std::mbrtoc32(3)

NAME
std::mbrtoc32 - std::mbrtoc32

Synopsis
   Defined in header <cuchar>
   std::size_t mbrtoc32( char32_t* pc32,
   const char* s, (since C++11)
   std::size_t n,
   std::mbstate_t* ps );
   Converts a narrow multibyte character to its UTF-32 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 32-bit character and stores it in *pc32 (if
   pc32 is not null).
   If the multibyte character in *s corresponds to a multi-char32_t sequence (not
   possible with UTF-32), then after the first call to this function, *ps is updated in
   such a way that the next calls to mbrtoc32 will write out the additional char32_t,
   without considering *s.
   If s is a null pointer, the values of n and pc32 are ignored and the call is
   equivalent to std::mbrtoc32(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
   pc32 - pointer to the location where the resulting 32-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 *pc32 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 char32_t from a multi-char32_t character has now been written to
       *pc32. 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 *pc32.
     * -1 if encoding error occurs. Nothing is written to *pc32, the value EILSEQ is
       stored in errno and the value of *ps is unspecified.

Examples
// Run this code
 #include <iostream>
 #include <iomanip>
 #include <clocale>
 #include <cstring>
 #include <cwchar>
 #include <cuchar>
 #include <cassert>
 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
     char32_t c32;
     const char *ptr = str.c_str(), *end = str.c_str() + str.size() + 1;
     while(std::size_t rc = std::mbrtoc32(&c32, ptr, end - ptr, &state))
     {
         std::cout << "Next UTF-32 char: " << std::hex
                   << static_cast<int>(c32) << " obtained from ";
         assert(rc != (std::size_t)-3); // no surrogates in UTF-32
         if(rc == (std::size_t)-1) break;
         if(rc == (std::size_t)-2) break;
         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-32 char: 0x7a obtained from 1 bytes [ 0x7a ]
 Next UTF-32 char: 0xdf obtained from 2 bytes [ 0xc3 0x9f ]
 Next UTF-32 char: 0x6c34 obtained from 3 bytes [ 0xe6 0xb0 0xb4 ]
 Next UTF-32 char: 0x1f34c obtained from 4 bytes [ 0xf0 0x9f 0x8d 0x8c ]

See also
   c32rtomb convert a 32-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,State>)