struct containing string value causes segmentation fault upon its assignment after its creation using dynamic memory allocation

Question

The compiler throws runtime segfault upon following code :

#include <iostream>
#include <string>
using namespace std;

struct Node{
  int data;
  void *next;   
  string nodeType;
};

Node* initNode(int data){
  Node *n = (Node*)malloc(sizeof(Node));
  n->data = data;
  n->next = NULL;
  n->nodeType = "Node";   //if this line is commented it works else segfault
  return n;
}

int main() {
  Node *n1 = initNode(10);
  cout << n1->data << endl;
}

Can someone please explain why string assignment does not work inside a struct which is dynamically allocated where in case of static allocation why it works ?

where as the following way it works :

Node initNode(string data){
  Node n;
  n.data = data;  //This works for node creation statically
  n.next = NULL;
  n.nodeType = "Node";  //and even this works for node creation statically
  return n;
}

and then in the main function:

int main() {
  Node n2 = initNode("Hello");
  cout << n2.data << endl;
}

malloc just allocates memory, it doesn't construct the object instance, which means the string constructor is not called. — Some programmer dude
– Some programmer dude, Commented Mar 17, 2015 at 9:53
General rule of thumb : don't use C's malloc in C++, use new, and actually don't use new either if possible, and use make_shared or std::unique_ptr (C++11) — Félix Cantournet
– Félix Cantournet, Commented Mar 17, 2015 at 10:06

TartanLlama · Accepted Answer · 2015-03-17 10:23:59Z

8

This doesn't work because you don't actually construct a Node instance into the memory which you malloc.

You should use new instead:

Node *n = new Node{};

malloc only allocates memory, it has no idea what a class is or how to instantiate one. You should generally not use it in C++.

new allocates memory and constructs an instance of the class.

edited Mar 17, 2015 at 10:23

answered Mar 17, 2015 at 9:52

TartanLlama

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3 Comments

Dhwanit Over a year ago

Hey yaa this works: Node *n = new Node(); But i thought that malloc would actually allocate memory by creating instance ? Isn't that true ? can you please explain or provide pointers to difference between doing "new" and "malloc" and when to use it ?

Dhwanit Over a year ago

yup right! thanks! well I actually had a feeling that the string's constructor would be called when I did malloc as it was string and not string*

TartanLlama Over a year ago

malloc doesn't call any constructors of any members of your class.

senfen · Accepted Answer · 2015-03-17 09:55:39Z

4

There is no place, where std::string constructor is executed.

You should use new

example *e = new example;

or placement new

void *example_raw = malloc(sizeof(example));
example *e = new(example_raw) example;

answered Mar 17, 2015 at 9:55

senfen

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Comments

David Schwartz · Accepted Answer · 2015-03-17 10:28:11Z

3

 Node *n = (Node*)malloc(sizeof(Node));

This cast is nonsense. You can't just tell the compiler to pretend that a chunk of data you just allocate contains a valid Node object and then manipulate it.

answered Mar 17, 2015 at 10:28

David Schwartz

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1 Comment

Dhwanit Over a year ago

yup right! well I actually had a feeling that the string's constructor would be called when I did malloc as it was string and not string* inside the Node

Santhosh Pai · Accepted Answer · 2015-03-17 10:09:34Z

1

string in c++ is an class and to create string objects use new instead of malloc as below .

Node *n = new Node{};

answered Mar 17, 2015 at 10:09

Santhosh Pai

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