Question

I am using C++ under Ubuntu 11.10 and the latest version of NetBeans. Let’s say I have the
following code:

class Node {}
class DerivedNode : public Node {}

class Graph {
    vector<Node*> nodes;
}

class DerivedGraph : public Graph { }

At the moment I’m storing DerivedNodes in the DerivedGraph class like this for example:

nodes.push_back(new DerivedNode());

When I need to use specific methods that only apply to DerivedNodes and DerivedGraphs
I am forced to use a dynamic_cast on my Node pointers first.

I would like to be able to have specific methods in DerivedGraph which apply only to DerivedNodes
and avoid the need of casting pointers. I do not mind having to redesign my classes if the end
result is better than what I have.

I am sure there must be a clean and simple method to achieve the same thing I’m trying to do.
Maybe something with specialized templates? Any thoughts on the matter would be greatly
appreciated. I’ll also provide any additional information required in the case I haven’t been too
clear.

EDIT: I don’t have two copies. I wanted to put emphasis on how it looks. I apologize for the presentation. What I want to obtain is:

class DerivedGraph: public Graph {
    vector<DerivedNode*> nodes;
}    

Answer 1

Are you sure that your interface in Node is appropriate? Sometimes when you find yourself needing to downcast (especially in a case like this where base pointers are stored in a container) that may be a signal that your abstract interface doesn’t cover all your needs properly. Often something like the Template Method pattern solves all your needs without needing a downcast at all.

However, assuming that your inheritance model really need work in such a way, what you probably want to do is have virtual methods that get overridden in DerivedGraph for adding and getting nodes. You will have to verify the node type and downcast it in this case.

One final approach is to have two separate containers, one in the parent that contains all nodes that aren’t DerivedNode and then another container in DerivedGraph that contains all the DerivedNode. Then you use overridden functions again to determine which container to access depending on your API needs.