Program crashes because of string? - c++

Question

I'm programming doubly linked list, everything was going fine but I faced with crash when reading a string value to the structure (code row is commented in the function "struct Node* GetNewNode()"):
#include <iostream>
#include <String>
#include <fstream>
#include <cstdlib>
using namespace std;
struct Node {
int sv;
double real;
bool log;
char simb;
string str;
struct Node* next;
struct Node* prev;
};
struct Node* head; // global variable - pointer to head node.
//----------------------------
struct Node* GetNewNode();
void Initialize(Node *stack);
void InsertAtTail(Node *stack);
void Print(Node *stack);
//----------------------------
//Creates a new Node and returns pointer to it.
ifstream fd("duom.txt");
struct Node* GetNewNode() {
struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
fd >> newNode->sv;
fd >> newNode->real;
string loginis;
fd >> loginis;
if (loginis == "TRUE")
newNode->log = true;
else
newNode->log = false;
fd >> newNode->simb;
//fd >> newNode->str; //uncommented code in this row crashes the program
newNode->prev = NULL;
newNode->next = NULL;
return newNode;
}
//Inserts a Node at head of doubly linked list
void Initialize(Node *stack) {
stack = head;
}
//Inserts a Node at tail of Doubly linked list
void InsertAtTail(Node *stack) {
struct Node* temp = stack;
struct Node* newNode = GetNewNode();
if(head == NULL) {
head = newNode;
return;
}
while(temp->next != NULL)
temp = temp->next; // Go To last Node
temp->next = newNode;
newNode->prev = temp;
}
//Prints all elements in linked list in reverse traversal order.
void Print(Node *stack) {
struct Node* temp = stack;
if(temp == NULL)
return; // empty list, exit
// Going to last Node
while(temp->next != NULL)
temp = temp->next;
// Traversing backward using prev pointer
while(temp != NULL) {
cout << temp->sv << " ";
cout << temp->real << " ";
if (temp->log == true)
cout << "TRUE " << " ";
else
cout << "FALSE " << " ";
cout << temp->simb << " ";
//cout << temp->str << "\n";
temp = temp->prev;
}
printf("\n");
}
int main() {
/*Driver code to test the implementation*/
head = NULL; // empty list. set head as NULL.
// Calling an Insert and printing list both in forward as well as reverse direction.
Initialize(head);
InsertAtTail(head);
Print(head);
InsertAtTail(head);
Print(head);
fd.close();
}
Input data is:
4 5.27 TRUE $ asdf
6 7.3 TRUE # qwer
9 8.8 FALSE # zxvc
7 6.35 FALSE ! vbmn
1 0.89 TRUE % ghjk
Can somebody explain what is wrong here?

Answer 1

Instead of using C standard function malloc
struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
you have to use operator new
In this case the compiler will call a constructor of class std::string that to create data member str
Othewise object str of type std::string will not be created and the program will chash.
Function malloc simply allocates a raw memory of a requested size. It knows nothing about constructors of classes.

Answer 2

malloc allocates a raw block of memory. This is sufficient for simple (POD) datatypes which only store data. A std::string however needs to have its constructor called to be initialized correctly. Therefore you must allocate the node using new:
Node* newNode = new Node();
In general, malloc is very rarely needed in C++ (it doesn't call any constructors). It is a C-function.
Note that you need to call delete instead of free to free memory allocated by new.