Please explain me these 2 codes

Question

https://code.sololearn.com/cyW4JEJesnTV/?ref=app
https://code.sololearn.com/ck5hJ76Onac6/?ref=app
I have added my questions in comment there .

Answer 1

Test 1:
To have all destructors being called when deleting a pointer to a base object the destructor has to be declared virtual. That's just how C++ works. Without declaring it virtual C++ has no idea which destructor to call when you delete a pointer to an object of a base class. Therefore it is good practice to always declare destructors of classes which are base classes virtual.
Test 2:
The problem is that you give the argument per value, effectively copying the object. Just change the declaration to
void foo(base& a);

Answer 2

12345 you are copying the object, which means in this case that you are constructing a completely new object of class base, which does not know that it was created from an object of a derived class. I extended your code to hopefully make it a bit clearer.
https://code.sololearn.com/cTisoEDgw7uD/?ref=app

Answer 3

Hape i slightly understood 1st test code.
But sir can you explain me this part
base &a=>calls derived class method
base a=>calls base class method