Linking error in C++ - c++

Question

Problem fixed. Thanks a lot!
I am having the following error in the code shown below:
Error is as follows:
$ g++ main.cpp Neighbor.cpp Graph.cpp
/tmp/ccclDcUN.o: In function main':
main.cpp:(.text+0xc1): undefined reference toGraph::add(int, Neighbor&)'
main.cpp:(.text+0xd3): undefined reference to `Graph::add(int, Neighbor&)'
collect2: ld returned 1 exit status
what could be going wrong?
// FILENAME: Graph.cpp
#include "Neighbor.h"
#include "Graph.h"
template <typename NS>
void Graph<NS>::add(int id,NS& n){
if(id>=adj_list.size())
while(adj_list.size()<id+1)
adj_list.push_back(list<NS>());
adj_list[id].push_back(n);
}
template <typename NS>
void Graph<NS>::remove(int id,NS& n){
if(id<adj_list.size()){
adj_list[id].remove(n);
}
}
// FILENAME: Graph.h
#ifndef GRAPH_H
#define GRAPH_H
#include "utils.h"
#include <vector>
#include <list>
class Neighbor;
template <typename NS>
class Graph {
private:
std::vector<std::list<NS> > adj_list;
public:
void add(int,NS&);
void remove(int,NS&);
inline typename std::vector<std::list<NS> >::iterator begin() { return adj_list.begin(); }
inline typename std::vector<std::list<NS> >::iterator end() { return adj_list.end(); }
};
#endif
// FILENAME: Neighbor.cpp
#include "Neighbor.h"
#include <iostream>
Neighbor::Neighbor(int id,float e,float p):id(id),edge_cost(e),price(p){}
bool operator==(const Neighbor& n1,const Neighbor& n2) {
if(&n1==&n2) return true;
return false;
}
ostream& operator<<(ostream& ostr,const Neighbor& n1) {
ostr<<"["<<n1.id<<","<<n1.price<<","<<n1.edge_cost<<"]";
return ostr;
}
// FILENAME: Neighbor.h
#ifndef NEIGHBOR_H
#define NEIGHBOR_H
#include <iosfwd>
class Neighbor {
private:
int id;
float edge_cost;
float price;
public:
Neighbor(int,float,float p=0.0);
friend bool operator==(const Neighbor&,const Neighbor&);
friend std::ostream& operator<<(std::ostream&,const Neighbor&);
};
#endif
// FILENAME: utils.h
#ifndef UTILS_H
#define UTILS_H
#include <iostream>
#include <fstream>
#include <stack>
#include <queue>
#include <vector>
#include <list>
#include <string>
#include <algorithm>
namespace utility {
typedef std::pair<int,int> ii;
typedef std::vector<int> vi;
typedef std::vector<ii> vii;
typedef std::vector<vii> vvii;
typedef std::stack<int> si;
typedef std::queue<int> qi;
}
#define UTILITY_TR(c,i) for(typeof((c).begin()) i = (c).begin() ; i!=(c).end() ; ++i )
#define UTILITY_ALL(c) (c).begin(),(c).end()
#define UTILITY_CPRESENT(c,x) (find(all(c),x) != (c).end())
#endif
// FILENAME: main.cpp
#include "utils.h"
#include "Neighbor.h"
#include "Graph.h"
using namespace std;
int main() {
Graph<Neighbor> graph;
Neighbor n1(1,10);
Neighbor n2(0,10);
graph.add(0,n1);
graph.add(1,n2);
cout<<"Printing graph"<<endl;
cout<<"--------------"<<endl;
UTILITY_TR(graph,it) {
UTILITY_TR(*it,n) {
cout<<*n<<endl;
}
}
};

Answer 1

What I usually do is manually verify the symbol exists in the library:
objdump --syms foo.o
This will output a list of symbols contained in the .o file... (since it's a link error, you should have .o files... (make sure you pass -c to g++ to get it to stop after compilation))... Then you can just visually verify the object has the symbols you think it does...

Answer 2

You need to have the definition of Graph's functions (add and remove) in the .h file so that the linker can find it.
I try to think of templates like envelopes. It's nonsensical to send it (compile) before you put in a letter (defined type). Seeing as cpp files are what is compiled, it makes sense that there shouldn't be cpp files for templated types.
HTH!