std::stringstream s{"123"};
std::cout << s.tellg();
But code below doesn't:
std::stringstream s{"123"};
std::println("{}", s.tellg());
The reason lies in that `std::fpos` can be implicitly converted to `std::streamoff`, which is one of the signed integer types (and overloaded operator<< will match it).
But it doesn't support `std::formatter` directly, so we have to code like:
std::println("{}", s.tellg() - std::streampos{});
// or
std::println("{}", (std::streamoff)s.tellg());
I'd like to post a small proposal for adding std::formatter for it :).