Inheritance and virtual methods questions

[zdzero]

I have something like this

class Repository
{
private:
 MyArray<Something> *myList;
 MyArray<MyArray<Something>> *undo;

public:
 // constructor, destructor, append, sort, undo last action, no point in writing them
}

Now I need to provide a way for the user to chose if the program will use files to save the lists or just keep it like this in memory.

I can write a new class, RepositoryInFile that will extend the Repository class

class RepositoryInFile: public Repository
private:
 void readFromFile();
 void writeToFile();
public:
 // again, no point in writing each function, just one to make my idea clear
 void append(Something newElement)
 {
  Repository::append(newElement);
  writeToFile();
 }

but if I do something like

 Repository *r = new RepositoryInFile();
 r.append(x);

the append from Repository will be used, not the one I want (the one from RepositoryInFile)

Do I need to make a entire new class, let's say VirtualRepository, that will have virtual methods for everything I need and then just make both Repository and RepositoryInFile extend that class?

Is there a quicker (and maybe more elegant) way of doing this?

[MiiNiPaa]

make all functions derived classes might need to overload virtual.

//in Repository class
virtual void append(Something newElement)

[zdzero]

So just make them virtual in Repository and make RepositoryInFile extend it?

That means that in headers I'll have

class Repository
{
 virtual void append(Something newElement);
 // everything like this
}

and my implementation of the Repository class will stay unchanged from what I have now?
That will make my life a lot easier as I'll have to do less changed to my actual code.

[MiiNiPaa]

Yes. Pretty much like that.