How to convert string from decimal code to cyrillic / unicode16 in C++?

Question

In my c++ program I have a "decimal code string" and I would like to print it in a file using unicode characters.

• This is the decimal code string: квартира
• This is the cyrillic decoded string: Квартира
• This is my code:

#include "stdafx.h"
#include<iostream>
#include<algorithm>
#include<cmath>
#include<cstdio>

using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
// string I have:
string decimal_code_string = "&#1082;&#1074;&#1072;&#1088;&#1090;&#1080;&#1088;&#1072;";

FILE *stream;
fopen_s( &stream, "C:\\Users\\Luze\\Desktop\\myFile.txt", "w+, ccs=UTF-8" );
//printf("--> %s", decimal_code_string.c_str()); 
// string I would like to write: "Квартира"
fwprintf(stream, L"%hs", decimal_code_string.c_str());
fclose(stream);

return 0;
}

[ EDIT ]

I've tried @MarkRansom solution but it doesn't seem to work:

#include "stdafx.h"
#include<iostream>
#include<algorithm>
#include<cmath>
#include<cstdio>

using namespace std;

std::string wchar_to_UTF8(const wchar_t * in)
{
    std::string out;
    unsigned int codepoint = 0;
    for (in;  *in != 0;  ++in)
    {
        if (*in >= 0xd800 && *in <= 0xdbff)
            codepoint = ((*in - 0xd800) << 10) + 0x10000;
        else
        {
            if (*in >= 0xdc00 && *in <= 0xdfff)
                codepoint |= *in - 0xdc00;
            else
                codepoint = *in;

            if (codepoint <= 0x7f)
                out.append(1, static_cast<char>(codepoint));
            else if (codepoint <= 0x7ff)
            {
                out.append(1, static_cast<char>(0xc0 | ((codepoint >> 6) & 0x1f)));
                out.append(1, static_cast<char>(0x80 | (codepoint & 0x3f)));
            }
            else if (codepoint <= 0xffff)
            {
                out.append(1, static_cast<char>(0xe0 | ((codepoint >> 12) & 0x0f)));
                out.append(1, static_cast<char>(0x80 | ((codepoint >> 6) & 0x3f)));
                out.append(1, static_cast<char>(0x80 | (codepoint & 0x3f)));
            }
            else
            {
                out.append(1, static_cast<char>(0xf0 | ((codepoint >> 18) & 0x07)));
                out.append(1, static_cast<char>(0x80 | ((codepoint >> 12) & 0x3f)));
                out.append(1, static_cast<char>(0x80 | ((codepoint >> 6) & 0x3f)));
                out.append(1, static_cast<char>(0x80 | (codepoint & 0x3f)));
            }
            codepoint = 0;
        }
    }
    return out;
}

std::wstring UTF8_to_wchar(const char * in)
{
    std::wstring out;
    unsigned int codepoint;
    while (*in != 0)
    {
        unsigned char ch = static_cast<unsigned char>(*in);
        if (ch <= 0x7f)
            codepoint = ch;
        else if (ch <= 0xbf)
            codepoint = (codepoint << 6) | (ch & 0x3f);
        else if (ch <= 0xdf)
            codepoint = ch & 0x1f;
        else if (ch <= 0xef)
            codepoint = ch & 0x0f;
        else
            codepoint = ch & 0x07;
        ++in;
        if (((*in & 0xc0) != 0x80) && (codepoint <= 0x10ffff))
        {
            if (sizeof(wchar_t) > 2)
                out.append(1, static_cast<wchar_t>(codepoint));
            else if (codepoint > 0xffff)
            {
                out.append(1, static_cast<wchar_t>(0xd800 + (codepoint >> 10)));
                out.append(1, static_cast<wchar_t>(0xdc00 + (codepoint & 0x03ff)));
            }
            else if (codepoint < 0xd800 || codepoint >= 0xe000)
                out.append(1, static_cast<wchar_t>(codepoint));
        }
    }
    return out;
}

int _tmain(int argc, _TCHAR* argv[])
{
string decimal_code_string = "&#1082;&#1074;&#1072;&#1088;&#1090;&#1080;&#1088;&#1072;";
printf("decimal_code: %s\n", decimal_code_string.c_str());
 
// string I would like to write: "Квартира"

wstring temp_string = wstring(decimal_code_string.begin(), decimal_code_string.end());
const wchar_t* result = temp_string.c_str();
string utf8_string=  wchar_to_UTF8( result );
printf("wchar_to_UTF8: %s\n", utf8_string);

const char* resultcp = decimal_code_string.c_str();
wstring wide_string = UTF8_to_wchar( resultcp );
printf("UTF8_to_wchar: %s\n", wide_string);
 
cout << "End\n";
return 0;
}

Answer 1

Simply extract the number entity inside each character: that is exactly the Unicode/UTF32 code of that character.

Once you've extract that cose you can use any UTF32-to-UTF8 string conversion algorithm, for example the experimental StringSuite C++20 library.