Q150576: FIX: C2682 Using dynamic_cast to Cast to a Const Pointer Type

Article: Q150576
Product(s): Microsoft C Compiler
Version(s): 4.0,4.1,4.2
Operating System(s): 
Keyword(s): kbCompiler kbVC500fix
Last Modified: 03-AUG-2001

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The information in this article applies to:

- Microsoft Visual C++, versions 4.0, 4.1, 4.2 
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SYMPTOMS
========

When casting a const pointer of derived class to a const pointer of base class,
the compiler generates the following error message:

  

  C2682: cannot use dynamic_cast to convert from "'const class 'name1'*

         to 'const class 'name2'*'"

where name1 and name2 refer to the names of your derived class and base class
respectively.

RESOLUTION
==========

Cast the derived class const pointer to a derived class non-const pointer. Then
use the derived class non-const pointer in the dynamic_cast expression. See the
MORE INFORMATION section of this article for sample code and a workaround.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug was corrected in Visual C++ version 5.0.

MORE INFORMATION
================

The following sample code demonstrates the problem and generates the compiler
error C2682. The workaround is shown in the comments in the sample code below.

Sample Code
-----------

        #define WORKAROUND  /* Uncomment this line to workaround */ 
        class A
        {
          virtual void f() { } ;
        } ;

        class B : public A
        {
        } ;

        void g(const B* pb)
        {
        #ifndef WORKAROUND
           const A* pa = dynamic_cast<const A*>(pb) ;
        #else
           B* tmpBp = const_cast<B*>(pb) ;
           const A* pa = dynamic_cast<const A*>(tmpBp) ;
        #endif
        }

        void main()
        {
           B bObj ;
           const B* pb = &bObj ;
           g(pb) ;
        }

Additional query words: kbVC400bug

======================================================================
Keywords          : kbCompiler kbVC500fix 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbVC410 kbVC420
Version           : :4.0,4.1,4.2
Issue type        : kbbug
Solution Type     : kbfix

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