Re: Virtualization of a 'protected interface'

From:

Michael Doubez <michael.doubez@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 4 May 2009 05:48:49 -0700 (PDT)

Message-ID:

<dc403cf0-990f-459b-9b21-c9956a0ec7e0@s28g2000vbp.googlegroups.com>

On 3 mai, 23:17, Marcel M=FCller <news.5.ma...@spamgourmet.org> wrote:

I have an abstract base class that provides public and protected pure
functions.

class MyBase
{protected:
   virtual void InternalService() = 0;

  public:
   virtual void PublicService() = 0;

   void CommonImplementation()
   { // Do something that depends on InternalService...
     InternalService();
   }

};

class MyImplementation : public MyBase
{protected:
   void InternalService();

  public:
   void PublicService();

};

class MyProxy : public MyBase
{private:
   MyBase& Backend;

  protected:
   void InternalService()
   { // Modify Backend.InternalService somehow...
     Backend.InternalService(); // <-- access denied
   }

  public:
   MyProxy(MyBase& backend) : Backend(backend) {}

   void PublicService()
   { // Modify Backend.PublicService somehow...
     Backend.PublicService();
   }

};

Unfortunately MyProxy cannot access the protected members of /another/
MyBase instance. Is there another way to do something like that, except
for making MyBase::InternalService public?

The idea behind that is, that references to MyBase objects (MyProxy) can
override some properties of the underlying instance and different
references may override different properties. The common implementation
part in MyBase contains a framework to deal with change notifications
and asynchronously requested informations. If MyProxy overrides a
property it must also intercept the methods to obtain that information.
But these Methods should only be called by the framework in MyBase and
by proxy classes.

An ugly way to do it is to add a static member function in MyBase that
calls InternalService on its parameter:

class MyBase
{
  protected:
  //...
  static void DoInternalService(MyBase& b)
  {
   b.InternalService();
  }
  //...
};

And use it in MyImplementation:
  protected:
    void InternalService()
    {
      DoInternalService(Backend);
    }

Note that this kind of design if brittle and it breaks encapsulation.

You can also use a static function to return the pointer on the member
function and use it to call InternalService.

--
Michael