std::c32rtomb(3)

NAME
std::c32rtomb - std::c32rtomb

Synopsis
   Defined in header <cuchar>
   std::size_t c32rtomb( char* s, char32_t c32, std::mbstate_t* ps ); (since C++11)
   Converts a UTF-32 character to its narrow multibyte representation.
   If s is not a null pointer, the function determines the number of bytes necessary to
   store the multibyte character representation of c32 (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::c32rtomb(buf, U'\0', ps) for
   some internal buffer buf.
   If c32 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
   c32 - the 32-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 char32_t in multi-char32_t-character
   sequence (does not occur in UTF-32).
   On failure (if c32 is not a valid 32-bit character), returns -1, stores EILSEQ in
   errno, and leaves *ps in unspecified state.

Example
// Run this code
 #include <iostream>
 #include <iomanip>
 #include <string_view>
 #include <clocale>
 #include <cuchar>
 #include <climits>
 int main()
 {
     std::setlocale(LC_ALL, "en_US.utf8");
     std::u32string_view strv = U"zß水🍌"; // or z\u00df\u6c34\U0001F34C
     std::cout << "Processing " << strv.size() << " UTF-32 code units: [ ";
     for(char32_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(char32_t c : strv)
     {
         std::size_t rc = std::c32rtomb(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 4 UTF-32 code units: [ 0x7a 0xdf 0x6c34 0x1f34c ]
 0x7a converted to [ 0x7a ]
 0xdf converted to [ 0xc3 0x9f ]
 0x6c34 converted to [ 0xe6 0xb0 0xb4 ]
 0x1f34c converted to [ 0xf0 0x9f 0x8d 0x8c ]

See also
   mbrtoc32 converts a narrow multibyte character to UTF-32 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,State>)