C++, function that can take either ifstream or istringstream

I have a function do_something that reads unsigned characters from a stream.

The stream can be created from a file given the file name. Or it can be created from the given string by considering it as data. I would like to reuse the function in both cases.

The code below gives an error in the second case: "error C2664: ‘do_something: cannot convert argument 1 from ‘std::basic_istringstream’ to ‘std::basic_istream’".

What is the proper way to do this?

static void do_something(std::basic_istream<unsigned char>& in)
{
   in.get();
}

static void string_read(unsigned char* in)
{
   std::basic_ifstream<unsigned char> file(std::string("filename"));
   do_something(file);

   std::basic_istringstream<unsigned char> str(std::basic_string<unsigned char>(in));
   do_something(str);
}

std::basic_istringstream<unsigned char> str(std::basic_string<unsigned char>(in));

std::basic_istringstream<unsigned char> str{std::basic_string<unsigned char>(in)};