why using namespace std is used here ?

it was a bit weird for me at first cuz idk what a class means

me either

I didn't know that you could turn a num negative by using -

Thank you for pointing it out.

Don't think it's a good practice to write some implementation in class declaration ( that is in a header file normally ).

Dunno why, but this solution throwed an error for me.

Nice. Somebody has read clean code and polymorphism. haha

The function returns a function not void
The solution is returning a lambda expression which is the shortcut version for writing a function