c++ - Template class calling template function - Linker Error -

i'm experimenting template classes , functions, , ran following error. i'm trying call template function class vertex template class class relationshipexpander. mwe below yields following linker error:

undefined symbols architecture x86_64:   "typeinfo pipetype", referenced from:       typeinfo relationshipexpander<in> in main.cpp.o   "vtable pipetype", referenced from:       pipetype::pipetype() in main.cpp.o   note: missing vtable means first non-inline virtual member function has no definition. ld: symbol(s) not found architecture x86_64 

#include <iostream> #include <vector> #include <forward_list>  using namespace std;  // abbrr class vertex; typedef forward_list<const vertex*> edge_list;  /*  * matching  */ struct match_first {     match_first(unsigned value) : value(value) {};      template<class a>     bool operator()(const pair<const unsigned, a> &a) {         return a.first == value;     }      unsigned value; };  /*  * edge indirection  */ struct in {     static edge_list& get_list(pair<edge_list, edge_list> &a) {         return a.first;     } };  struct out {     static edge_list& get_list(pair<edge_list, edge_list> &a) {         return a.second;     } };   class vertex { public:     template<class direction>     edge_list &get_edges(unsigned relation_id) {         auto = find_if(neigh_set.begin(), neigh_set.end(), match_first(relation_id));          if (i != neigh_set.end()) {             return direction::get_list(i->second);         }          // throw except.     }  private:     vector<pair<const unsigned, pair<edge_list, edge_list>>> neigh_set; };    class pipetype { public:     /*      * mutators      */     virtual void pipe(vertex& gremlin); };    template <class direction> class relationshipexpander: pipetype { public:     /*      *  constructor      */     relationshipexpander(unsigned relationship_id) : relationship_id(relationship_id) {};      /*      *  mutators      */     void pipe(vertex& gremlin) {         gremlin.get_edges<direction>(relationship_id);     }; private:     unsigned relationship_id; };    int main() {     relationshipexpander<in> expand_in(1); } 

pipetype is different in member function seems not have definition. compiler says:

"vtable pipetype", referenced from:
pipetype::pipetype() in main.cpp.o
note: missing vtable means first non-inline virtual member function has no definition.

if intend pipetype interface, can make function pure virtual adding = 0 instead of function body. cannot skip implementation, if implemented in derived class.

class pipetype { public:     virtual void pipe(vertex& gremlin) = 0; }; 

if intend class have function implemented, of course have add implementation somewhere. either in header or in 1 of .cpp files.