strange error compiling main module using g++

Question

I am trying to compile the below code using "g++ main.cpp -c" but it gives me this strange error .. any ideas?

main.cpp: In function ‘int main()’:
main.cpp:9:17: error: invalid conversion from ‘Graph*’ to ‘int’
main.cpp:9:17: error:   initializing argument 1 of ‘Graph::Graph(int)’
main.cpp:10:16: warning: deprecated conversion from string constant to ‘char*’

This is my main module which i am trying to compile and below that is the graph class i have in graph.hpp

#include <iostream>
#include "graph.hpp"

using namespace std;

int main()
{
  Graph g;
  g = new Graph();
  char* path = "graph.csv";
  g.createGraph(path);
  return 0;
}

AND this is my Graph class

    /*
 * graph.hpp
 *
 *  Created on: Jan 28, 2012
 *      Author: ajinkya
 */

#ifndef _GRAPH_HPP_
#define _GRAPH_HPP_

#include "street.hpp"
#include "isection.hpp"
#include <vector>

class Graph
{
 public:
  Graph(const int vertexCount = 0);
  //void addIsection(Isection is);
  //void removeIsection(int iSectionId);
  Isection* findIsection(int);
  void addStreet(int iSection1, int iSection2, int weight);
  void createGraph(const char *path); //uses adj matrix stored in a flat file
  //void removeStreet(int streetID);
  void printGraph();
  ~Graph();
 private:
  //Isection *parkingLot;
  //Isection *freeWay;
  int** adjMatrix;
  std::vector <Street*> edgeList;
  std::vector <Isection*> nodeList;
  int vertexCount;
};

    #endif

Answer 1

This is C++, not Java or C#. new doesn't work the same way here.

new expressions return pointers. You cannot assign a pointer to a Graph (i.e. a Graph*) to a variable of type Graph:

Graph g;
g = new Graph(); // Graph = Graph* ? nope

Seems like the compiler is trying to be "helpful" and trying to use your constructor take takes an int argument to make a value of type Graph, but it can't convert a Graph* to an int.

When you write Graph g; you already have a Graph object. You don't need to create one with new. In fact, you probably don't even want to do that, as it will lead to memory leaks.

Then there's this line:

char* path = "graph.csv";

"graph.csv" has type char const[10] so you should not assign it to a char*. In the past you could, but that turned out to be a bad idea. That feature was marked as deprecated, and now it was completely removed in C++. Instead of doing that you can:

• Make an array out of it: char path[] = "graph.csv";;
• Make a pointer to it with the proper type: char const* path = "graph.csv"; (this works because array types decay to pointers);

Answer 2

That should probably be g = Graph(); and forget the g = new Graph;. The reason is that new returns a pointer to the created object (e.g. a Graph*), not an object value.

Better still, just do Graph g; and forget about assigning anything to g. This will automatically create a Graph for you by calling Graph's no-arg constructor.

Answer 3

Graph* g;

It has to be a pointer.

Answer 4

First of all new Graph() returns a Graph*. In c++, a constructor with one argument can be implicitly called, so currently your code is really meaning:

g = Graph(new Graph());

What you want is

g = Graph();