Returning a pointer to a structure and accessing its fields C++

Question

I am implementing a skiplist for my self and I'm experiencing some problems with C++. I have two structures:

A node of the skiplist - it holds its int value, and a pointer to an array of pointers to other nodes.
```
struct node{ int val; node** next; }; 
```
Skiplist which holds pointers to the head and tail of the list (sentinels).
```
struct skiplist{ node *head, *tail; }; 
```

Also, I have a function which returns a pointer to the skiplist structure (I use this function to initialize the skiplist):

skiplist* createSkipList(){ skiplist* l = new skiplist; node* listHead = new node; node* listTail = new node; node* headNext[MAX_LEVEL]; //array of pointers listHead->next = headNext; for(int i=0; i<MAX_LEVEL; i++){ listHead->next[i] = listTail; } l->head=listHead; l->tail=listTail; }

And in the main() function I call:

skiplist* skiplist=createSkipList();

Everything works fine in the createSkipList() function, but if I want to refer to the skiplist in the main() i.e. by accessing skiplist->tail the program crashes. I've been searching for related posts, but they didn't help me.

As mentioned in a similar post I should't experience dangling pointers because I am using the new operator to allocate the structures. I would be grateful for any hints ;)

Turn on compiler warnings and stop wasting your time and everyone else's. The compiler would have told you about the missing return. — Jonathan Wakely
– Jonathan Wakely, Commented May 11, 2013 at 10:24

Community · Accepted Answer · 2017-05-23 10:32:10Z

First problem:

You are not returning anything from createSkipList(), which means your program has undefined behavior. Add a return statement:

skiplist* createSkipList(){ skiplist* l = new skiplist; // ... return l; // ^^^^^^^^^ }

Per paragraph 6.6.3/2 of the C++11 Standard:

[...] Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function.

Second problem:

As mentioned in a similar post I should't experience dangling pointers because I am using the new operator to allocate the structures [...]

Unfortunately, you do experience dangling pointers. As mentioned by Angew in the comments, what you are doing here:

node* headNext[MAX_LEVEL]; //array of pointers listHead->next = headNext;

Is to create a local array object and let listHead->next point to its first element, without considering that the array (as well as the objects it contains) will be destroyed once createSkipList() returns - objects with automatic storage duration get destroyed when they fall out of scope.

Moreover, as a general advice, consider using smart pointers for modeling ownership rather than doing manual memory management through raw pointers, new, and delete.

Not to mention the fact that listHead->next is set to point to a local variable and will become dangling once the function exits.
@Angew: Correct, I stopped at the first problem ;) Thank you for mentioning it, I am going to add it to the answer
@First problem I was inattentive enough to omit the return statement in my code snippet. Of course you're right and thank you for commenting on that. @Second problem: I've changed the line in which I create the array, and now it works fine. Thank you both for helping me on that ;) In case somebody experienced similar problem: node** headNext = new node*[MAX_LEVEL]; Now the array will not be lost after the function exits.

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