Linking error in 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 to Graph::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!