std::mbrtoc16(3)

NAME
std::mbrtoc16 - std::mbrtoc16

Synopsis
   Defined in header <cuchar>
   std::size_t mbrtoc16( char16_t* pc16,
                        const char* s, (since C++11)
                        std::size_t n,
                        std::mbstate_t* ps );
   Converts a narrow multibyte character to UTF-16 character representation.
   If s is not a null pointer, inspects at most n bytes of the multibyte character
   string, beginning with the byte pointed to by s to determine the number of bytes
   necessary to complete the next multibyte character (including any shift sequences).
   If the function determines that the next multibyte character in s is complete and
   valid, converts it to the corresponding 16-bit character and stores it in *pc16 (if
   pc16 is not null).
   If the multibyte character in *s corresponds to a multi-char16_t sequence (e.g. a
   surrogate pair in UTF-16), then after the first call to this function, *ps is
   updated in such a way that the next call to mbrtoc16 will write out the additional
   char16_t, without considering *s.
   If s is a null pointer, the values of n and pc16 are ignored and the call is
   equivalent to std::mbrtoc16(nullptr, "", 1, ps).
   If the wide character produced is the null character, the conversion state *ps
   represents the initial shift state.
   The multibyte encoding used by this function is specified by the currently active C
   locale.

Parameters
   pc16 - pointer to the location where the resulting 16-bit character will be written
   s - pointer to the multibyte character string used as input
   n - limit on the number of bytes in s that can be examined
   ps - pointer to the conversion state object used when interpreting the multibyte
          string

Return value
   The first of the following that applies:
     * 0 if the character converted from s (and stored in *pc16 if non-null) was the
       null character.
     * the number of bytes [1...n] of the multibyte character successfully converted
       from s.
     * -3 if the next char16_t from a multi-char16_t character (e.g. a surrogate pair)
       has now been written to *pc16. No bytes are processed from the input in this
       case.
     * -2 if the next n bytes constitute an incomplete, but so far valid, multibyte
       character. Nothing is written to *pc16.
     * -1 if encoding error occurs. Nothing is written to *pc16, the value EILSEQ is
       stored in errno and the value of *ps is unspecified.

Example
// Run this code
 #include <clocale>
 #include <cstring>
 #include <cuchar>
 #include <cwchar>
 #include <iomanip>
 #include <iostream>
 int main()
 {
     std::setlocale(LC_ALL, "en_US.utf8");
     std::string str = "z\u00df\u6c34\U0001F34C"; // or u8"zß水🍌"
     std::cout << "Processing " << str.size() << " bytes: [ " << std::showbase;
     for (unsigned char c: str)
         std::cout << std::hex << +c << ' ';
     std::cout << "]\n";
     std::mbstate_t state{}; // zero-initialized to initial state
     char16_t c16;
     const char *ptr = &str[0], *end = &str[0] + str.size();
     while (std::size_t rc = std::mbrtoc16(&c16, ptr, end - ptr + 1, &state))
     {
         std::cout << "Next UTF-16 char: " << std::hex
                   << static_cast<int>(c16) << " obtained from ";
         if (rc == (std::size_t)-3)
             std::cout << "earlier surrogate pair\n";
         else if (rc == (std::size_t) - 2)
             break;
         else if (rc == (std::size_t) - 1)
             break;
         else
         {
             std::cout << std::dec << rc << " bytes [ ";
             for (std::size_t n = 0; n < rc; ++n)
                 std::cout << std::hex << +static_cast<unsigned char>(ptr[n]) << ' ';
             std::cout << "]\n";
             ptr += rc;
         }
     }
 }

Output:
 Processing 10 bytes: [ 0x7a 0xc3 0x9f 0xe6 0xb0 0xb4 0xf0 0x9f 0x8d 0x8c ]
 Next UTF-16 char: 0x7a obtained from 1 bytes [ 0x7a ]
 Next UTF-16 char: 0xdf obtained from 2 bytes [ 0xc3 0x9f ]
 Next UTF-16 char: 0x6c34 obtained from 3 bytes [ 0xe6 0xb0 0xb4 ]
 Next UTF-16 char: 0xd83c obtained from 4 bytes [ 0xf0 0x9f 0x8d 0x8c ]
 Next UTF-16 char: 0xdf4c obtained from earlier surrogate pair

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