Cody Pritchard Cody Pritchard - 1 month ago 14
C Question

Converting linked list from raw pointers to smart pointers

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

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.

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