Question

Why is it impossible to use NULL as default pointer argument inside templated function?
Lets concider the following code:

template<class Graph, class NodeAttribs, class ArcAttribs> string
graphToGraphviz(Graph       &graph,
                NodeAttribs *nattribs = NULL,
                ArcAttribs  *aattribs = NULL,
                string      name      = ""){
   /*...*/
}

I want to be able to call it like this:

graphToGraphviz(g);

I have suspections, that compiler thinks it cannot resolve the types for NULL, but these types are not used if the attribute is NULL (there are if conditions). But maybe this case could not be resolved the proper way by compiler. If yes, how could I write such overloaded function, which will allow me to use the short form?

I have an idea of overloading it like that:

class Empty{}

template<class Graph> string
graphToGraphViz(Graph       &graph,
                string      name      = ""){
    return graphToGraphviz<Graph, Empty, Empty>(graph, NULL, NULL, name)
}

but then the compiler gives me errors, among others, that class Empty has no operator [] defined. This is again understable, but do I have to make all these “dummy” operator overloadings and empty functions to satify compiler or is there a better way to do this?

EDIT:
Please take a look at the full source code – it converts Lemon graph to graphviz format:
I’ve tried to use the new syntax from C++11 (as the answers below suggest), but without success.

#ifndef GRAPHTOGRAPHVIZ_H_
#define GRAPHTOGRAPHVIZ_H_

#include <lemon/list_graph.h>

using namespace lemon;
using namespace std;

/* USAGE:
 * ListDigraph::NodeMap<unordered_map<string, string>> nodeAttribs(g);
 * ListDigraph::ArcMap<unordered_map<string, string>> arcAttribs(g);
 * nodeAttribs[node]["label"] = "node_label";
 * string dot = graphToGraphviz(g, &nodeAttribs, &arcAttribs, "hello");
 */

template<class Map>
string getAttribs(Map &map){
    string attribs = "";
    for (const auto &el : map){
        if (el.second != "")
            attribs += "\"" + el.first + "\"=\"" + el.second + "\",";
    }
    if (attribs != "")
        attribs = " [" + attribs + "]";
    return attribs;
}


template<class Graph, class NodeAttribs, class ArcAttribs> string
graphToGraphviz(Graph       &graph,
                NodeAttribs *nattribs = NULL,
                ArcAttribs  *aattribs = NULL,
                string      name      = ""){

    typedef typename Graph::template NodeMap<string> NodeMap;
    typedef typename Graph::NodeIt NodeIterator;
    typedef typename Graph::ArcIt  ArcIterator;

    NodeMap labels(graph);
    ostringstream layout;
    layout << "strict digraph \""+name+"\" {\n";

    // prepare labels
    for (NodeIterator node(graph); node != INVALID; ++node){
        string label = "";
        if (*nattribs != NULL)
            label = (*nattribs)[node]["label"];
        if (label == "") label = static_cast<ostringstream*>( &(ostringstream() << graph.id(node)) )->str();
        label = "\"" + label + "\"";
        labels[node] = label;
    }

    // initialize nodes
    for (NodeIterator node(graph); node != INVALID; ++node){
        layout << labels[node];
        if (*nattribs != NULL)
            layout << getAttribs((*nattribs)[node]);
        layout << ";" << std::endl;
    }

    // initialize arcs
    for (ArcIterator arc(graph); arc != INVALID; ++arc){
        layout << labels[graph.source(arc)] << "->" << labels[graph.target(arc)];
        if (*aattribs != NULL)
            layout << getAttribs((*aattribs)[arc]);
        layout << ";" << std::endl;
    }
    layout << "}";
    return layout.str();
}


#endif /* GRAPHTOGRAPHVIZ_H_ */

with C++11 syntax the function header will look like:

template<class Graph, class NodeAttribs=ListDigraph::NodeMap<string>, class ArcAttribs=ListDigraph::NodeMap<string> > string
graphToGraphviz(Graph       &graph,
                NodeAttribs *nattribs = NULL,
                ArcAttribs  *aattribs = NULL,
                string      name      = "")

but it does not compile and gives a tons of strange errors.

Answer 1

If you are using C++11, you can do this:

template<class Graph, class NodeAttribs=Empty, class ArcAttribs=Empty> ...

I have not found relevant language in the standard, but gcc accepts it.