FIX: C2682 Using dynamic_cast to Cast to a Const Pointer Type (150576)

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) ;
      }