C++ undefined reference to template class method [duplicate]

Question

This question already has an answer here:

• Why do I get “unresolved external symbol” errors when using templates? [duplicate] 3 answers
• Undefined reference to template members 1 answer

I always get

undefined reference to `Graph::InsertVertex(std::string)'

if I compile my project! Any hints why he cant resolve this reference? (all Files are in the netbeans project folder)

// main.cpp

#include <cstdlib>
#include <string>
#include "Graph.h"

using namespace std;

int main(int argc, char** argv)
{
    Graph<string> *graph = new Graph<string>(); // <--- ERROR

    graph->InsertVertex("A");

    return 0;
}

// Node.h

#include <iostream>
#include "Graph.h"

template<class T> 
class Node
{   

friend class Graph;    

public:
    Node(T val)
    {
        this->data = val;
        this->vertList = NULL;
        this->next = NULL;
    }

    Node(const Node& orig);
    virtual ~Node();

private:
    T data;
    Node<T> *vertList;
    Node<T> *next;
    int status;

};

// Graph.h

#include <iostream>
#include "Node.h"

template <class T> 
class Graph 
{    
public:
    Graph()
    {
        head = NULL;        
    }

    void InsertVertex(T val);    
    void InsertEdge(T v_val, T e_val);

    void PrintVertices();
    void PrintEdges(T v_val);

    void DeleteEdge(T v_val, T e_val);   
    void DeleteVertex(T val);

    void bfs();    

private:
    Node<T> *head;

};

// Graph.cpp

#include "Graph.h"

template <class T>
void Graph<T>::InsertVertex(T val)
{
    Node<T> *temp = new Node<T>(val);

    if(head == NULL) head = temp;
    else
    {
        Node<T> node = head;

        while(node->vertList != NULL)
            node = node->vertList;

        node->vertList = temp;
    }   
}

template <class T>
void Graph<T>::InsertEdge(T v_val, T e_val)
{
    if (head != NULL)
    {
        Node<T> *k = head;
        Node<T> *t = head;
        Node<T> *temp = new Node<T> (e_val);        

        while (t != NULL)
        {
            if (t->data == v_val)
            {
                Node<T> *s = t;

                while (s->next != NULL)
                    s = s->next;

                s->next = temp;

                while (k != NULL)
                {
                    if(k->data == e_val) break;

                    k = k->vertList;
                }

                temp->vertList = k;
                return;
            }

            t = t->vertList;
        } // end while loop        
    }
    else std::cout << "Add first vertices to the graph" << std::endl;
}

template <class T>
void Graph<T>::PrintEdges(T v_val)
{
    Node<T>* t = head;

    while (t != NULL)
    {
        if (t->data == v_val)
        {
            while (t->next != NULL)
            {
                std::cout << t->next->vertList->data << "   ";
                t = t->next;
            }
        }
        t = t->vertList;
    }
}

template <class T>
void Graph<T>::PrintVertices()
{
    Node<T>* t = head;

    while (t != NULL)
    {
        std::cout << t->data << "   ";
        t = t->vertList;
    }
}

Answer 1

Typically you want your template methods in the header, so they are compiled when needed. In case you really want to hide it in the implementation file, you have to explicitly instantiate the template in Graph.cpp like

template class  Graph<string>;

Since you have to do that for every type T you intend to use with Graph<T>, the point of the template class is somewhat defeated and you better put everything into the header

Answer 2

You need to define member functions in a header file, because when instantiating a template, the compiler needs to have access to the implementation of the methods, to instantiate them with the template argument.

In your example:

template <class T> 
class Graph  {    
public:
    void InsertVertex(T val) {
        Node<T> *temp = new Node<T>(val);

        if(head == NULL) 
            head = temp;

        // ... 
    }

    // ...

private:
    Node<T> *head;
};