Hi Thorsten, On Fri, Oct 08, 2004 at 08:11:28PM +0200, Thorsten Ottosen wrote:
Then I compared code like
for_each(.ptr_container_with_null_objects ) { i->foo(); // non-virtual i->bar(); // virtual }
with
for_each( standard_container_with_0s ) { if( *i ) { (*i)->foo(); // non-virtual (*i)->bar(); // virtual } }
on vc7.1
In this setting a if the we have 2 : 5 nulls (2 nulls, 3 normal), checking for 0 wins. If we have 1 : 5 nulls or better the null object method wins. Basically I would guess this ratio depends entirely on how many if() we can execute for each virtual function dispatch since the body of null-object function is always trivial.
Conclusion: disallowing 0 will not only lead to simpler, safer programs, but it will probably also boost performance.
I'm sorry, but I must disagree with you analysis. You are looking to the problem only from the smart containers perspective. This leads to fundamentaly wrong assumptions and wrong results. Lets us think for a while what is the difference between null_object and 0 pointer. If we want to apply your semantics, null object must have fully defined interface. Although its function will probably do nothing, still they must behave correctly. So it is no longer a null_object. It is an object with a special state. If you relax this requiremnt you will get to the exactly same scenario as with 0 poiner, only with slightly bigger overhead. Why? Becase if I designate something as null, I will have to check for it anyway. So your example will look like this: for_each(.ptr_container_with_null_objects ) { if(!i->is_null) { i->foo(); // non-virtual i->bar(); // virtual } } Otherwise, you will have pay the comparison cost every time you perform an operation on the null object, and that might be on far more places. In other words. If I designate an object as null, I want to threat it as an invalid null object. Therefor I will have to check every operation I would with 0 pointer. Also, if the design of my application needs to put only non-null objects into the container, I will NOT HAVE to check anything. At most I will put asserts there, but definitely not exceptions. So your arguments are quite misleading. If you allow 0 pointers, it is still possible to uses null_object pattern without any additional cost. The oposite is obviosly not feasible. Regards, Pavol