std::c16rtomb(3)

NAME
std::c16rtomb - std::c16rtomb

Synopsis
   Defined in header <cuchar>
   std::size_t c16rtomb( char* s, char16_t c16, std::mbstate_t* ps ); (since C++11)
   Converts a single code point from variable-length 16-bit character representation
   (typically, UTF-16) to a narrow multibyte character representation.
   If s is not a null pointer and c16 is the last 16-bit code unit in a valid
   variable-length encoding of a code point, the function determines the number of
   bytes necessary to store the multibyte character representation of that code point
   (including any shift sequences, and taking into account the current multibyte
   conversion state *ps), and stores the multibyte character representation in the
   character array whose first element is pointed to by s, updating *ps as necessary.
   At most MB_CUR_MAX bytes can be written by this function.
   If s is a null pointer, the call is equivalent to std::c16rtomb(buf, u'\0', ps) for
   some internal buffer buf.
   If c16 is not the final code unit in a 16-bit representation of a wide character, it
   does not write to the array pointed to by s, only *ps is updated.
   If c16 is the null wide character u'\0', a null byte is stored, preceded by any
   shift sequence necessary to restore the initial shift state and the conversion state
   parameter *ps is updated to represent the initial shift state.
   The multibyte encoding used by this function is specified by the currently active C
   locale.

Parameters
   s - pointer to narrow character array where the multibyte character will be stored
   c16 - the 16-bit character to convert
   ps - pointer to the conversion state object used when interpreting the multibyte
         string

Return value
   On success, returns the number of bytes (including any shift sequences) written to
   the character array whose first element is pointed to by s. This value may be 0,
   e.g. when processing the first char16_t in a surrogate pair.
   On failure (if c16 is not a valid 16-bit character), returns -1, stores EILSEQ in
   errno, and leaves *ps in unspecified state.

Notes
   The C++ standard defers to the C standard for the semantics of this function. In C11
   as published, unlike std::mbrtoc16, which converts variable-width multibyte (such as
   UTF-8) to variable-width 16-bit (such as UTF-16) encoding, this function can only
   convert single-unit 16-bit encoding, meaning it cannot convert UTF-16 to UTF-8
   despite that being the original intent of this function. This was corrected by the
   post-C11 defect report DR488.

Example
   Note: this example assumes the fix for the defect report 488 is applied. // Run this code
 #include <climits>
 #include <clocale>
 #include <cuchar>
 #include <iomanip>
 #include <iostream>
 #include <string_view>
 int main()
 {
     std::setlocale(LC_ALL, "en_US.utf8");
     std::u16string_view strv = u"zß水🍌"; // or z\u00df\u6c34\U0001F34C
     std::cout << "Processing " << strv.size() << " UTF-16 code units: [ ";
     for (char16_t c : strv)
         std::cout << std::showbase << std::hex << static_cast<int>(c) << ' ';
     std::cout << "]\n";
     std::mbstate_t state{};
     char out[MB_LEN_MAX]{};
     for (char16_t c : strv)
     {
         std::size_t rc = std::c16rtomb(out, c, &state);
         std::cout << static_cast<int>(c) << " converted to [ ";
         if (rc != (std::size_t) - 1)
             for (unsigned char c8 : std::string_view{out, rc})
                 std::cout << +c8 << ' ';
         std::cout << "]\n";
     }
 }

Output:
 Processing 5 UTF-16 code units: [ 0x7a 0xdf 0x6c34 0xd83c 0xdf4c ]
 0x7a converted to [ 0x7a ]
 0xdf converted to [ 0xc3 0x9f ]
 0x6c34 converted to [ 0xe6 0xb0 0xb4 ]
 0xd83c converted to [ ]
 0xdf4c converted to [ 0xf0 0x9f 0x8d 0x8c ]

See also
   mbrtoc16 converts a narrow multibyte character to UTF-16 encoding
   (C++11) (function)
   do_out converts a string from InternT to ExternT, such as when writing to file
   [virtual] (virtual protected member function of
             std::codecvt<InternT,ExternT,StateT>)
   C documentation for
   c16rtomb