Question

I have defined a class

template <class T> class NodeMap {

NodeMap(int n, T defaultEntry = NULL);

virtual ~NodeMap();

T& operator[](const node& u);

...

}

which maps an object of type node to an object of parameter type T.

Now I’d like to have a class Matching, which is essentially a NodeMap<node>. For convenience, I’d like to add methods like isProperMatching(Graph& G) and match(node u, node v). Can Matching inherit from NodeMap<node>? Is it possible (and if yes, is it a good idea) to extend a template class with a fixed template parameter?

Answer 1

You have the following possibilities as far at the code you’ve shown tells me what you are doing:

1. Specialize Nodemap for T=node, to include the additional methods you want to have. For convenience you can then typedef Nodemap<node> to Matching. However this might add some code duplication of the Methods you have in the nonspecialized template.
2. Derive from Nodemap<node> as you suggested. This is a valid approach, if you design Nodemap in a way that makes it a proper base class. The virtual destructor suggests you did that already, although it might not be necessary to make methods virtual at all.
3. If possible, provide the additional functionality as free functions, so there is no need to derive from or specialize Nodemap<node>

I would consider a mixed approach of the first two: Make a base class template (I’ll call it NodeMapBase) that contains the common functionality of NodeMaps and Matching, then derive the NodeMap template from the corresponding base class template (probably without adding much functionality) and derive Matching from NodeMapBase<node>. If you have a look at MSVC’s standard library implementation you see that a lot there.