String Doubly Linked List of 4 characters each node - c++

Question

I want to accept a long string of numbers and insert in to a doubly linked list , with each node having 4 characters(numbers).
Below is my code. It takes the input as number, but says "Program finished with exist code 0"
Please help what did I am miss here?
#include <iostream>
#include <string>
#include <conio.h>
using namespace std;
struct Node {
string data;
struct Node* prev;
struct Node* next;
};
struct Node* head = NULL;
void insert(string newdata) {
struct Node* newnode = (struct Node*)malloc(sizeof(struct Node));
newnode->data = newdata;
newnode->prev = NULL;
newnode->next = head;
if (head != NULL)
head->prev = newnode;
head = newnode;
cout << "\nNode inserted";
}
void display() {
struct Node* ptr;
ptr = head;
while (ptr != NULL) {
cout << ptr->data << " ";
ptr = ptr->next;
}
}
int main() {
string n1, temp;
cout << "Enter the number\n";
cin >> n1;
int len, i;
len = n1.size();
cout << "\n Length is " << len;
getch();
// temp= n1;
// cout<<"\n line is "<<temp.substr(len);
for (i = 0; i < len; i = i + 4) {
// temp = n1.substr(i,4);
insert(n1.substr(i, 4));
}
cout << "\nThe doubly linked list is: ";
display();
return 0;
}

Answer 1

As already pointed out in the comments section, the problem is that the constructor of the std::string object is not getting called, so that this object is not initialized properly.
The most straightforward fix to this would be to use placement new, which effectively does nothing else than to call the constructor. In order to do this, you can change the line
newnode->data = newdata;
to
new (&newnode->data) string( newdata );
This will call the copy (or move) constructor on the std::string object.
However, a more C++ style solution to the problem would be not to use malloc at all, but to instead use new, and to write a proper constructor for struct Node, which invokes the copy or move constructor of the string object. In order to do this, you could define struct Node like this:
struct Node {
//copy and move constructor
Node( const string &data, Node* prev = nullptr, Node* next = nullptr )
: data(data), prev(prev), next(next) {}
Node( const string &&data, Node* prev = nullptr, Node* next = nullptr )
: data(data), prev(prev), next(next) {}
string data;
struct Node* prev;
struct Node* next;
};
Now you can replace the lines
struct Node* newnode = (struct Node*)malloc(sizeof(struct Node));
newnode->data = newdata;
newnode->prev = NULL;
newnode->next = head;
with this single line:
Node* newnode = new Node( newdata, nullptr, head );