BUG: this Pointer Expands Incorrectly for Multiple Inheritance

ID: Q104676

4.10 | 1.00 1.50 1.51 1.52 4.10 MS-DOS | WINDOWS kbtool kbcode kbbuglist

The information in this article applies to:

SYMPTOMS

Expanding the "this" pointer of a class with multiple inheritance displays incorrect information.

CAUSE

The class type is derived from more than one classes.

STATUS

Microsoft has confirmed this to be a problem in Microsoft CodeView version 4.1 for MS-DOS and Windows and in the Visual Workbench Debugger that ships with Visual C++ for Windows, version 1.0.

The problem does not occur with the Visual Workbench Debugger that ships with Visual C++ 32-bit Edition or with CodeView for Win32s version 4.25.

MORE INFORMATION

When you debug the sample code below, expanding the this pointer of the class collection in a Watch window displays the following:

-this = 0x45CA:0x09A2 

  -base1 = {...}
    -base1 = CXX0030: Error: expression cannot be evaluated
      +base1 = CXX0030: Error: expression cannot be evaluated
      +base2 = CXX0030: Error: expression cannot be evaluated
       c = CXX0030: Error: expression cannot be evaluated
    -base2 = CXX0030: Error: expression cannot be evaluated
      +base1 = CXX0030: Error: expression cannot be evaluated
      +base2 = CXX0030: Error: expression cannot be evaluated
       c = CXX0030: Error: expression cannot be evaluated
     c = CXX0030: Error: expression cannot be evaluated
  -base2 = {...}
    +base1 = CXX0030: Error: expression cannot be evaluated
    +base2 = CXX0030: Error: expression cannot be evaluated
     c = CXX0030: Error: expression cannot be evaluated
   c = 12870
The base1 and base2 classes incorrectly appear more than once in the expanding process.

Sample Code

/*
 * Compiler options needed: /Zi /Od
 */
class base1{ 
   public:
   int a;
};

class base2{ 

   public:
   int b;
};

class collection:public base1,public base2 { 

   public:
   int c;
   void sub();
}; 

void collection::sub()
{ 
   a=100;  // Set a breakpoint here and watch this pointer.
   b=200;
   c=300;
} 

void main()
{ 
   collection k;
   k.sub();
}

Additional reference words: 4.10 1.00 1.50 KBCategory: kbtool kbcode kbbuglist KBSubcategory: WBDebug CvwIss Keywords : kb16bitonly

Last Reviewed: July 23, 1997