std::mbrtowc(3)

NAME
std::mbrtowc - std::mbrtowc

Synopsis
   Defined in header <cwchar>
   std::size_t mbrtowc( wchar_t* pwc,
   const char* s,
   std::size_t n,
   std::mbstate_t* ps );
   Converts a narrow multibyte character to a wide character.
   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 wide character and stores it in *pwc (if pwc
   is not null).
   If s is a null pointer, the values of n and pwc are ignored and call is equivalent
   to std::mbrtowc(nullptr, "", 1, ps).
   If the wide character produced is the null character, the conversion state stored in
   *ps is the initial shift state.

Parameters
   pwc - pointer to the location where the resulting wide 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 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 pwc if non-null) was
       the null character
     * the number of bytes [1...n] of the multibyte character successfully converted
       from s
     * static_cast<std::size_t>(-2) if the next n bytes constitute an incomplete, but
       so far valid, multibyte character. Nothing is written to *pwc.
     * static_cast<std::size_t>(-1) if encoding error occurs. Nothing is written to
       *pwc, the value EILSEQ is stored in errno and the value of *ps is left
       unspecified.

Example
// Run this code
 #include <iostream>
 #include <clocale>
 #include <cstring>
 #include <cwchar>
 void print_mb(const char* ptr)
 {
     std::mbstate_t state = std::mbstate_t(); // initial state
     const char* end = ptr + std::strlen(ptr);
     int len;
     wchar_t wc;
     while((len = std::mbrtowc(&wc, ptr, end-ptr, &state)) > 0) {
         std::wcout << "Next " << len << " bytes are the character " << wc << '\n';
         ptr += len;
     }
 }
 int main()
 {
     std::setlocale(LC_ALL, "en_US.utf8");
     // UTF-8 narrow multibyte encoding
     const char* str = "z\u00df\u6c34\U0001d10b"; // or u8"zß水𝄋"
                       // or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9d\x84\x8b";
     print_mb(str);
 }

Output:
 Next 1 bytes are the character z
 Next 2 bytes are the character ß
 Next 3 bytes are the character 水
 Next 4 bytes are the character 𝄋

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_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>)