What are the accessibility rules for using dynamic_cast to access a sibling class? The information I have found about dynamic_cast is articles in C++ Report, and in Bjarne Stroustrup's latest book do not cover this point.

The test program at the end of this message, which I tried with Borland C++ 4.0, shows that BCC 4.0 does NOT allow a dynamic_cast to the private bnase of a derived class, even when the dynamic_cast is performed from a friend function.

I would consider allowing only global (public) accessibility in cases like this, but this should explicitly be in the standard. I doubt that the standards committee would wish to require run-time accessability determination.

*****    Test Program follows   *****

#include <iostream.h>
#include <typeinfo.h>

class Base1
{
  virtual void f() {}
};

class Base2
{
  virtual void f() {}
};

class Derived : public Base1, Base2
{
  friend Base2* b12(Base1* pb);
};

Base2* b12(Base1* pb)
{
  return dynamic_cast<Base2 *>(pb);
}

int main()
{
  try
  {
    Derived d, *pd;
    Base1 *b1 = &d;

    Base2 *b2;
    if (b2 = b12(b1))
    {
      cout << "Pointer points to type: " << typeid(*b2).name() << endl;
    }
    else throw Bad_cast();
  }
  catch(Bad_cast)
  {
    cout << "Dyamic cast failed" << endl;
    return 1;
  }
  catch(...)
  {
    cout << "Exception handling error" << endl;
    return 2;
  }
  return 0;
}