< cpp‎ | string‎ | multibyte
Defined in header <cuchar>
std::size_t mbrtoc16( char16_t* pc16,

                      const char* s,
                      std::size_t n,

                      std::mbstate_t* ps );
(since C++11)

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(NULL, "", 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.


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.


#include <iostream>
#include <iomanip>
#include <clocale>
#include <cstring>
#include <cwchar>
#include <cuchar>
int main()
    std::setlocale(LC_ALL, "en_US.utf8");
    std::string str = u8"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 << c16 << " obtained from ";
        if(rc == (std::size_t)-3)
            std::cout << "earlier surrogate pair\n";
        else if(rc == (std::size_t)-2)
        else if(rc == (std::size_t)-1)
        else {
            std::cout << std::dec << rc << " bytes [ ";
            for(std::size_t n = 0; n < rc; ++n)
                std::cout << std::hex << +(unsigned char)ptr[n] << ' ';
            std::cout << "]\n";
            ptr += rc;


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

convert a 16-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,State>)