Question

I have the following three files in the same directory:

citysim.cpp

#include "utils.h"
using namespace std;

int main()
{
    City *c;
    c = new City();
    Graph<City *> g;
    g.addVertex(c);
}

utils.h

#include <iostream>
#include <map>
#include <vector>
using namespace std;

class City {
    public:
        City() {}
    private:
        string name;
};

template <typename Tkey>
class Graph {
    public:
        Graph() {}
        void addVertex(Tkey);
    private:
        vector<Tkey> v;
        vector< vector<int> > e;
        map<Tkey, int> key_map;
};

utils.cpp

#include "utils.h"

template <typename Tkey>
void Graph<Tkey>::addVertex(Tkey vertex)
{
    v.push_back(vertex);
}

And I am really perplexed as to why the following compilation sequence produces the result indicated:

test> g++ -c citysim.cpp
test> g++ -c utils.cpp
test> g++ -o citysim citysim.o utils.o
citysim.o: In function `main':
citysim.cpp:(.text+0x4a): undefined reference to `Graph<City*>::addVertex(City*)'
collect2: ld returned 1 exit status

Any ideas or insights are appreciated!

Answer 1

Define everything of your templated class in your header file, not in a cpp file. Instead of having your utils.cpp have everything like this in your header file:

template <typename Tkey>
class Graph {
public:
    Graph() {}
    void addVertex(Tkey vertex)
    {
         v.push_back(vertex);
    }
private:
    vector<Tkey> v;
    vector< vector<int> > e;
    map<Tkey, int> key_map;
};

Here is the related reading in the faq…

EDIT:
(But you can define it later on like you did it in your cpp in the header file as well…)