32_t,char,mbstate_t>
<char32_t>
<char32_t>
system wchar_t: UTF-32 (non-Windows)
UCS-2 (Windows)
/
<
<wchar_t,char,mbstate_t>>(
)
<wchar_t>
<wchar_t>