How to create a dynamic 2D array inside a class in C++ ?

Difficulty Level : Hard
Last Updated : 29 May, 2017

Suppose we want to create a class for Graph. The class stores adjacency matrix representation of the graph. Therefore, our class structure would be something like below.

class Graph 
{
  int V; 
  int adj[V][V];  // This line doesn't work
    
  /* Rest of the members */
};
  
int main()
{
}

Output :

error: invalid use of non-static data
       member 'Graph::V'.

Even if we make V static, we get error “array bound is not an integer constant”

C++ doesn’t allow to create an stack allocated array in a class whose size is not constant. So we need to dynamically allocate memory. Below is a simple program to show how to dynamically allocate 2D array in a C++ class using a class for Graph with adjacency matrix representation.

// C++ program to show how to allocate dynamic 2D
// array in a class using a Graph example.
#include<bits/stdc++.h>
using namespace std;
  
// A Class to represent directed graph
class Graph
{
    int V;    // No. of vertices
  
    // adj[u][v] would be true if there is an edge
    // from u to v, else false
    bool **adj;
  
public:
    Graph(int V);   // Constructor
  
    // function to add an edge to graph
    void addEdge(int u, int v)  { adj[u][v] = true; }
    void print();
};
  
Graph::Graph(int V)
{
    this->V = V;
  
    // Create a dynamic array of pointers
    adj = new bool* [V];
  
    // Create a row for every pointer
    for (int i=0; i<V; i++)
    {
       // Note : Rows may not be contiguous
       adj[i] = new bool[V];
  
       // Initialize all entries as false to indicate
       // that there are no edges initially
       memset(adj[i], false, V*sizeof(bool));
    }
}
  
// Utility method to print adjacency matrix
void Graph::print()
{
   for (int u=0; u<V; u++)
   {
      for (int v=0; v<V; v++)
         cout << adj[u][v] << " ";
      cout << endl;
   }
}
  
// Driver method
int main()
{
    // Create a graph given in the above diagram
    Graph g(4);
    g.addEdge(0, 1);
    g.addEdge(0, 2);
    g.addEdge(1, 2);
    g.addEdge(2, 0);
    g.addEdge(2, 3);
    g.addEdge(3, 3);
  
    g.print();
  
    return 0;
}

Output :

A note on call of memset():
memset() is used separately for individual rows. We can’t replace these calls with one call because rows are allocated at different addresses and making a memset call like below would be disastrous.

       // Wrong!! (Rows of matrix at different addresses)
       memset(adj, false, V*V*sizeof(bool));

This article is contributed by Dheeraj Gupta. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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