How to initialize a vector of pointers?

Question

My program crashes when trying to run the function initialize_graph. I have omitted out all the other functions. This is one of my first times working with vectors. The graph.h header will be below main. I just need someone to help me initialize the vector with a graph object whose head pointer is null. I will be able to take it from there. Thanks for looking.

#include <cstdlib>
#include <iostream>
#include <vector>
#include "graph.h"

using namespace std;

vector<graph*> adj_list; // one dimensional vector; each position in the vector stores a pointer to the head of a linked list


void graph::initialize_graph(int num_of_vertices, int num_of_edges)
{
    cout << "Doing push back"; // want to push back the same obj. call it graph* graph_obj (where head is null)
    //adj_list.resize(num_of_vertices); 
    graph *graph_obj;
    graph_obj -> head = NULL;
    for(int k = 0; k < num_of_vertices; k++)
    {
        adj_list.push_back(graph_obj); 
    }
    cout << "Pushback complete";
}

int main()
{
    int num_of_vertices, num_of_edges, vertex1, vertex2, function;
    graph graph_obj;


    while(1)
    {

     cout<<"1 - initialize graph" <<endl;
     cout<<"2 - insert an edge to the graph" <<endl;
     cout<<"3 - delete an edge from the graph" <<endl;
     cout<<"4 - list all edges in the graph" <<endl;
     cout<<"5 - list all of the neighbors for a particular vertex" << endl;
     cout<<"6 - list all of the vertices with no incoming edges" << endl << endl;

    cout<<"Choose a function (1 - 6): ";
    cin>>function;
    cout<<endl<<endl;

    switch(function)
    {
      case 1: 
             cout<<"Enter the number of vertices in the graph: ";
             cin>>num_of_vertices;
             cout<<endl<<"Enter the number of edges in the graph: ";
             cin>>num_of_edges;
             cout<<endl<<endl;
             cin.get();
             graph_obj.initialize_graph(num_of_vertices, num_of_edges);
             break;

     case 2: 
            cout<<"To enter an edge X -> Y (an edge from node X to node Y), use the following format: X Y (the names of the two vertices separated by a single space)" << endl;
            cout<<"Enter the edge to insert into the graph: ";
            cin>>vertex1>>vertex2;
           cout<<endl<<endl;
           graph_obj.insert_edge(vertex1, vertex2);         
           break;

     case 3: 
            cout<<"To enter an edge X -> Y (an edge from node X to node Y), use the following format: X Y (the names of the two vertices separated by a single space)" << endl;
            cout<<"Enter the edge to delete from the graph: ";
            cin>>vertex1>>vertex2;
            cout<<endl<<endl;
           graph_obj.delete_edge(vertex1, vertex2);         
           break;

     case 4:
             graph_obj.list_all_edges(num_of_vertices);
             break;

     case 5:   
            cout<<"Enter the vertex to list all of the neighbors for: ";
            cin>>vertex1;
            cout<<endl<<endl;
            graph_obj.list_all_neighbors(vertex1, num_of_vertices);
            break;

     case 6:
           graph_obj.no_incoming_edges(num_of_vertices);


    } //end switch



    }  //end while

    system("PAUSE");
    return 0;
}

class graph //class for the graph data structure
{
    public:  
     void initialize_graph(int num_of_vertices, int num_of_edges); //creates a new directed graph
     void insert_edge(int vertex1, int vertex2);  // inserts a directed edge (V1 - > V2) into the graph
     void delete_edge(int vertex1, int vertex2);    // deletes an edge (V1 -> V2) from the graph
     void list_all_edges(int num_of_vertices); // lists all of the edges in the graph
     void list_all_neighbors(int vertex1, int num_of_vertices); // lists all of the neighbors for a particular vertex
     void no_incoming_edges(int num_of_vertices);  // lists all of the vertices with no incoming edges

    private: 
    graph *prev; //pointer to the previous node in the linked list (used in the adjacency list implementation only)
    graph *next;  //pointer to the next node in the linked list    (used in the adjacency list implementation only)
    graph *head; //pointer to the head of a linked list            (used in the adjacency list implementation only)
    int edge;   // the id of the vertex that there is an edge to    (used in both implementations)

};

Answer 1

You never actually allocated new memory from the heap for the pointer at:

graph *graph_obj;

It's pointer value is otherwise undefined, which is why it's crashing.

Initialize it with:

graph *graph_obj = new graph(); // Depending on your constructor.

Edit: Also if you're going to be re-using this pointer and pushing it back onto a container, you'll need to create a new pointer per element in container (Don't forget to delete it later!).

Answer 2

A bit of a problem here:

graph *graph_obj;
graph_obj -> head = NULL;

You're creating an uninitialized pointer and then immediately dereferincing it. You'll need to create an object for that pointer to point to.