Question

This question is different than ‘When/why should I use a virtual destructor?‘.

struct B {
  virtual void foo ();
  ~B() {}  // <--- not virtual
};
struct D : B {
  virtual void foo ();
  ~D() {}
};
B *p = new D;
delete p;  // D::~D() is not called

Questions:

1. Can this be classified as an undefined behavior (we are aware that ~D() is not going to be called for sure)?
2. What if ~D() is empty. Will it affect the code in any way?
3. Upon using new[]/delete[] with B* p;, the ~D() will certainly not
   get called, irrespective of virtualness of the destructor. Is it
   an undefined behavior or well defined behavior?

Answer 1

when/why should I use a virtual destructor?
Follow Herb Sutters guideline:

A base class destructor should be either public and virtual, or protected and nonvirtual

Can this be classified as an undefined behavior (we are aware that ~D() is not going to be called for sure) ?

It is Undefined Behavior as per the standard, which usually results in the Derived class destructor not being called and resulting in a memory leak, but it is irrelevant to speculate on after effetcs of an Undefined Behavior because standard doesn’t gaurantee anything in this regard.

C++03 Standard: 5.3.5 Delete

5.3.5/1:

The delete-expression operator destroys a most derived object (1.8) or array created by a new-expression.
delete-expression:
::opt delete cast-expression
::opt delete [ ] cast-expression

5.3.5/3:

In the first alternative (delete object), if the static type of the operand is different from its dynamic type, the static type shall be a base class of the operand’s dynamic type and the static type shall have a virtual destructor or the behavior is undefined. In the second alternative (delete array) if the dynamic type of the object to be deleted differs from its static type, the behavior is undefined.73)

What if ~D() is empty. Will it affect the code in any way ?
Still it is Undefined Behavior as per the standard, The derived class destructor being empty may just make your program work normally but that is again implementation defined aspect of an particular implementation, technically, it is still an Undefined Behavior.

Note that there is no gaurantee here that not making the derived class destructor virtual just does not result in call to derived class destructor and this assumption is incorrect. As per the Standard all bets are off once you are crossed over in Undefined Behavior land.

Note what he standard says about Undefined Behavior.

The C++03 Standard: 1.3.12 undefined behavior [defns.undefined]

behavior, such as might arise upon use of an erroneous program construct or erroneous data, for which this International Standard imposes no requirements. Undefined behavior may also be expected when this International Standard omits the description of any explicit definition of behavior. [Note: permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during
translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message). Many erroneous program constructs do not engender undefined behavior; they are required to be diagnosed. ]

If only derived destructor will be not called is governed by the bold text in the above quote, which is clearly left open for each implementation.