How to convert between BSTR and 32-bit Unicode strings in Visual C++?

I have 3rd party code which punycodes strings (escapes and unescapes). As Unicode input/output, it uses 32-bit Unicode strings (uint32_t-based), not 16-bit. My own input/output is BSTR (UTF 16-bit). How should I convert between 32-bit Unicode char array and BSTR (both directions)?

The code should work in Visual C++ 6.0 and later versions.

UTF16 is same as UTF32 for characters less than 0xFFFF. You can use the following conversion to display UTF-32 codes in Windows.

Note, this is based on Wikipedia UTF16 article. I didn't add any error checks, it expects valid codes.

void get_utf16(std::wstring &str, int ch32)
{
    const int mask = (1 << 10) - 1;
    if(ch32 < 0xFFFF)
    {
        str.push_back((wchar_t)ch32);
    }
    else
    {
        ch32 -= 0x10000;
        int hi = (ch32 >> 10) & mask;
        int lo = ch32 & mask;

        hi += 0xD800;
        lo += 0xDC00;

        str.push_back((wchar_t)hi);
        str.push_back((wchar_t)lo);
    }
}

For example the following code should display a smiley face in Windows 10:

std::wstring str;
get_utf16(str, 0x1f600);
::MessageBoxW(0, str.c_str(), 0, 0);

---

Edit:

Obtaining UTF-16 from array of UTF-32 code points, and the reverse operation:

UTF-16 string can be one wchar_t character long (2 bytes per code point), or 2 wchar_t characters joined together (4 bytes per code point). If the first character is between 0xD800 and 0xE000 that indicates 4 bytes per code point.

bool get_str_utf16(std::wstring &dst, const std::vector<unsigned int> &src)
{
    const int mask = (1 << 10) - 1;
    for(size_t i = 0; i < src.size(); i++)
    {
        unsigned int ch32 = src[i];
        ////check for invalid range
        //if(ch32 > 0x10FFFF || (ch32 >= 0xD800 && ch32 < 0xE000))
        //{
        //  cout << "invalid code point\n";
        //  return false;
        //}

        if(ch32 > 0x10000)
        {
            ch32 -= 0x10000;
            int hi = (ch32 >> 10) & mask;
            int lo = ch32 & mask;
            hi += 0xD800;
            lo += 0xDC00;
            dst.push_back((wchar_t)hi);
            dst.push_back((wchar_t)lo);
        }
        else
        {
            dst.push_back((wchar_t)ch32);
        }
    }
    return true;
}

void get_str_utf32(std::vector<unsigned int> &dst, const std::wstring &src)
{
    for(unsigned i = 0; i < src.size(); i++)
    {
        const wchar_t ch = src[i];
        if(ch >= 0xD800 && ch < 0xE000)
        {
            //this character is joined with the next character
            if(i < src.size() - 1)
            {
                unsigned int hi = src[i]; i++;
                unsigned int lo = src[i];
                hi -= 0xD800;
                lo -= 0xDC00;
                unsigned int u32 = 0x10000 + (hi << 10) + lo;
                dst.push_back(u32);
            }
        }
        else
        {
            dst.push_back(ch);
        }
    }
}

Example:

std::wstring u16 = L"123🙂456";

std::vector<unsigned int> u32;
get_str_utf32(u32, u16);
cout << "\n";

cout << "UTF-32 result: ";
for(auto e : u32)
    printf("0x%X ", e);
cout << "\n";

std::wstring test;
get_str_utf16(test, u32);
MessageBox(0, test.c_str(), (u16 == test) ? L"OK" : L"ERROR", 0);