FIX: Destructor Called on Non Constructed Temporary

• Microsoft C/C++ Compiler (CL.EXE), included with:
  • Microsoft Visual C++ for Windows, versions 1.5, 1.51, 1.52
  • Microsoft Visual C++ 32-bit Edition, 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.

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 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 :
Version : :1.0,1.5,1.51,1.52,2.0,2.1
Platform : NT WINDOWS
Issue type :