An object of a class cannot be set to NULL; however, you can set a pointer (which contains a memory address of an object) to NULL.
Example of what you can’t do which you are asking:
Cat c;
c = NULL;//Compiling error
Example of what you can do:
Cat c;
//Set p to hold the memory address of the object c
Cat *p = &c;
//Set p to hold NULL
p = NULL;