DOCUMENT:Q129617  23-JUL-2001  [visualc]
TITLE   :FIX: Destructor Called on Non Constructed Temporary
PRODUCT :Microsoft C Compiler
PROD/VER::1.0,1.5,1.51,1.52,2.0,2.1
OPER/SYS:
KEYWORDS:kbbuglist kbfixlist

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - Microsoft C/C++ Compiler (CL.EXE), included with:
    - Microsoft Visual C++, versions 1.5, 1.51, 1.52 
    - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, versions 1.0, 2.0, 2.1 
-------------------------------------------------------------------------------

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;
   }

Additional query words: 1.00 1.50 1.51 1.52 2.00 2.10 8.0 8.00 8.0c 8.00c 9.0 9.00 GPF

======================================================================
Keywords          :  kbbuglist kbfixlist
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :1.0,1.5,1.51,1.52,2.0,2.1
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

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