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:

A data holder class CDataHolder, which uses a Pimpl pattern

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

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();
};
```
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.

Abhay Abhay · Accepted Answer · 2009-05-12T03:45:42

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?

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?

2 Answers