Private inheritance and dynamic cast issue

Question

Hello. I am hitting a problem combining private inheritance and dynamic casting, using Sun Studio 12 (Sun C++ 5.9 SunOS_sparc Patch 124863-01 2007/07/25):
I have three related classes. Let's call them:
Handle: The basic Handle class.
DspHandle. Handle implementation, able to add itself to a Receiver.
IODriver. Implemented in terms of DspHandle, It is the actually instantiated object.
Consider the following code. Sorry, but I've tried my best trying to minimize it:
#include <iostream>
using namespace std;
class Handle {
public:
virtual ~Handle() {};
class Receiver {
public:
void add(Handle &a);
class DspHandle : public Handle {
public:
virtual ~DspHandle() {};
void run(Receiver &recv);
class IODriver : private DspHandle {
public:
void start(Receiver &recv) {
DspHandle::run(recv);
void DspHandle::run(Receiver &recv) {
cout << "Calling Receiver::add(" << typeid(this).name() << ")" << endl;
recv.add(*this);
void Receiver::add(Handle &a) {
cout << "Called Receiver::add(" << typeid(&a).name() << ")" << endl;
DspHandle d = dynamic_cast<DspHandle>(&a);
cout << "a= " << &a << ", d=" << d << endl;
int main(int argc, char *argv[]) {
Receiver recv;
IODriver c;
c.start(recv);
Compiling and running this code with Sun Studio 12:
CC -o test test.cc
./test
Calling Receiver::add(DspHandle*)
Called Receiver::add(Handle*)
a= ffbffd54, d=0
The dynamic cast in Receiver::add, trying to downcast Handle to DspHandle, fails.
This same code works, for example with GNU g++ 4.1.3:
Calling Receiver::add(P9DspHandle)
Called Receiver::add(P6Handle)
a= 0xbfe9c898, d=0xbfe9c898
What is the reason of the dynamic_cast being rejected. Since the pointer is actually a DspHandle* , even when it is part of a private class, shouldn't it be downcastable to DspHandle? I think that perhaps the pointer should be rejected by Receiver::add(Handle &a) as it could be seen as a IODriver, that can't be converted to its private base. But since it's accepted, shouldn't the dynamic_cast work?
Changing the inheritance of IODriver to public instead of private avoids the error, but it's not an option in my design.
So, questions: Why is it failing? Any workarround?
Best wishes.
Manuel.

Answer 1

Thanks for your fast answer.
But could you please provide a deeper answer? I would like to know where do you think the problem is. Shouldn't the reference be accepted by Receiver:add, since it can only be seen as a Handle using its private base, or should the dynamic_cast work?
Aren't we actually trying to cast to a private base? However, casting to private bases directly uses to be rejected in compile time. Should the *this pointer passed from the DspHandle class to Receiver be considered a pure DspHandle or a IODriver?
Thanks a lot.