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

Answer 1

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;
};

Answer 2

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