致命错误lnk11201未解决的外部

我想弄清楚为什么我的代码不编译，这说未解决的外部。我正试图找出问题是什么，为什么它不能编译。

错误LNK2001:未解析的外部符号"public: virtual bool__thiscall BST，类std::allocator>>::isThere(类std:: basic_string类std::分配器>)"(isThere@ BST@V美元? basic_string@DU ?美元$ char_traits@D@std@@V ? allocator@D@2@@std@@@@UAE_NV美元? basic_string@DU美元? char_traits@D@std@@V美元? allocator@D@2@@std@@@Z美元)1> c: jordin 文档 visual studio用户2012 ConsoleApplication16 项目调试 ConsoleApplication16.exe:致命错误LNK1120: 1个未解决的外部

#include <iostream>
#include <sstream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
#include <ctime>
#include <cstdlib>
#include "BST.h"
SearchableADT<string> *dictionary = new BST<string>;
int main()
{
char choice = 'z';
string tempdata,
       fileloc;
cout<<"Welcome to Jordin Ray's Spell Check Applicationn"
    <<"Please pick a tree typen"
    <<"'b' -Binary Search Treen"
    <<"'a' -AVL treen"
    <<"'h' -hash tablen";
cin>>choice;
cout<<"Please give the exact location of the file for downloadn";
cin>>fileloc;
if(choice == 'b')
{
    dictionary->loadFromFile(fileloc);
    char ch = 'z';
    while(ch != 'q')
    {
        string word = "";
        if(ch == 'e')
        {
            cout<<"Please enter the word you would like to search forn";
            cin>>word;
            dictionary->isThere(word);
        }
        else if (ch == 'p')
        {
            cout<<"Please enter the partial word you would like to look for with a '?' where the missing letter is: n"
                <<"Ex. - '?nd' will search for words in the dictionary that have those last two lettersn";
        //  dictionary->partialIsThere(word);
        }
        else if (ch == 'c')
        {
            //dictionary.clear();
        }
        else if (ch == 's')
        {
            int entries;
            entries = dictionary->numEntries();
            cout<<"nThe amount of entries logged in the database is: "<<entries<<".n";
        }
        cout<<"Would you like to:n"
            <<"Clear the dictionary      - 'c'n"
            <<"Check for an entry        - 'e'n"
            <<"Check for a partial entry - 'p'n"
            <<"Report Statistics         - 's'n"
            <<"Quit                      - 'q'";
        cin>>ch;
    }//end of while loop
}
return 0;
}//end of main()

