CGAL users discussion list

[Leon Nan <>]

Comparing an iterator to a NULL value

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Hello, everyone:

I have a problem with iterators in a fairly complex polygonal mesh data
structure which is implemented using lists of geometric entities.
However, the problem in itself is fairly simple:

I need to define special iterator values. In particular, I want a null
iterator. This is different from end() which means the end of this
list. I want an iterator value that is known to just not correspond to
any element. It must be possible to compare using == and != and, of
course, this special value must be invariant to changes to the data
structure.

Say the type is

list<int>

and my special iterator is NULL_INT_LIST_ITERATOR

I want to compare like this

a == NULL_INT_LIST_ITERATOR

and this comparison should return true only if a has previously been
assigned my null value. Previously, I did this using

----------------------------------------------
typedef list<int>::iterator IntIter;
IntIter NULL_INT_LIST_ITER(0);
----------------------------------------------

This just defines an iterator with an internal null pointer. That works
fine in g++ but it is not documented that it should work. Arguably it
is ugly. A different solution is this

----------------------------------------------
inline IntIter get_null_int_iter()
{
static list<int> l;
return l.end();
}
#define NULL_INT_LIST_ITER get_null_int_iter()
-----------------------------------------------

However, this solution does not work in recent version of Microsoft
visual studio. The reason *appears* to be that when I compare this
iterator value to a different iterator value it is detected that
NULL_INT_LIST_ITER belongs to a different list.

It seems you can make it work by defining HAS_ITERATOR_DEBUGGING=0.
However, the program still does not work - but possibly for different
reasons.

If you have a better solution, I'd be grateful to hear about it.

Thanks

 

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Leon Nan

2007-10-30