How to achieve forward declaration of class template

Question

I am trying to create a Graph class by using Nested Classes Vertex and Edge. I want to make my Vertex class to accept generic parameters. I have forward declared my Vertex class so that I can use it in my Edge Class.

When I use templates, I get few errors that I am not sure how to resolve.

Here is what I tried. The errors are commented out next to each line.

class Graph
{
    private:
        template <class T>
        class Vertex; // Forward Declaration

        template <class T>
        vector<Vertex<T> > vertices; // Err: member 'vertices' declared as a template

        class Edge
        {
            public:
                template <class T>
                Vertex<T>& _orig; // Err: member '_orig' declared as a template

                template <class T>
                Vertex<T>& _dest; // Err: member '_dest' declared as a template

                template <class T>
                Edge(Vertex<T>& orig, Vertex<T>& dest) : _orig(orig), // Err: member initializer '_orig' does not name a non-static data member or base class
                                                         _dest(dest) { }

                template <class T>
                Vertex<T>& getOrig() { return _orig; } // Err: use of undeclared identifier '_orig'

                template <class T>
                Vertex<T>& getDest() { return _dest; } // Err: use of undeclared identifier '_dest'
        };

        template <typename T>
        class Vertex
        {
            public:
                T _data;
                vector<Edge> _edges;

                Vertex(T data) : _data(data) { }

                void addEdgeToVertex(Edge& edge)
                {
                    _edges.push_back(edge);
                }
        };

    public:
        template <typename T>
        void addEdge(Vertex<T>& orig, Vertex<T>& dest)
        {
            Edge edge(orig, dest);
            orig.addEdgeToVertex(edge);
            dest.addEdgeToVertex(edge);
        }
};

Can you please help me in pointing out what I am doing wrong? How can I fix this code?

Answer 1

The forward declaration looks fine. The problem is that you can't declare "template member variable", how could you specify the template argument for them?

You should make class template instead.

template <class E>
class Graph
{
    private:
        template <class T>
        class Vertex; // Forward Declaration

        vector<Vertex<E> > vertices;
    ...
};

It's same for class Edge too.

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