Creation of LInked List - c++

Question

I want to display the string "Jimmy" but nothing appears. What is my mistake?
#include<iostream>
#include<string>
struct Node
{
std::string s;
Node* next;
};
struct Node* head = NULL;
void insert(const std::string& name)
{
struct Node* newnode = (struct Node*)malloc(sizeof(struct Node));
newnode->s = name;
newnode->next = NULL;
head = newnode;
}
void display()
{
struct Node* ptr;
ptr = head;
while (ptr != NULL) {
std::cout << ptr->s << std::endl;
}
}
int main()
{
insert("Jimmy");
display();
return 0;
}
No outputs are shown in this code. Please give some recommendation. I am still new to this data structure.

Answer 1

The standard C function malloc allocates raw memory knowing nothing about the object that will be placed in the memory.
So neither constructor of the object will be called.
The structure Node contains data member of the type std::string for which a constructor shall be called.
In C++ use the operator new instead of calling the C function malloc. The operator not only allocates memory but also calls a constructor for the created object.
It is a bad idea to use global objects in function definitions without passing them through parameters.
The function display can have an infinite loop in case when head is not equal to null pointer because the variable ptr (that is assigned with head) used in the loop is not changed.
void display()
{
struct Node* ptr;
ptr = head;
while (ptr != NULL) {
std::cout << ptr->s << std::endl;
}
}
The function insert can be called only once
void insert(const std::string& name)
{
struct Node* newnode = (struct Node*)malloc(sizeof(struct Node));
newnode->s = name;
newnode->next = NULL;
head = newnode;
}
because otherwise it can result in memory leaks
You should free nodes before exiting the program.
Here is a modified your program that does not have the drawbacks of the original program.
#include <iostream>
#include <string>
struct Node
{
std::string s;
Node *next;
};
void insert( Node * &head, const std::string &s )
{
head = new Node { s, head };
}
std::ostream & display( const Node * head, std::ostream &os = std::cout )
{
for ( const Node *current = head; current != nullptr; current = current->next )
{
os << current->s << '\n';
}
return os;
}
void clear( Node * &head )
{
while ( head )
{
Node *tmp = head;
head = head->next;
delete tmp;
}
}
int main()
{
Node *head = nullptr;
insert( head, "Jimmy" );
display( head );
clear( head );
return 0;
}
Its output is
Jimmy