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#include <iostream>
#include <vector>
using namespace std;
 
template<class V>
class GraphInterface {
	public:
	// Abstract GraphIterator class, defined elsewhere
	template<class VE> class GraphIteratorInterface;
 
	virtual ~GraphInterface();
 
	// BFS that uses the graph iterator class to perform work on the discovered
	// vertices and edges
	virtual void BFS(V src_vertex, GraphIteratorInterface<V> *) = 0;
 
};
 
template<class V>
class Graph : public GraphInterface<V> {
	public :
	template<class VE> class GraphIterator; // implements the graph-iter interface
 
	~Graph();
 
	void BFS(V src_vertex, GraphIterator<V> *);
};
 
template<class V> 
template <class VE>
class GraphInterface<V>::GraphIteratorInterface {
 
	virtual void examine_edge(VE &) = 0;
	virtual void discover_vertex(const V &) = 0;
	// ...
};
 
template<class V> template <class VE>
class Graph<V>::GraphIterator : public GraphInterface<V>::template GraphIteratorInterface<VE> {
	std::vector<V> vertices;
	//.. other members not in the interface
	void examine_edge(VE &);
	void discover_vertex(const V &);
};
 
int main() {
	Graph<int> g;
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:47:13: error: cannot declare variable ‘g’ to be of abstract type ‘Graph<int>’
  Graph<int> g;
             ^
prog.cpp:20:7: note:   because the following virtual functions are pure within ‘Graph<int>’:
 class Graph : public GraphInterface<V> {
       ^
prog.cpp:15:15: note: 	void GraphInterface<V>::BFS(V, GraphInterface<V>::GraphIteratorInterface<V>*) [with V = int]
  virtual void BFS(V src_vertex, GraphIteratorInterface<V> *) = 0;
               ^

Standard output is empty