C++: Two Dimensional Array as class member - c++

Question

in my C++ class at university, i have to implement a Directed, weighted graph. As internal representation i have to implement a two-dimensional array, which stores the information about the edges between the vertices in the graph.
okay, i´ve implemented a C++ class "TwoDimArray" with an overloaded [] operator.
it works well as long as i instantiate objects of TwoDimArray in main(). But it does not as class member.
My Class for representation of the graph is "DirectedGraph" and has a private member "adjacencyMatrix" of Type TwoDimArray*.
In the constructor of my DirectedGraph class i intend to fill the array with zeros initially, indicating "there is no edge between the nodes i and j".
okay, and this is where it all goes wrong. I can write till coordinate [0][2] (when initializing the graph with 3 nodes, so the array should have 3x3 cells). when trying to write at adress [1][0] the assignment operation crashes with segmentation fault. So the assignment operations succeed n times and fail beginning at n+1 (where n is the number of vertices).
Any ideas what i am doing wrong?
My TwoDimArray Class (first header, then implementation):
#ifndef TWODIMARRAY_H_INCLUDED
#define TWODIMARRAY_H_INCLUDED
class TwoDimArray{
private:
int* pArr;
int rows;
int cols;
public:
TwoDimArray(int rows, int cols);
int* operator[](int row);
~TwoDimArray();
};
#endif // TWODIMARRAY_H_INCLUDED
The implementation:
#include <TwoDimArray.h>
TwoDimArray::TwoDimArray(int nrOfRows, int nrOfCols){
rows = nrOfRows;
cols = nrOfCols;
//allocate memory
pArr = new int[rows * cols];
}
int* TwoDimArray::operator [](int row){
return &pArr[row * cols];
}
TwoDimArray::~TwoDimArray(){
delete[] pArr;
}
Directed Graph header:
#define DIRECTEDGRAPH_H_INCLUDED
#include <string>
#include <list>
#include <Vertex.h>
#include <TwoDimArray.h>
using namespace std;
/**
* DOCUMENTATION
* ======================
* object oriented Implementation
* of the abstract
* Datatype Directed Graph
* as C++ class
*/
class DirectedGraph{
private:
int maxVertices;
list<Vertex> vertices;
TwoDimArray* adjacencyMatrix;
bool edgeExists(string srcName, string tgtName);
int vertexExists(string vName);
public:
//DirectedGraph();
DirectedGraph(int maxVertices);
~DirectedGraph();
void AddVertex(Vertex& v);
void AddEdge(Vertex& source, Vertex& target, int weight);
int getMaxVertices() const;
list<Vertex> getVertexNames()const;
void PrintGraph();
};
#endif // DIRECTEDGRAPH_H_INCLUDED
Directed Graph Implementation (only the constructor):
DirectedGraph::DirectedGraph(int maxV){
this->maxVertices = maxV;
//initialize the array
this->adjacencyMatrix = new TwoDimArray(maxV, maxV);
int i = 0;
int j = 0;
for(i = 0; i <= maxVertices - 1; i++){
for(j = 0; j <= maxVertices - 1; j++){
// ==> the fatal assignment
//fails at i = 1 and j = 0
*adjacencyMatrix[i][j]=0;
cout << "assigned " << i << " " << j << "with 0"<<endl;
}
}
}
any suggestions?
i guess its not okay to declare the class member as TwoDimArray* instead of TwoDimArray, but otherwise it does not compile.
What i´ve also tried is:
DirectedGraph::DirectedGraph(int maxV){
this->maxVertices = maxV;
//try to instantiate TwoDimArray
TwoDimArray myArr(maxV, maxV);
this->adjacencyMatrix = myArr;
int i = 0;
int j = 0;
for(i = 0; i <= maxVertices - 1; i++){
for(j = 0; j <= maxVertices - 1; j++){
// ==> the fatal assignment
//fails at i = 1 and j = 0
myArr[i][j]=0;
cout << "assigned " << i << " " << j << "with 0"<<endl;
}
}
}
but it fails at the same point.
i am not very familiar with pointer logic in c++ i must admit...
any suggestions?
thanks in advance
Roland

Answer 1

You have violated the Rule of Three. The easiest way to solve that is to avoid directly allocating memory:
class TwoDimArray{
private:
std::vector<int> arr;
int rows;
int cols;
public:
TwoDimArray(int rows, int cols) : arr(rows * cols);
int* operator[](int row) { return &arr[cols*row]; }
};

Answer 2

One problem is that you're not providing a copy constructor and assignment operator for TwoDimArray.
This breaks the following:
TwoDimArray myArr(maxV, maxV);
this->adjacencyMatrix = myArr;
and possibly other code.
See What is The Rule of Three?