Buenas a quien lo lea!

Estoy programando un árbol binario, pero al momento de compilar el programa, la función virtual no la reconoce o no se linkea, ¿a qué se debe? o que hago mal?   ojalá puedan ayudarme, gracias!

BTNode.h :

#define BTNODE_H_INCLUDED
 
#include <sstream>
 
using namespace std;
 
template<typename IT>
struct BTNode {
 
  IT data;
  BTNode<IT>* left;
  BTNode<IT>* right;
 
  BTNode(const IT &Data, BTNode<IT>* lv = NULL, BTNode<IT>* rv = NULL) :
    data(Data), left(lv), right(rv) {}
 
  virtual ~BTNode() {}
 
  virtual string toString() const {
 
    ostringstream os;
 
    os << data;
    return os.str();
  }
};
 
template <typename IT>
ostream &operator<<(ostream &out, const BTNode<IT> &node) {
 
  return out << node.toString();
}
 
#endif // BTNODE_H_INCLUDED
 
 

Binary_Tree.h:

 
#ifndef BINARY_TREE_H_INCLUDED
#define BINARY_TREE_H_INCLUDED
 
#include <cstddef>
#include <sstream>
#include <stdexcept>
#include <string>
#include "BTNode.h"
 
using namespace std;
 
template <typename IT>
class Binary_Tree {
 
public:
 
  Binary_Tree(): root(NULL) {}
  Binary_Tree(const IT &Data, const Binary_Tree<IT> &lc = Binary_Tree(), const Binary_Tree<IT> &rc = Binary_Tree()):
    root(new BTNode<IT>(Data, lc.root, rc.root)) {}
 
  virtual ~Binary_Tree() {}
 
  Binary_Tree<IT> getLeftSubtree() const;
  Binary_Tree<IT> getRigthSubtree() const;
  const IT &getData() const;
 
  bool isNull() const;
  bool isLeaf() const;
 
  virtual string toString() const; //<---esta es la función!!!!
 
  static Binary_Tree<IT> readBinaryTree(istream& in);
Binary_Tree(BTNode<IT>* newRoot): root(newRoot) {}
protected:
 
 
  BTNode<IT>* root;
};
 
template<typename IT>
ostream &operator<<(ostream &out, const Binary_Tree<IT> &tree) {
 
  return out << tree.toString();
}
 
template<typename IT>
istream &operator>>(istream &in, Binary_Tree<IT> &tree) {
 
  tree = Binary_Tree<IT>::readBinaryTree(in);
  return in;
}
 
#endif // BINARY_TREE_H_INCLUDED
 
 

Binary_Tree.cpp

#include "Binary_Tree.h"
 
using namespace std;
 
template<typename IT>
Binary_Tree<IT> Binary_Tree<IT>::getLeftSubtree() const {
 
  if(root == NULL) {
 
    throw invalid_argument("getLeftSubtree on empty tree");
  }
 
  return Binary_Tree<IT>(root->left);
}
 
template<typename IT>
Binary_Tree<IT> Binary_Tree<IT>::getRigthSubtree() const {
 
  if(root == NULL) {
 
    throw invalid_argument("getLeftSubtree on empty tree");
  }
 
  return Binary_Tree<IT>(root->right);
}
 
template<typename IT>
bool Binary_Tree<IT>::isLeaf() const {
 
  if(root != NULL) {
 
    return root->left == NULL && root->right == NULL;
  } else {
 
    return true;
  }
}
 
template<typename IT>
string Binary_Tree<IT>::toString() const {
 
  ostringstream os;
 
  if(isNull()) {
 
    os << "NULL\n";
  } else {
 
    os << *root << "\n";
    os << getLeftSubtree().toString();
    os << getRigthSubtree().toString();
  }
 
  return os.str();
}