getting a weird value using nodes in c++ - c++

Question

I hope this question wasn't asked before. I tried searching for an answer but couldn't find any. I'm trying to learn pointers and linked lists in C++. this is my code so far
#include <iostream>
using namespace std;
struct node{
int value;
node* next;
};
node createNode(int value){
node temp;
temp.value = value;
temp. next = NULL;
return temp;
}
void add( node*& head, int value){
node temp = createNode(value);
// the problem is in this if statement
if (head->next == NULL){
head->next = &temp;
}
}
int main() {
node* head;
add(head,4);
cout<<head->next->value;
return 0;
}
if I try to grab the address of temp into the head, in my main function, the cout will print this value "1634545454" and If I move the code line "head->next = &temp;" outside of the if statement, it'll print the correct result which is 4. I don't understand why is that happening! what is that value? is the condition inside the if statement doing anything to the address, or the value of head->next?

Answer 1

You're declaring temp as a node (note the lack of *), i.e. a local variable. So it's automatically being freed at the end of the function.
Roughly speaking, you need to decide how you want to manage your memory. If you're going to do it all yourself, you need to use node* everywhere and remember to free your data structures when they go out of scope. But the more modern, safer approach would be to use unique_ptr<node>, which correctly captures that your node owns its next pointer. In this way, the data will be freed automatically when the time comes, but not before.

Answer 2

as #Silvio Mayolo mentioned you have two choices of using self managing memory or smart pointers , but as your base code was written in the self managing way ive fixed some of the pointers in your code.
the node pointer named head was not memory allocated and the temp variable most be a pointer so that the lifecycle dosent exceeds ,and if your using c++11 and above using nullptr would be better.
#include <iostream>
using namespace std;
struct node{
int value;
node* next=nullptr;
};
node* createNode(int value){
node* temp=new node;
temp->value = value;
temp->next = nullptr;
return temp;
}
void add( node* head, int value){
node* temp = createNode(value);
if (head->next == nullptr){
head->next = temp;
}
}
int main() {
node* head=new node;
add(head,4);
cout<<head->next->value;
return 0;
}

Answer 3

#include<iostream>
using namespace std;
struct node{
int value;
node* next=NULL; //node* next;
};
node* createNode(int value){ //node createNode(int value){
node* temp=new node; //node temp;
temp->value = value;
temp->next = NULL;
return temp;
}
void add( node* head, int value){ //void add( node*& head, int value){
node* temp = createNode(value); //node temp = createNode(value);
if (head->next == NULL){
head->next = temp; //head->next = &temp;
}
}
int main() {
node* head=new node; //node* head;
add(head,4);
cout<<head->next->value;
return 0;
}