Programming Forums > Application Development > C++

xaosai · Mar 26th, 2005, 2:49 PM

I'm building a linked list class and have a question about one of my memeber functions. This is what i have now:

//Removes the last item from a linked list
void linked_list::pop_tail()
{
     ll_item *cursor;
     cursor = head;

     while (true)
     {
          if(cursor->next_item->next_item == NULL)
          {
               delete cursor->next_item;
               cursor->next_item = NULL;

               break;
          }

          cursor = cursor->next_item;
      }
size--;
}

It works fine but I'm curious about the necessity of the first statement inside the if clause. Do I really need to delete the memory contents of next_item if I'm setting it to null, i.e. can I get away with just setting it to null without a memory leak?

Dizzutch · Mar 26th, 2005, 3:47 PM

if you free next_item you'll have to reallocate next time you want to use it, so you can just leave it NULL, it should be allocated already, so you should be fine.
Dizz

mackenga · Mar 31st, 2005, 10:25 AM

Ew, no! next_item is a pointer. Setting it to NULL will leave the memory allocated, sure, but you'll have no record of the address, meaning the structure exists but your program has no way of getting back at it. If you try to indirect through a null pointer, you'll get a Segmentation Fault (on Unix; some equivalent, probably more serious, and more vague error on Windows toys).

In other words, if you were to omit delete, you'd still need to allocate again next time with new, but you'd leak memory each time this happened. If you want to keep a pool of memory on the heap so you don't have to keep allocating and deallocating, you have to work harder than this and keep a list of references to your available allocated memory that you can look up - effectively implement your own mini memory manager on top of the existing one.

Mar 26th, 2005, 2:49 PM	#1
xaosai Newbie Join Date: Mar 2005 Posts: 8 Rep Power: 0	memory leak question I'm building a linked list class and have a question about one of my memeber functions. This is what i have now: (Toggle Plain Text) //Removes the last item from a linked list void linked_list::pop_tail() { ll_item cursor; cursor = head; while (true) { if(cursor->next_item->next_item == NULL) { delete cursor->next_item; cursor->next_item = NULL; break; } cursor = cursor->next_item; } size--; } //Removes the last item from a linked list void linked_list::pop_tail() { ll_item cursor; cursor = head; while (true) { if(cursor->next_item->next_item == NULL) { delete cursor->next_item; cursor->next_item = NULL; break; } cursor = cursor->next_item; } size--; } It works fine but I'm curious about the necessity of the first statement inside the if clause. Do I really need to delete the memory contents of next_item if I'm setting it to null, i.e. can I get away with just setting it to null without a memory leak?

Mar 26th, 2005, 3:47 PM	#2
Dizzutch Professional Programmer Join Date: Dec 2004 Location: Worcester, MA Posts: 441 Rep Power: 4	if you free next_item you'll have to reallocate next time you want to use it, so you can just leave it NULL, it should be allocated already, so you should be fine. Dizz __________________ naked pictures of you \| PFO F@H stats

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Mar 31st, 2005, 10:25 AM	#3
mackenga Professional Programmer Join Date: Mar 2005 Location: Glasgow, Scotland Posts: 314 Rep Power: 4	Ew, no! next_item is a pointer. Setting it to NULL will leave the memory allocated, sure, but you'll have no record of the address, meaning the structure exists but your program has no way of getting back at it. If you try to indirect through a null pointer, you'll get a Segmentation Fault (on Unix; some equivalent, probably more serious, and more vague error on Windows toys). In other words, if you were to omit delete, you'd still need to allocate again next time with new, but you'd leak memory each time this happened. If you want to keep a pool of memory on the heap so you don't have to keep allocating and deallocating, you have to work harder than this and keep a list of references to your available allocated memory that you can look up - effectively implement your own mini memory manager on top of the existing one.

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Programming Forums > Application Development > C++

memory leak question