Path: utzoo!utgpu!watmath!watcgl!andrewt
From: andrewt@watsnew.waterloo.edu (Andrew Thomas)
Newsgroups: gnu.g++.bug
Subject: Destructor called too soon, Destructor not called - G++1.35
Message-ID: <ANDREWT.89Aug3001106@watsnew.waterloo.edu>
Date: 3 Aug 89 04:11:06 GMT
Sender: daemon@watcgl.waterloo.edu
Distribution: world
Organization: University of Waterloo, Waterloo, Ontario, Canada
Lines: 88

System:	uVax II , Ultrix 2.0

G++ 1.35

Problem: The destructors for class instances passed as return values
(rather than returning a pointer or reference) get called too soon if
only a member of the class is of interest.  Also, sometimes no
destructor is called at all.

Program: The following program exhibits both behaviours described
above:

----------------- cut ----------------------------
#include	<stream.h>

class String
{
  char		*_ptr;
public:
  String (String&);
  String (char *);
  ~String();

  char*		ptr()			{ return (_ptr); }
};

String::String (String& old)
{
  char* a = new char[strlen(old._ptr) + 1];
  strcpy (a, old._ptr);
  _ptr = a;
  printf ("Constructing: %s (%x)\n", a, a);
}

String::String (char* old)
{
  char* a = new char[strlen(old) + 1];
  strcpy (a, old);
  _ptr = a;
  printf ("Constructing: %s (%x)\n", a, a);
}

String::~String()
{
  printf ("deleting %s (%x)\n", _ptr, _ptr);
  delete _ptr;
}

String my_func ()
{
  String	x = "Hello";
  return (x);
}

main ()
{
  printf ("Function: %s (%x)\n", my_func(), my_func());
  printf ("Function: %s (%x)\n", my_func().ptr(), my_func().ptr());
}

------------------------- cut -------------------------
The output of this program looks like:

Constructing: Hello (1c00)
Constructing: Hello (1c10)
deleting Hello (1c00)
Constructing: Hello (1c00)
Constructing: Hello (1c20)
deleting Hello (1c00)
Function: Hello (1c10)
Constructing: Hello (1c00)
Constructing: Hello (1c30)
deleting Hello (1c00)
deleting Hello (1c30)
Constructing: Hello (1c30)
Constructing: Hello (1c00)
deleting Hello (1c30)
deleting Hello (1c00)
Function: Hello (1c30)

Note only 6 destructors, and 8 constructors.  Also note that memory
address 1c30 is deleted with an automatically called destructor before
the String instance is actually used.
--

Andrew Thomas
andrewt@watsnew.waterloo.edu	Systems Design Eng.	University of Waterloo
"If a million people do a stupid thing, it's still a stupid thing." - Opus