Where in derived class or base class should I declare friend class?

Question

I have a derived class that implements an interface(abstract class) in C++. This interface has a couple of protected functions whose implementation are defined in derived class.

Now I'm trying to access these protected functions in derived class from an external class, by declaring the external class as a friend in derived class. However, the compiler still complains that I cannot access the protected member functions.

For example: I have class ConnectSession implementing IConnectSession . And an external class called SystemProcess who wants to access protected member function HandlePriviledgedRequest .

In IConnectSession.h

class IConnectSession{
protected:
    virtual void HandlePriviledgedRequest() = 0;
}

In ConnectSession.h

class ConnectSession : public IConnectSession{
protected:
    void HandlePriviledgedRequest() override {/* func definition */}
    friend class SystemProcess;
}

In friend class I try to access HandlePriviledgedRequest()

SystemProcess.cpp

SystemProcess::ApplyConfiguration(){
    std::shared_ptr<IConnectSession> sessionPtr = new ConnectSession();
    sessionPtr->HandlePriviledgedRequest(); // compile error
}

It throws an error from compiler saying I cannot access protected members even if I have already declared SystemProcess as a friend. Since friend declarations are not inherited I particularly declare them in ConnectSession . Why does it not work and how to fix it? Thanks.

Answer 1

这是因为您的访问是通过IConnectSession接口sessionPtr ，并且您的友谊仅与派生类有关，而与基类无关。

Answer 2

Access control is something that only applies at compile time. In

SystemProcess::ApplyConfiguration(){
    std::shared_ptr<IConnectSession> sessionPtr = new ConnectSession();
    sessionPtr->HandlePriviledgedRequest(); // compile error
}

even though sessionPtr gets set to an instance of ConnectSession , its static type is IConnectSession and that is what access control is ran against. Since SystemProcess isn't a friend of IConnectSession it is not allowed to access HandlePriviledgedRequest() .

You'll need to make it a friend of IConnectSession to get it to compile.