Convert std::string from UTF8, UTF16, ISO88591 to Hexadecimal - encoding

Question

I try to Encoding the std::string from UTF8,... to Hexadecimal. What I can't do right now is that I can't get the decimal value of each character of the input string to convert it if the input string contains special character which is from the Code Page Identifiers(windows-1258) include Vietnamese character.
At first, I will get the decimal value and then convert it to Binary and then to Hexadecimal. s is my input string. s = "Ồ".
void StringUtils::strToBinary(wstring s, string* result)
{
int n = s.length();
for (int i = 0; i < n; i++)
{
wchar_t c = s[i];
long val = long(c);
std::string bin = "";
while (val > 0)
{
(val % 2) ? bin.push_back('1') :
bin.push_back('0');
val /= 2;
}
reverse(bin.begin(), bin.end());
result->append(convertBinToHex(bin));
}
}
std::string StringUtils::convertBinToHex(std::string temp) {
long long binaryValue = atoll(temp.c_str());
long long dec_value = 0;
int base = 1;
int i = 0;
while (binaryValue) {
long long last_digit = binaryValue % 10;
binaryValue = binaryValue / 10;
dec_value += last_digit * base;
base = base * 2;
}
char hexaDeciNum[10];
while (dec_value != 0)
{
int temp = 0;
temp = dec_value % 16;
if (temp < 10)
{
hexaDeciNum[i] = temp + 48;
i++;
}
else
{
hexaDeciNum[i] = temp + 55;
i++;
}
dec_value = dec_value / 16;
}
std::string str;
for (int j = i - 1; j >= 0; j--) {
str = str + hexaDeciNum[j];
}
return str;
}
If my input only contain "Ồ" this is what my expected output
UTF8 : E1BB92
UTF16 : FEFF 1ED2
UTF16BE : 1ED2
UTF16LE : D21E
This how I do it in Java
Charset charset = Charset.forName(Enum.valueOf(Encoding.class, encodingType).toString());
ByteBuffer buffer = charset.newEncoder().encode(CharBuffer.wrap(inputString.toCharArray()));
byte[] bytes = new byte[buffer.limit()];
buffer.get(bytes, 0, buffer.limit());
result = new ByteField(bytes);
return result;
}