pointer to pointer assignment

Question

I have a linked list like this:

typedef struct _LIST_ELEMENT
{
    T* data;
    _LIST_ELEMENT* nextItem;
    int index;

}LITEM,*P_LITEM,**PP_LITEM;

and four pointers to pointer

PP_LITEM _head;
PP_LITEM _tail;
PP_LITEM _current;
PP_LITEM _previous;

The situation is like this:

I have 4 elements in the list and _head points to the first and _tail to the last, _current and _previous work together as a cursor. After i erase element with index 2, the list contains 3 elements with indexes 0,1 and 3 and all pointers point right. At this point if i do this (to move the cursor to the start):

(*_previous) = (*_tail);
(*_current) = (*_head);

The list is missing element with index 1 but all pointers point correctly. If i do this:

_previous = &(*_tail);
_current = &(*_head);

everything is fine and nothing missing.

If anyone has any idea why is this happening please enlighten me.

(_previous) = (*_tail) - this shouldn't even compile! _previous is a pointer-to-pointer, but *_tail is not. I suggest turning up your compiler's warning level. — Oliver Charlesworth
– Oliver Charlesworth, Commented Apr 23, 2012 at 13:26
More generally, it's usually considered bad practice to hide pointers behind typedefs (regardless of whether Microsoft used to do that a lot in Win32 stuff)... — Oliver Charlesworth
– Oliver Charlesworth, Commented Apr 23, 2012 at 13:27
Why do people tend to write C and then tag the question with c++? — Griwes
– Griwes, Commented Apr 23, 2012 at 13:29

Oliver Charlesworth · Accepted Answer · 2012-04-23 13:35:06Z

4

(*_previous) = (*_tail) and _previous = &(*_tail) mean completely different things.

(*_previous) = (*_tail) means "set the value of the thing pointed to by _previous to be equal to the value of thing pointed to by _tail.

_previous = &(*_tail) is equivalent to _previous = _tail, and so it means "set _previous to point at the same thing as _tail.

answered Apr 23, 2012 at 13:35

Oliver Charlesworth

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2 Comments

Atlantrix Over a year ago

I know that but probably my question was not very clear. When i set the values the nextItem of the element with index 0 is pointing to the element with index 3, so the element with index 1 is missing. in the second case i don't have the index 1 element missing. What i don't understand is why when i set the values one element is missing.

Atlantrix Over a year ago

I have elem1->nextItem pointing to elem2, elem2->nextItem to elem3 and elem3->nextItem to elem1. When i do his (_previous) = (_tail) elem1->nextItem is pointing to elem3 and elem3->nextItem is pointing to elem1, *_tail points to elem3 and _head to elem1 so elem2 is gone. if i do this _previous = _tail everything is fine. i don't understand why in the first case elem1->nextItem is changing to point to elem3 even though i made no changes to the nextItem of any element.

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