What is wrong in inheritance?

Question

What is wrong in inheritance in follow code?

https://code.sololearn.com/cu1ZxuH9T1iW/?ref=app

Answer 1

thanks buddy

Answer 2

I test this code, why it is run when type in main function this follow code:
test. f () ;

this does not return any thing but defined int for it's output?.

Answer 3

when there is "f (7)" so there is a error.
but when there is " f ( )" so there is not any error.

what is reason?
both of them don't return any thing

Answer 4

this might help.
https://code.sololearn.com/cDFcb240qi8D/?ref=app