Map Insert Error

[StMick]

Hi there! Im trying to insert two objects (actually 1 object and a an object list) into a map and am getting an error "no matching function for call to std::map<Node, std::...". Anyone able to see what I am doing wrong here?

  // DiGraph.h file

#ifndef DIGRAPH_H
#define DIGRAPH_H
#include<string>
#include <map>
#include <vector>
#include <list>
#include <Node.h>
#include <Edge.h>


class DiGraph
{
    public:
        DiGraph();
        ~DiGraph();

        int getSize() const;

        void add(Node node);

    private:
        std::map< Node, std::list<Edge> > graph;
        int graphSize;

};

#endif // DIGRAPH_H



// DiGraph.cpp file begins here:

#include "DiGraph.h"

DiGraph::DiGraph()
{
    graphSize = 0;
}

DiGraph::~DiGraph(){}

int DiGraph::getSize()const{
    return graphSize;
}

void DiGraph::add(Node node){

    std::map<Node, std::list<Edge> >::iterator iter;
    iter = graph.find(node);
    if(!(iter == graph.end())){

        std::list<Edge> edgeList;
        graph.insert(node, edgeList);  // Error message pointing here
    }

}

[lastchance]

Have you defined any ordering for class Node? Also, node is not an iterator, so which overload of the insert member function are you trying to use?

[StMick]

The only ordering of the Node class I've defined is with the operator< function. Is there a specific function in the node class I am supposed to define for ordering?

Regarding node as not being an iterator - I thought the insert method took 2 parameters - the (key, value) tuple, yes? Where I have overridden the operator< function in the node class, I thought that the insert method would work in this case. But for some reason it does not. Please see below for the operator< function:

// Within the Node.cpp file

bool Node::operator<(const Node &node)const{
     if(this->state == node.getState()){

        return true;
    } else {

        return false;
    }
}

[lastchance]

Why are you defining a “less than” operator with an “equals” statement?

As for your insert call,
- you can insert a single “pair” object, but you are trying to supply two arguments
- why are you trying to insert at a point that already exists? (Your slightly contorted logic runs this loop when a value for node is already found in the map.)

[StMick]

Aah, good point with the "less than" operator. I'll fix that up.

What is an example of a "pair" object?

What do you mean by - trying to insert at a point that already exists? Is this referring to the if statement? If so - I am trying to add to the map if the Node object does not already exist is the map.

[lastchance]

I suspect (tentatively - I have no way of testing in your code) that you can just use
graph.insert( {node, edgeList} );
to insert the relevant pair. Note the curly brackets within the curved brackets. Otherwise you will have to look up a constructor for pair or use make_pair.

At present you are incorrectly trying to add to the map if the node DOES exist in the map - remove the '!' symbol in line 52.

[dhayden]

I am trying to add to the map if the Node object does not already exist is the map.

But your if statement says if(!(iter == graph.end())){ If should be if (iter == graph.end())

But really there's a much easier way to do this:

void DiGraph::add(Node node){
    graph[node];  // http://www.cplusplus.com/reference/map/map/operator[]/
}

In fact, you probably don't need the add() method at all. graph[node] returns a reference to the mapped value at node (the list of edges), creating it if it doesn't already exist. So, for example, to add an edge to a node, you can simply say:

Edge newEdge = whatever;
graph[node].push_back(newEdge);

[StMick]

Thank you all - the curly brackets were all that was needed to successfully insert the pair into the map! Also, I appreciate the attention brought to the faulty if statement logic.