std::wcsrtombs(3)

NAME
std::wcsrtombs - std::wcsrtombs

Synopsis
   Defined in header <cwchar>
   std::size_t wcsrtombs( char* dst,
   const wchar_t** src,
   std::size_t len,
   std::mbstate_t* ps );
   Converts a sequence of wide characters from the array whose first element is pointed
   to by *src to its narrow multibyte representation that begins in the conversion
   state described by *ps. If dst is not null, converted characters are stored in the
   successive elements of the char array pointed to by dst. No more than len bytes are
   written to the destination array.
   Each character is converted as if by a call to std::wcrtomb. The conversion stops
   if:
     * The null character was converted and stored. src is set to a null pointer and
       *ps represents the initial shift state.
     * A wchar_t was found that does not correspond to a valid character in the current
       C locale. src is set to point at the first unconverted wide character.
     * the next multibyte character to be stored would exceed len. src is set to point
       at the first unconverted wide character. This condition is not checked if dst is
       a null pointer.

Parameters
   dst - pointer to narrow character array where the multibyte characters will be
         stored
   src - pointer to pointer to the first element of a null-terminated wide string
   len - number of bytes available in the array pointed to by dst
   ps - pointer to the conversion state object

Return value
   On success, returns the number of bytes (including any shift sequences, but
   excluding the terminating '\0') written to the character array whose first element
   is pointed to by dst. If dst is a null pointer, returns the number of bytes that
   would have been written (again, excluding the terminating null character '\0').
   On conversion error (if invalid wide character was encountered), returns
   static_cast<std::size_t>(-1), stores EILSEQ in errno, and leaves *ps in unspecified
   state.

Example
// Run this code
 #include <iostream>
 #include <vector>
 #include <clocale>
 #include <string>
 #include <cwchar>
 void print_wide(const wchar_t* wstr)
 {
     std::mbstate_t state = std::mbstate_t();
     std::size_t len = 1 + std::wcsrtombs(nullptr, &wstr, 0, &state);
     std::vector<char> mbstr(len);
     std::wcsrtombs(&mbstr[0], &wstr, mbstr.size(), &state);
     std::cout << "multibyte string: " << &mbstr[0] << '\n'
               << "Length, including '\\0': " << mbstr.size() << '\n';
 }
 int main()
 {
     std::setlocale(LC_ALL, "en_US.utf8");
     // UTF-8 narrow multibyte encoding
     const wchar_t* wstr = L"z\u00df\u6c34\U0001d10b"; // or L"zß水𝄋"
     print_wide(wstr);
 }

Output:
 multibyte string: zß水𝄋
 Length, including '\0': 11

See also
   wcrtomb converts a wide character to its multibyte representation, given state
             (function)
   mbsrtowcs converts a narrow multibyte character string to wide string, 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>)