Linker errors

I have a header file which contains the class with only function declarations. I then define the function in a separate cpp file. However, I get linker errors when I do this. If I define the function in the header file (but outside the class) I don't get linker errors. Why would it matter whether the function was defined inside the header or cpp file?

The function insertVertex is giving me problems.

GraphEdgeList.h

Code:

#ifndef GRAPHEDGELIST_H
#define GRAPHEDGELIST_H

#include <vector>
#include <list>

template <typename Object>
class Vertex {
private:
    template <typename> friend class GraphEdgeList;
    Object o;
    int undirectedDegree;
    int inDegree;
    int outDegree;
public:
    Vertex(const Object& oo = Object()) : o(oo), undirectedDegree(0),
        inDegree(0), outDegree(0)
    {
    }
    const Object& element() const {
        return o;
    }
    void element(const Object& oo) {
        o = oo;
    }
};

template <typename Object>
class Edge {
private:
    template <typename> friend class GraphEdgeList;
    typedef Vertex<Object> Vertex;
    Object o;
    Vertex* origin;
    Vertex* destination;
    bool isDirected;
public:
    Edge(const Object& oo = Object(), Vertex* org = NULL, Vertex* dst = NULL,
        bool directed = false)
        : o(oo), origin(org), destination(dst), isDirected(directed)
    {
    }
    const Object& element() const {
        return o;
    }
    void element(const Object& oo) {
        o = oo;
    }
};

/*
 * A graph represented by an edge list
 */
template <typename Object>
class GraphEdgeList {
    typedef Vertex<Object> Vertex;
    typedef Edge<Object> Edge;
    std::list<Vertex> V;
    std::list<Edge> E;
public:
    Vertex& insertVertex(const Object& o);
};

#ifdef DEFINEINHEADER

template <class Object>
typename GraphEdgeList<Object>::Vertex&
GraphEdgeList<Object>::insertVertex(const Object& o) {
    typedef typename GraphEdgeList<Object>::Vertex Vertex;
    V.push_back(Vertex(o));
    return V.back();
}

#endif

#endif

GraphEdgeList.cpp

Code:

#include "GraphEdgeList.h"

#ifdef DEFINEINCPP

template <class Object>
typename GraphEdgeList<Object>::Vertex&
GraphEdgeList<Object>::insertVertex(const Object& o) {
    typedef typename GraphEdgeList<Object>::Vertex Vertex;
    V.push_back(Vertex(o));
    return V.back();
}

#endif

main.cpp

Code:

#define DEFINEINCPP // this gives linker errors
//#define DEFINEINHEADER // this works
#include "GraphEdgeList.h"
#include <iostream>
#include <utility>
#include <string>

int main() {
    GraphEdgeList<std::string> g;
    g.insertVertex("SFO");
}

You might want to read: Why can't I separate the definition of my templates class from it's declaration and put it inside a .cpp file?

From what I read, I just have to add another line to the cpp file. However, I'm still getting linker errors. What am I doing wrong?

Code:

#include "GraphEdgeList.h"

#ifdef DEFINEINCPP

template <class Object>
typename GraphEdgeList<Object>::Vertex&
GraphEdgeList<Object>::insertVertex(const Object& o) {
    typedef typename GraphEdgeList<Object>::Vertex Vertex;
    V.push_back(Vertex(o));
    return V.back();
}

template class GraphEdgeList<std::string>; // added this line

#endif

You have to add

Code:

#define DEFINEINCPP

into GraphEdgeList.cpp as well, otherwise it won't be defined. Of course, this is duplication of code, so I'd suggest adding the DEFINEIN* to another .h file which you then include in GraphEdgeList.h. That should solve the problem.

Also, by the way, I had compiler errors when compiling your code with g++ 4.3 and g++ 4.4. It doesn't like you doing things like this:

Code:

typedef Vertex<Object> Vertex;

Erros:

Code:

GraphEdgeList.h:56: error: declaration of ‘typedef class Vertex<Object> GraphEdgeList<Object>::Vertex’
GraphEdgeList.h:8: error: changes meaning of ‘Vertex’ from ‘class Vertex<Object>’

If I were you I'd pick a different name.

[edit] In case you're interested, here's the code I got to compile with g++ 4.3.

Code:

$ for file in *.cpp *.h; do echo === $file; cat $file; done
=== GraphEdgeList.cpp
#define DEFINEINCPP // this gives linker errors
#include "GraphEdgeList.h"

#ifdef DEFINEINCPP

template <class Object>
typename GraphEdgeList<Object>::Vertex_&
GraphEdgeList<Object>::insertVertex(const Object& o) {
    //typedef typename GraphEdgeList<Object>::Vertex Vertex;
    V.push_back(Vertex_(o));
    return V.back();
}

#include <string>

template class GraphEdgeList<std::string>; // added this line

#endif
=== main.cpp
#define DEFINEINCPP // this gives linker errors
//#define DEFINEINHEADER // this works
#include "GraphEdgeList.h"
#include <iostream>
#include <utility>
#include <string>

int main() {
    GraphEdgeList<std::string> g;
    g.insertVertex("SFO");
}
=== GraphEdgeList.h
#ifndef GRAPHEDGELIST_H
#define GRAPHEDGELIST_H

#include <vector>
#include <list>

template <typename Object>
class Vertex {
private:
    template <typename> friend class GraphEdgeList;
    Object o;
    int undirectedDegree;
    int inDegree;
    int outDegree;
public:
    Vertex(const Object& oo = Object()) : o(oo), undirectedDegree(0),
        inDegree(0), outDegree(0)
    {
    }
    const Object& element() const {
        return o;
    }
    void element(const Object& oo) {
        o = oo;
    }
};

template <typename Object>
class Edge {
private:
    template <typename> friend class GraphEdgeList;
    typedef Vertex<Object> Vertex_;
    Object o;
    Vertex_* origin;
    Vertex_* destination;
    bool isDirected;
public:
    Edge(const Object& oo = Object(), Vertex_* org = NULL, Vertex_* dst = NULL,
        bool directed = false)
        : o(oo), origin(org), destination(dst), isDirected(directed)
    {
    }
    const Object& element() const {
        return o;
    }
    void element(const Object& oo) {
        o = oo;
    }
};

/*
 * A graph represented by an edge list
 */
template <typename Object>
class GraphEdgeList {
    typedef Vertex<Object> Vertex_;
    typedef Edge<Object> Edge_;
    std::list<Vertex_> V;
    std::list<Edge_> E;
public:
    Vertex_& insertVertex(const Object& o);
};

#ifdef DEFINEINHEADER

template <class Object>
typename GraphEdgeList<Object>::Vertex&
GraphEdgeList<Object>::insertVertex(const Object& o) {
    //typedef typename GraphEdgeList<Object>::Vertex Vertex_;
    V.push_back(Vertex(o));
    return V.back();
}

#endif

#endif
$

Notice the #include <string> and the typedefs changed to Vertex_ etc. [/edit]