How to convert std::string to std::u32string in C++11?

I am working with Unicode in C++11 & I am right now unable to convert std::string to std::u32string.

My code is as follows:

#include <iostream>
#include <string>
#include <locale>
#include "unicode/unistr.h"
#include "unicode/ustream.h"

int main()
{
    constexpr char locale_name[] = "";
    setlocale( LC_ALL, locale_name );
    std::locale::global(std::locale(locale_name));
    std::ios_base::sync_with_stdio(false);
    std::wcin.imbue(std::locale());
    std::wcout.imbue(std::locale());

    std::string str="hello☺😆";

    std::u32string s(str.begin(),str.end());

    icu::UnicodeString ustr = icu::UnicodeString::fromUTF32(reinterpret_cast<const UChar32 *>(s.c_str()), s.size());
    std::cout << "Unicode string is: " << ustr << std::endl;

    std::cout << "Size of unicode string = " << ustr.countChar32() << std::endl;

    std::cout << "Individual characters of the string are:" << std::endl;
    for(int i=0; i < ustr.countChar32(); i++)
      std::cout << icu::UnicodeString(ustr.char32At(i)) << std::endl;

    return 0;
}

On executing the output is: (which is not expected)

Unicode string is: hello�������
Size of unicode string = 12
Individual characters of the string are:
h
e
l
l
o
�
�
�
�
�
�
�

Please suggest if any ICU library function exists for this