C++, Allocate memory, Segmentation fault: 11 - c++

Question

#include <iostream>
using namespace std;
class Node
{
public:
Node(int N, Node *l, Node *r);
int value; // stored value
Node *left; // left node
Node *right; // right node
};
Node::Node(int N, Node *l, Node *r){
value = N;
left = l;
right = r;
}
void insert(Node *x){
if (x == nullptr) {
Node newNode(5, nullptr, nullptr);
*x = newNode;
}
}
int main(){
Node *root;
root = nullptr;
insert(root);
cout << root->value << endl;
return 0;
}
This is the beginning of a binary search tree. In the insert function, I am trying to change a nullptr to a pointer pointing to a Node object. When I run this c++ code, I get the error: "Segmentation fault: 11". After doing some research, I believe I need to (re)allocate memory. How can I allocate the memory inside of the insert function, if possible?

Answer 1

First up I presume
if (x == nullptr) {
Is a typo? Didn't you mean
if (x != nullptr) {
??
If it is null you shouldn't be going ahead and dereferencing it.
How you fix the crash really depends on how you want the interface to your BST to be.
You are passing in a nullptr and attempting to assign to it. This wont work. You can't assign to nothing.
So you could do something like.
#include <iostream>
using namespace std;
class Node
{
public:
Node(int N, Node *l, Node *r);
int value; // stored value
Node *left; // left node
Node *right; // right node
};
Node::Node(int N, Node *l, Node *r){
value = N;
left = l;
right = r;
}
void insert(Node *x){
if (x != nullptr) {
Node newNode(5, nullptr, nullptr);
*x = newNode;
}
}
int main(){
Node root(2, nullptr, nullptr);
insert(&root);
cout << root.value << endl;
return 0;
}
Here you have an initial object, allocated on the stack, which you can assign to in insert. If you use this method you are wasting some time doing the initial construction of root in function main, when you are always going to go and assign over it.
If you wanted to persist with heap allocation of the Node. Pass a pointer to pointer to insert e.g.
#include <iostream>
using namespace std;
class Node
{
public:
Node(int N, Node *l = nullptr, Node *r = nullptr);
int value; // stored value
Node *left; // left node
Node *right; // right node
};
Node::Node(int N, Node *l, Node *r)
: value(N), left(l), right(r)
{}
void insert(Node **x)
{
if (x != nullptr)
{
Node* n = new Node(5);
*x = n;
}
}
int main()
{
Node *root = nullptr;
insert(&root);
if(root)
{
cout << root->value << endl;
delete root;
}
return 0;
}
This lets insert manage the allocation of the node.

Answer 2

Have you tried to run valgrind? It's a good idea with these kind of errors (and even if you don't see them), it sometime detects error before the symptoms gets visible (the segmentation fault might be just a consequence of an earlier error - if it's not a normal debugger will stop where the segmentation fault occurs).
It points at the fault directly:
void insert(Node *x){
if (x == nullptr) {
Node newNode(5, nullptr, nullptr);
*x = newNode; <<<--- here
}
}
So you basically check if x is null and if it is you tries to dereference and write to the pointed object? That sounds really bad. You're supposed to do the opposite - check and if it's null you do not dereference the pointer.