BUG: Debugger Shows Incorrect "this" Pointer

• The C/C++ Compiler (CL.EXE), used with:
  • Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2, 5.0, 6.0

SYMPTOMS

In Visual C++ 4.0, when you debug a class that uses multiple inheritance, the "this" pointer for any of the base classes that reside at an offset in the class are displayed incorrectly. Data for derived classes is not displayed correctly either because the debugger attempts to reference the data by using the invalid "this" pointer.

In Visual C++ 4.1 or 4.2, only the value of "this" pointer is not shown correctly. All the members are displayed correctly.

CAUSE

The compiler produces incorrect debugging information causing a display error in the debugger only. The vtable entries themselves are correct, and programmatic access to the data produces the proper values.

RESOLUTION

The only way to work around this problem is to output the addresses to an output device. You may use a TRACE statement or cout() as shown in the sample code below.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.

MORE INFORMATION

Sample Code to Demonstrate Problem

/* This code demonstrates the problem.
   Compile options needed: none
*/ 

#include <iostream.h>

// Class definitions
class A
{
protected:
   int aint;
public:
    virtual void fA(void) = 0;
};

class B
{
protected:
   int bint;
public:
    virtual void fB(void) = 0;
};

class AB : public A, public B
{
protected:
   int abint;
public:
    virtual void fAB(void);
};

class DAB : public AB
{
protected:
   int dabint;
public:
   DAB() {aint = 1; bint = 2; abint = 3; dabint = 4;}
    void fA(void)
    {
        cout << "DAB::fA - this=" << this << endl;
    }
    void fB(void)
    {
   // The "this" pointer will be displayed incorrectly in the
   // locals window during the execution of this function. The
   // address displayed in the output window will still be correct.

        cout << "DAB::fB - this=" << this << endl;

   // The base class member variables "aint" and "bint" will be
   // displayed incorrectly in the locals window, but the derived
   // class member variables "abint" and "dabint" will be
   // incorrect.  All will show correct values in the final output.

      cout << "aint: " << aint << " bint: " << bint
         << " abint: " << abint << " dabint: " << dabint <<
 endl;
    }
};

void AB::fAB(void)
{
    cout << "AB::fAB - this=" << hex << this << endl;
    fA();
    fB();
}

void main()
{
    DAB dab;
    dab.fAB();
} 

Additional query words: kbVC400bug

Keywords : kbCompiler kbDebug kbide kbVC500bug kbVC600bug
Version : winnt:4.0,4.1,4.2,5.0,6.0
Platform : winnt
Issue type : kbbug