Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ihnp4!alberta!calgary!lomow
From: lomow@calgary.UUCP (Greg Lomow)
Newsgroups: comp.lang.c++
Subject: Virtual Destructors
Message-ID: <1297@vaxb.calgary.UUCP>
Date: 19 Jan 88 21:36:33 GMT
Organization: U. of Calgary, Calgary, Ab.
Lines: 54


/*
A friend of mine (Darrin West) posted a message asking about the use
  of virtual destructors. The answer was that the use of virtual
  destructors is legal. Having used them, I can atest to the fact
  that they are very useful.

However, if you call delete AA when AA is NULL and the type of AA is
  "pointer to class a" where a's destructor is declared as virtual, a
  segmentation fault occurs. The C++ manual says that delete 
  can be applied to any pointer, NULL or non-NULL.

The example program shown below demonstrates this problem. If both
  pieces of code that are commented out are included in the program,
  a segmentation fault occurs.

We conjecture that delete is trying to access the destructor for AA
  like C++ would access any other virtual routine; i.e., by accessing a
  table with pointers to virtual functions, however if AA is NULL then 
  no such table exists.

Is this a bug or are we using virtual destructors wrong?

We are using version 1.2 of C++ on a Vax 11/780 running Unix 4.3.
*/
/*
error if virtual dtor and delete AA
ok    if non-virtual dtor and delete AA
ok    if virtual dtor and delete AA commented out
*/
#include <stream.h>
#define NULL	0

class a {
public:
  a();
  /* virtual*/ ~a();
};

a::a(){}
a::~a(){ cout << "deleted\n";}

main()
{ a *A = new a();
  delete A;

  a *AA = NULL;
//  delete AA;
}
-- 
Greg Lomow
	lomow@cpsc.calgary.cdn
      or
	....![ubc-vision,ihnp4]!alberta!calgary!lomow