Passing pointers to function in c++

Question

Could someone please tell me why are pointers head and tail different when exiting function reverse ?

struct elem{
    int val;
    elem* prev;
    elem* next;
    ...
};
void print(elem* head,elem* tail){...}
void insertAtEnd(elem* e,elem* tail){...}
void reverse(elem* head,elem* tail){
    elem* headref = head;
    elem* temp = head;
    while(temp != NULL){
        elem* t = temp->prev;
        temp->prev = temp->next;
        temp->next = t;
        temp = temp->prev;
    }
    head = tail;
    tail = headref;
    print(head,tail);
}
int main(){
    elem* head = new elem();
    elem* tail = new elem();
    ...
    print(head,tail);
    reverse(head,tail);
    print(head,tail);
    return 0;
}

print() inside function reverse works fine. Next print (just before return 0 in main ) causes segmentation fault ( head->next points to NULL ).

Answer 1

With void reverse(elem* head,elem* tail) , you don't modify pointer (you may modify content).

You probably mean

void reverse(elem*& head, elem*& tail)

to modify head and tail .

Answer 2

The pointers in main() themselves don't change after calling reverse() , because you pass them to reverse() by value, and the code in the function only modifies its own copies of the pointers . However, the contents of the elem objects they point to have been changed by the reverse() function.

That is, head in main() still points to the same elem object it used to point to before, but now that elem object is the tail of the list (because you changed its contents in the reverse() function and now its next member is nullptr ). Similarly, tail in main() points to an elem that is now the head of the list.

Answer 3

Calling reverse doesn't change head and tail because they are passed by value (reverse only modifies its private copies). If you change the declaration of reverse to

void reverse(elem *&head, elem *&tail)

It should work.

Answer 4

Without analysing your code in detail, When you are reversing you are passing pointers to your elements, so the one called head is now the tail and the one called tail is now the head.

So after you will need to print( tail, head )

Your print function obviously does not check for null that is, it will cause an access violation if next is nullptr before it reaches your tail . And it will be because head is the tail so its next will indeed be nullptr.

If you actually want the reverse function to change the pointers themselves as well as what they point to, so you get a reversed list with the names head and tail swapped, pass them by reference.

Answer 5

Your exchange of pointers inside reverse() is local. It will be discarded once you leave the function scope. To achieve the exchange, yo0u have to use double pointers:

void reverse(elem** head, elem** tail){
    elem* headref = *head;
    elem* temp = *head;
    while(temp != NULL){
        elem* t = temp->prev;
        temp->prev = temp->next;
        temp->next = t;
        temp = temp->prev;
    }
    *head = *tail;
    *tail = headref;
    print(*head,*tail);
}

And pass addresses of head and tail:

reverse(&head, &tail);

PS Sorry for possible bugs - I didn't test the code