FIX: Destructor Called on Non Constructed Temporary (129617)


The information in this article applies to:

  • Microsoft Visual C++ 1.5
  • Microsoft Visual C++ 1.51
  • Microsoft Visual C++ 1.52
  • Microsoft Visual C++, 32-bit Professional Edition 2.0
  • Microsoft Visual C++, 32-bit Professional Edition 2.1
This article was previously published under Q129617

SYMPTOMS

The destructor of a class is called on a temporary that was never constructed if all the following conditions exist:

  • Two classes A and B are created.
  • A global function takes an argument of class B by value.
  • A conversion operator is used to convert from class A to class B.
  • The global function is called with an item of class A.
The destructor being called for an object that was never constructed can cause a general protection (GP) fault or other memory errors at run time.

WORKAROUND

Listed below in order of preference are three workarounds to this problem:

  • Define a conversion constructor in class B instead of a conversion operator in class A. -or-

  • Define the function to take an argument of "const B &" instead of taking class B by value. -or-

  • Explicitly instantiate a B object and call the global function with the instance of the B object.
The first workaround produces much cleaner code than the other two workarounds. See the sample code listed below for further details.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was fixed in Microsoft Visual C++, 32-bit Edition, version 4.0.

MORE INFORMATION

The sample code below can be used to reproduce this problem. If the program is complied with no options, the following output is displayed when the program is run:
-- A ctor called
-- Class A -> Class B conversion called
-- B ctor called, Created B: 1245064
-- B copy ctor, Created B: 1245096
-- B dtor called, Destroyed B: 1245064
-- Calling function test(B) on B: 1245076
-- B dtor called, Destroyed B: 1245076
This B was never constructed!
-- B dtor called, Destroyed B: 1245096
-- A dtor called

Sample Code to Reproduce Problem

/* Compile options needed, choose one of the following:
          none           - To demonstrate the problem.
          /DWORKAROUND1  - For workaround 1.
          /DWORKAROUND2  - For workaround 2.
          /DWORKAROUND3  - For workaround 3.
*/ 

#include <iostream.h>

class A;

class B
{
   B *pBthis;
public:
   B() {
      pBthis = this;
      cout << "-- B ctor called, Created B: " << (long)this << endl;
   }
#ifdef WORKAROUND1
   B(const A&) {
      pBthis = this;
      cout << "-- B(A) ctor called, Created B: " << (long)this << endl;
   }
#endif
   B( const B& b ) {
      pBthis = this;
      cout << "-- B copy ctor, Created B: " << (long)this << endl;
   }
   ~B() {
      cout << "-- B dtor called, Destroyed B: " << (long)this << endl;
     if (pBthis != this)
         cout << "    This B was never constructed!" << endl;
   }
};

class A
{
public:

   A() { cout << "-- A ctor called" << endl; }
   ~A() { cout << "-- A dtor called" << endl; }

#ifndef WORKAROUND1
   operator B() {
      cout << "-- Class A -> Class B conversion called" << endl;
      B b1;
      return b1;
   }
#endif

};

#ifdef WORKAROUND2
void test(const B &b)
#else
void test(B b)
#endif
{
   cout << "-- Calling function test(B) on B: " << (long)(&b) << endl;
}

int main()
{
   A a;
#ifdef WORKAROUND3

   B b(a);
   test(b);
#else
   test(a);
#endif
   return 0;
}
Modification Type:MinorLast Reviewed:7/5/2005
Keywords:kbbug kbfix KB129617
©2004 Microsoft Corporation. All rights reserved.