C++, an empty destructor for a class

I've read one of your answers (Will an 'empty' constructor or destructor do the same thing as the generated one?). You wrote:
"Let's assume the object of type C is created in the definition of A's constructor in the .cpp file, which also contains the definition of struct C. Now, if you use struct A, and require destruction of an A object, the compiler will provide an implicit definition of the destructor, just like in the case above. That destructor will also implicitly call the destructor of the auto_ptr object. And that will delete the pointer it holds, that points to the C object - without knowing the definition of C! That appeared in the .cpp file where struct A's constructor is defined". Could you please explain 2 things for me:
1. why should the destructor of A know C's definition?

2. How does adding A's empty destructor help the sityation?
Thank you