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It is very possible! I had to do a lot of reimplementation in college. If you plan on remaking something like ios library, I suggest looking through the headers first because theres a reason they were never remade or altered.
I guessed it is not a good idea. But still, is it possible for "home made" classes to become same level, to #include them in a same manner as the "core" libraries such as iostream etc.?
Oh so, for example, make an Employee class and #include "employee.h"? Yes, thats how it works! If you want to keep it saved for any project, put the .h file in your compiler's include folder