Defined in header <codecvt>
template< class Elem,
          unsigned long Maxcode = 0x10ffff,
          std::codecvt_mode Mode = (std::codecvt_mode)0 >
class codecvt_utf8_utf16 : public std::codecvt<Elem, char, std::mbstate_t>;
(since C++11)
(deprecated in C++17)

std::codecvt_utf8_utf16 is a std::codecvt facet which encapsulates conversion between a UTF-8 encoded byte string and UTF-16 encoded character string. If Elem is a 32-bit type, one UTF-16 code unit will be stored in each 32-bit character of the output sequence.

This is an N:M conversion facet, and cannot be used with std::basic_filebuf (which only permits 1:N conversions, such as UTF-32/UTF-8, between the internal and the external encodings). This facet can be used with std::wstring_convert.

Template Parameters

Elem - either char16_t, char32_t, or wchar_t
Maxcode - the largest value of Elem that this facet will read or write without error
Mode - a constant of type std::codecvt_mode

Member functions

constructs a new codecvt_utf8_utf16 facet
(public member function)
destroys a codecvt_utf8_utf16 facet
(public member function)


explicit codecvt_utf8_utf16( std::size_t refs = 0 );

Constructs a new std::codecvt_utf8_utf16 facet, passes the initial reference counter refs to the base class.


refs - the number of references that link to the facet



Destroys the facet. Unlike the locale-managed facets, this facet's destructor is public.

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

invokes do_out
(public member function of std::codecvt)
invokes do_in
(public member function of std::codecvt)
invokes do_unshift
(public member function of std::codecvt)
invokes do_encoding
(public member function of std::codecvt)
invokes do_always_noconv
(public member function of std::codecvt)
invokes do_length
(public member function of std::codecvt)
invokes do_max_length
(public member function of std::codecvt)

Protected member functions

converts a string from internT to externT, such as when writing to file
(virtual protected member function of std::codecvt)
converts a string from externT to internT, such as when reading from file
(virtual protected member function of std::codecvt)
generates the termination character sequence of externT characters for incomplete conversion
(virtual protected member function of std::codecvt)
returns the number of externT characters necessary to produce one internT character, if constant
(virtual protected member function of std::codecvt)
tests if the facet encodes an identity conversion for all valid argument values
(virtual protected member function of std::codecvt)
calculates the length of the externT string that would be consumed by conversion into given internT buffer
(virtual protected member function of std::codecvt)
returns the maximum number of externT characters that could be converted into a single internT character
(virtual protected member function of std::codecvt)

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


#include <iostream>
#include <string>
#include <codecvt>
#include <cassert>
#include <locale>
int main()
    std::string u8 = u8"z\u00df\u6c34\U0001f34c";
    std::u16string u16 = u"z\u00df\u6c34\U0001f34c";
    // UTF-8 to UTF-16/char16_t
    std::u16string u16_conv = std::wstring_convert<
        std::codecvt_utf8_utf16<char16_t>, char16_t>{}.from_bytes(u8);
    assert(u16 == u16_conv);
    std::cout << "UTF-8 to UTF-16 conversion produced "
              << u16_conv.size() << " code units:\n";
    for (char16_t c : u16_conv)
        std::cout << std::hex << std::showbase << c << ' ';
    // UTF-16/char16_t to UTF-8
    std::string u8_conv = std::wstring_convert<
        std::codecvt_utf8_utf16<char16_t>, char16_t>{}.to_bytes(u16);
    assert(u8 == u8_conv);
    std::cout << "\nUTF-16 to UTF-8 conversion produced "
              << std::dec << u8_conv.size() << " bytes:\n" << std::hex;
    for (char c : u8_conv)
        std::cout << +(unsigned char)c << ' ';


UTF-8 to UTF-16 conversion produced 5 code units:
0x7a 0xdf 0x6c34 0xd83c 0xdf4c 
UTF-16 to UTF-8 conversion produced 10 bytes:
0x7a 0xc3 0x9f 0xe6 0xb0 0xb4 0xf0 0x9f 0x8d 0x8c

See also

locale-defined multibyte
(UTF-8, GB18030)
UTF-16 mbrtoc16 / c16rtomb(with C11's DR488) codecvt<char16_t, char, mbstate_t>
UCS2 c16rtomb(without C11's DR488) codecvt_utf8<char16_t>





mbrtoc32 / c32rtomb.

codecvt<char32_t, char, mbstate_t>


system wide:

mbsrtowcs / wcsrtombs
<wchar_t, char, mbstate_t>>(locale).

No No
converts between character encodings, including UTF-8, UTF-16, UTF-32
(class template)
(C++11)(deprecated in C++17)
tags to alter behavior of the standard codecvt facets
(C++11)(deprecated in C++17)
converts between UTF-8 and UCS2/UCS4
(class template)
(C++11)(deprecated in C++17)
converts between UTF-16 and UCS2/UCS4
(class template)

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