Q129617: FIX: Destructor Called on Non Constructed Temporary

Article: Q129617
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1
Operating System(s): 
Keyword(s): kbbuglist kbfixlist
Last Modified: 23-JUL-2001

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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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