FIX: Destructor Called on Non Constructed Temporary

WINDOWS        | WINDOWS NT

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE) included with:

      - Microsoft Visual C++ for Windows, versions 1.5, 1.51, and 1.52
      - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, and 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.

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.

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 B {

   B *pBthis;

   B() {
      pBthis = this;
      cout << "-- B ctor called, Created B: " << (long)this << endl;
   }

   B(const A&) {
      pBthis = this;
      cout << "-- B(A) ctor called, Created B: " << (long)this << endl;
   }

   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;
   }
};

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

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

};

#ifdef WORKAROUND2

void test(const B &b)

void test(B b)

   cout << "-- Calling function test(B) on B: " << (long)(&b) << endl;

int main()

   A a;

   B b(a);
   test(b);

   test(a);

   return 0;