How to serialize generic object without using external libraries (C++ 11)

I tried to find with no success how to serialize a generic object. Every solution that was suggested knew the type/ used some external library (i.e boost/cereal). I have a template and in it I need to update a file. The file needs to hold unordered_map<std::string, T>. Is there a way to serialize an object T without knowing it's type/it's members without using an external library?