BST.h

#ifndef BST_H
#define BST_H
#pragma once
#include "SearchableADT.h"
#include <cstring>
using namespace std;
template <typename T>
class BST : public SearchableADT<T>
{
  private:
      struct t_node
      {
          string data;
          t_node *L;
          t_node *R;
      };
      int numnodes;
      t_node* head; 
      t_node* cPTR; //current pointer
      t_node* pPTR; //parent pointer
      t_node* tPTR; //temporary pointer  
 public:
  BST(void);
  ~BST(void);
  int loadFromFile(string filename);
  //void clear(void);
  void insertEntry(T info); 
  void deleteEntry(T info); 
  bool isThere(T info);
  int numEntries(void); 
  //needed for comparison to AVL 
  int BST<T>::height(t_node* tPTR);
}; // end of class BST
//constructor
template <typename T>
BST<T>::BST(void)
{
    head     = NULL;
    numnodes = 0;
}
//destructor
template <typename T>
BST<T>::~BST(void)
{
    this->clear();
}
template <typename T>
void BST<T>::insertEntry(T info)
{
    t_node* t = new t_node;
    t->data = info;
    t->L    = NULL;
    t->R    = NULL;
    pPTR    = NULL;
    if(head == NULL)
        head = t;
    else
    {
        t_node* cPTR; //current pointer
        cPTR = head;
        //checks to see if the data just entered is
        //greater than the head 
        while(cPTR)
        {
            pPTR = cPTR;
            if( t->data > cPTR->data)
                cPTR = cPTR->R;
            else
            cPTR = cPTR->L;
    }
    //checks to see if the data just entered is
    // less than the parent data that was 
    // set equal to cPTR;
    if(t->data < pPTR->data)
    {
        pPTR->L = t;
    }
    else
    {
        pPTR->R = t;
    }
}
}  //end of insertEntry()
template <typename T>
void BST<T>::deleteEntry(T info)
{
    //i use this as an example of abstraction
    //after using this function the cPTR and pPTR are already pointing to the data needed to be found
    if(!isThere(info))
    {
        cout<<"The data: '"<<info<<"' could not be found in the ADT, sorry!n";
        return;
    }
//one has to account for all possibilities when deleting data
//1.) - Removing just a leaf node (no children) **easy  
//2.) - Removing a leaf node with 1 child **medium
//3.) - Removing a leaf node with 2 children **hard
//case 1
if( cPTR->L == NULL && cPTR ->R == NULL)
{
    if(pPTR-> L == cPTR)
        pPTR->L = NULL;
    else
        pPTR->R = NULL;
    delete cPTR;
    return;
}//end of case 1
//case 2
else if((cPTR->L == NULL && cPTR->R != NULL) ||
        (cPTR->L != NULL && cPTR->R == NULL))
{
    //if the left data of cptr has data and the right data is NULL
    if(cPTR->L != NULL && cPTR->R == NULL)
    {
        if(pPTR->L == cPTR)
        {
            pPTR->L = cPTR->R;
            delete cPTR;
        }
        else
        {
            pPTR->R = cPTR->L;
            delete cPTR;
        }
    }
    else //right child present, and left child is NULL 
    {
        if(pPTR->L == cPTR)
        {
            pPTR->L = cPTR->R;
            delete cPTR;
        }
        else
        {
            pPTR->R = cPTR->R;
            delete cPTR;
        }
    }
    //case 3
    if((cPTR->L != NULL && cPTR->R != NULL))
    {
        tPTR=cPTR->R;
        //if the right node doesn't have a right leaf or left leaf, delete that node
        if((tPTR->L == NULL && tPTR->R == NULL))
        {
            cPTR = tPTR;
            delete cPTR;
            cPTR->R = NULL;
        }
        else
        {
            if(tPTR->L != NULL)
            {
                t_node* secPTR;
                secPTR = tPTR->L;
                //cycle through left nodes until you find the lowest left node
                while(secPTR->L != NULL)
                {
                    secPTR = secPTR->L;
                }
                //because you're only setting the data equal to eachother the cPTR keeps its left and right nodes
                //therefore it correctly replaces the data without unwanted loss of other information
                cPTR->data = secPTR->data;
                delete secPTR;
                cPTR->L    = NULL;
            }
            else
            {
                cPTR->data = tPTR->data;
                cPTR->R    = tPTR->R;
                delete tPTR;
            }
        }
    }
}
}//end of deleteEntry() function
template <typename T>
int BST<T>::numEntries(void)
{
    cout << "Tree Height: " << height(head) << endl;
    return numnodes;
}
template <typename T>
int BST<T>::loadFromFile(string filename)
{
    int count = 0;
    string tempdata;
    ifstream fin(filename);
    if(!fin)
    {
        cout<< "Error: Could no open filen";
        count--;
    }
    while(fin)
    {
        fin>>tempdata;
        if(fin)
    {
        insertEntry(tempdata);
        count++;
    }
}
fin.close();
return count;       
}//end of loadFromFile() function
template <typename T>
int BST<T>::height(t_node* tPTR)
{
   if (head == NULL)
       return -1;
    else
         return 1 + max(height(tPTR->L), height(tPTR->R));
    tPTR=head;
}
#endif

SearchableADT.h

#ifndef SEARCHABLEADT_H
#define SEARCHABLEADT_H
#include <string>
using namespace std;
template <typename T>
class SearchableADT
{
public:
    virtual int loadFromFile(string filename) = 0;
    //virtual void clear(void) = 0;
    virtual void insertEntry(T value) = 0;
    virtual void deleteEntry(T value) = 0;
    virtual bool isThere(T value) = 0 ;
    virtual int numEntries(void) = 0;
};
#endif