TITLE: why have virtual destructors

[ Robert Martin had originally posted to a thread on interviewing
  job candidates on C++ knowledge. He stated that one question he
  would ask is roughly "what are the three problems you can run into
  by lacking a virtual destructor". -adc ]


RESPONSE: kanze@gabi-soft.fr (J. Kanze)

   1. The wrong address will be passed to operator delete.  This will
   generally result in a crash.

   2. The wrong size will be passed to an overloaded operator delete.
   Depending on how operator delete is implemented, this could also result
   in a crash, or it might only result in not all of the memory being
   freed.

   3. The wrong destructor will be called.  Generally, this will not cause
   a crash, but will likely result in some resorces not being freed.

   I suppose that Robert was exagerating a little when he said that all
   three can make the program crash.  Except that all three can leak
   memory, and in a lot of applications, a memory leak will eventually
   cause a crash.


RESPONSE: rmartin@oma.com (Robert C. Martin), 16 Jan 96

A bit more than that.  Whenever a constructor/destructor pair are not
properly conjugated the results are unpredictable.   Memory leaks are
the most prevalent problem, however other possibilities obtain.  

	e.g.:  Opened files not closed.
               Siezed Mutexes not Released
               File Locks not unlocked.
               Sentinel flags not cleared.
	           etc, etc.....