How to access a data class's private member variable from another derived class whose parent class is a friend class of the data class?

Question

I have three classes:

1. A data holder class CDataHolder, which uses a Pimpl pattern

   class CDataHolder
   {
   public:
     // ...
   private:
   friend class CBase;
   struct PImpl;
   PImpl* iPimpl;
   };
   

2. A base class CBase, which need to access the iPImpl member in CDataHolder, so it is a friend class of CDataHolder

   class CBase: 
   {
   protected:
   CDataHolder::Pimpl* getDataHolderPimpl();
   };
   

3. A derived class CDerived from CBase, which need to access the same iPimpl member also. Here occurs a problem. The derived class cannot use the iPimpl member although its parent class is a friend class. like this:

   class CDerived : public CBase
   {
   public:
   void doSth() {
      CDataHolder::Pimpl *pImpl = getDataHolderPimpl(); // this line raises an error: 
                                                        // "illegal access from CDataHolder to protected/private member CDataHolder::PImpl"
   }
   };
   

   There are plenty of derived classes, so it's not a good way for each derived class to put a "friend class CDerivedXXX" line in CDataHolder class. How to overcome this issue? Is there a better way to do this? Thanks in advance.

Answer 1

Since you have declared struct PImpl in the private part of CDataHolder class, only friends of CDataHolder can access the same. Why don't you put a forward declaration struct PImpl in the public section or even better before the CDataHolder class?