Defined in header <cuchar>
std::size_t mbrtoc32( char32_t* pc32,
                      const char* s,
                      std::size_t n,
                      std::mbstate_t* ps );
(since C++11)

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.


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.


#include <cassert>
#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
    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)
        if (rc == (std::size_t) - 2)
        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;


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

convert a 32-bit wide character to narrow multibyte string
converts a string from ExternT to InternT, such as when reading from file
(virtual protected member function of std::codecvt<InternT,ExternT,StateT>)
C documentation for mbrtoc32

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