std::wctomb(3)

NAME
std::wctomb - std::wctomb

Synopsis
   Defined in header <cstdlib>
   int wctomb( char *s, wchar_t wc );
   Converts a wide character wc to multibyte encoding and stores it (including any
   shift sequences) in the char array whose first element is pointed to by s. No more
   than MB_CUR_MAX characters are stored. The conversion is affected by the current
   locale's LC_CTYPE category.
   If wc is the null character, the null byte is written to s, preceded by any shift
   sequences necessary to restore the initial shift state.
   If s is a null pointer, resets the global conversion state and determines whether
   shift sequences are used.

Parameters
   s - pointer to the character array for output
   wc - wide character to convert

Return value
   If s is not a null pointer, returns the number of bytes that are contained in the
   multibyte representation of wc or -1 if wc is not a valid character.
   If s is a null pointer, resets its internal conversion state to represent the
   initial shift state and returns 0 if the current multibyte encoding is not
   state-dependent (does not use shift sequences) or a non-zero value if the current
   multibyte encoding is state-dependent (uses shift sequences).

Notes
   Each call to wctomb updates the internal global conversion state (a static object of
   type std::mbstate_t, only known to this function). If the multibyte encoding uses
   shift states, this function is not reentrant. In any case, multiple threads should
   not call wctomb without synchronization: std::wcrtomb may be used instead.

Example
// Run this code
 #include <iostream>
 #include <iomanip>
 #include <clocale>
 #include <string>
 #include <cstdlib>
 void print_wide(const std::wstring& wstr)
 {
     bool shifts = std::wctomb(nullptr, 0); // reset the conversion state
     std::cout << "shift sequences are " << (shifts ? "" : "not" )
               << " used\n" << std::uppercase << std::setfill('0');
     for (const wchar_t wc : wstr) {
         std::string mb(MB_CUR_MAX, '\0');
         const int ret = std::wctomb(&mb[0], wc);
         const char* s = ret > 1 ? "s" : "";
         std::cout << "multibyte char '" << mb << "' is " << ret
                   << " byte" << s << ": [" << std::hex;
         for (int i{0}; i != ret; ++i) {
             const int c = 0xFF & mb[i];
             std::cout << (i ? " " : "") << std::setw(2) << c;
         }
         std::cout << "]\n" << std::dec;
     }
 }
 int main()
 {
     std::setlocale(LC_ALL, "en_US.utf8");
     // UTF-8 narrow multibyte encoding
     std::wstring wstr = L"z\u00df\u6c34\U0001d10b"; // or L"zß水𝄋"
     print_wide(wstr);
 }

Output:
 shift sequences are not used
 multibyte char 'z' is 1 byte: [7A]
 multibyte char 'ß' is 2 bytes: [C3 9F]
 multibyte char '水' is 3 bytes: [E6 B0 B4]
 multibyte char '𝄋' is 4 bytes: [F0 9D 84 8B]

See also
   mbtowc converts the next multibyte character to wide character
             (function)
   wcrtomb converts a wide character to its multibyte representation, given state
             (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>)