Getting segmentation fault core dumped error [closed] - c++

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When I try to create the object of graph and pointer object of Edge it gives me segmentation fault while when I only create the graph object it does not gives me the error but when I create the graph object and Edge pointer object simultaneously it gives me the segmentation error. Why is it so? When I delete the line where the edge pointer object is declared in the main function then the code works fine but I need an edge object to display the edges...
The same code I run in any online c++ compiler gives me the correct output without any error.
#include <iostream>
using namespace std;
template <class B>
struct Edge
{
B vertex;
Edge<B> *next;
};
template <class A>
struct VertexNode
{
Edge<A> *edgeHead;
A vertex;
VertexNode *nextVertex;
};
template <class A>
class Graph
{
private:
VertexNode<A> *head;
public:
Graph();
void insertVertex(A vertex);
bool insertEdge(A vertex1, A vertex2); // this would return true if the both the vertices exists otherwise false
bool deleteVertex(A vertex); // this would return true if the vertex exists otherwise false
bool deleteEdge(A vertex1, A vertex2); // this would return true if the both the vertices exists and the edge between them exists otherwise false
bool isEmpty();
Edge<A> *Adjacent(A vertex);
};
template <class A>
Graph<A>::Graph()
{
head->edgeHead = NULL;
head->nextVertex = NULL;
head = NULL;
}
template <class A>
Edge<A> *Graph<A>::Adjacent(A vertex) // would return the list head of the edges
{
for (VertexNode<A> *temp = head; temp != NULL; temp = temp->nextVertex)
{
if (temp->vertex == vertex)
{
return temp->edgeHead;
}
}
return NULL;
}
template <class A>
bool Graph<A>::isEmpty()
{
return (head == NULL);
}
int main()
{
Graph<char> obj;
Edge<char> *edgeHead; // for displaying the list of edges
// adding vertices
obj.insertVertex('A');
obj.insertVertex('B');
obj.insertVertex('C');
obj.insertVertex('D');
obj.insertVertex('E');
obj.insertEdge('A', 'C');
// now displaying adjacent edges for checking that our graph is implemented correctly
edgeHead = obj.Adjacent('A');
}

Answer 1

Removing unnecessary code, we get:
template <class A>
class Graph {
VertexNode<A> *head;
Graph();
};
template <class A> Graph<A>::Graph() {
head->edgeHead = NULL;
head->nextVertex = NULL;
head = NULL;
}
At no point is memory allocated for head, so you are dereferencing an uninitialized value. That will often lead to a segmentation fault.