"John Torjo" <john.lists@torjo.com> wrote in message news:41640494.6000403@torjo.com... | | > | | > | Another problem, IMO, is that client code will need to be more | > | complicated, in order to find out if a value is null or not. | > | > I don't understand that. One of the objectives of a null object is that you | > don't have to | > worry about a value being null. | | really? | I thought the purpose of null was to say "there's no object here. You | can't do anything with me other than compare other pointers to me." in the above a "null object" is an intance of a class and "null" is 0. have you read the referenced paper? (http://www.two-sdg.demon.co.uk/curbralan/papers/europlop/NullObject.pdf) | > | > | And, for each new (virtual) function you add, you should update the | > | > | null_object class - which I think can turn into a maintenance nightmare | > :( | > | > | > | > if your inheritance hierarchy contains N distinct classes, you would need | > to | > | > add N implementations of that virtual function anyway, right? | > | | > | Not necessary... I can add one implementation, and some of those | > | distinct N classes don't have to override it ;) | > | > you could insert the null object class between the abstract base class and the | > leaf classes. | | I've never seen a design like this, would never do something like this | myself, why not? It is quite common to have some class like this since we want to keep the base class fully abstract (as you would like to ensure minimal rebuilds). | and I think I would really dislike any design that exibits this. why? Anyway, I don't really see how adding *one* extra function is a maintenance nightmare. Keeping track of 0's seems much closer to a maintenance nightmare since it affects *all* function calls on the container. | > It might be worth to have the existing interface if users want to try changing | > from vector<string> to ptr_vector<string> | > just to see what happens. | > | | I don't like phrases like "hey, let's change from std::vector to | std::map, just to see what happens" (I know I exagerated a bit). When | you change something, you should have a reason, and know what you're | doing (tradeoffs, etc.). Ross Boylan thought it was important. br Thorsten