Converting linked list from raw pointers to smart pointers

Question

I need to convert a C implementation of a doubly linked list that uses raw pointers, to an implementation using smart pointers instead.

I have some small experience with smart pointers.

Im working on converting the insertFirst() function to get my bearings and understand how this will come together.

struct node {
  int data;
  int key;

  std::shared_ptr<node> next;
  std::weak_ptr<node> prev;
};

void insertFirst(int key, int data){

  //create a link
  //struct node *link = (struct node*) malloc(sizeof(struct node));

  std::shared_ptr<node> link = (std::shared_ptr<node>) malloc(sizeof(struct node));

  link->key = key;
  link->data = data;

  if(isEmpty()){
    //make it the last link
    last = link;
  }else {
    //update first prev link
    head->prev = link;
  }

  //point it to old first link
  link->next = head;

  //point first to new first link
  head = link;
}

Im having trouble with this line:

struct node *link = (struct node*) malloc(sizeof(struct node));

I thought doing like so:

std::shared_ptr<node> link = (std::shared_ptr<node>) malloc(sizeof(struct node));

was all I would need. But i am less familiar with C and what exactly is happening and why this is not allowed.

I get the error:

no matching conversion for C-style cast from 'void *' to 'std::shared_ptr<node>'

Can anyone offer some tips and explanation?

Answer 1

When constructing C++ class instances, you must use new and delete , instead of malloc and free . malloc and free are C library functions, that know absolutely nothing about C++ class constructors, destructors, and everything else that goes with what a C++ class is all about.

The shown code is attempting to construct an instance of the node class by using malloc . that won't work. new must be used to construct it:

std::shared_ptr<node> link = new node;

That's even shorter and neater, than the C-style concoction comprised of malloc , and an ugly cast.

You mentioned that you are in the process of converting C code to C++. A mandatory part of that conversion is to replace all malloc and free calls with new and delete . This is not optional, this is required for proper C++ code.