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Talk:Null object: Difference between revisions
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== '''C''' and '''C+''' languages ==
The C anc C++ examples don't really fit the task description (and it's not possible to do so, because C and C++ don't know a generic null object). What you can have is a null ''pointer'', which is a pointer which doesn't point to any object (and that is indeed what <code>NULL</code> is for). The closest to a null object in C++ would be an empty <code>boost::optional</code> from the boost library.
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I don't know Ada, but I suspect the Ada example to be incorrect as well. --[[User:Ce|Ce]] 12:53, 5 August 2008 (UTC)
I think that insofar as this is a problem, we should fix it by changing the task, not the examples. The basic idea of null is similar enough for this collection to be informative, whether the language has everything-is-a-reference-including-null (Python), maybe-null references and primitives (Java, C), type systems which can include or exclude null (Common Lisp, E, C#), or data structures which explicitly add a case (Haskell) --[[User:Kevin Reid|Kevin Reid]] 22:42, 5 August 2008 (UTC)
:I agree with Kevin, but I'm not sure how it should be reworded to make everyone happy without changing the examples. Any suggestions? --[[User:Mwn3d|Mwn3d]] 00:06, 6 August 2008 (UTC)
::I also agree. The problem with [[Ada]]'s null and [[C++]]'s 0/NULL is that neither is ''the'' object. They are values of a reference type. I cannot tell what was the intention of the task, thinkable interpretations:
::* An [ideal/imaginary/exceptional] object substitutable for anything. "Null object" sounds like an instance of the root type, the common ancestor of all types, provided that the language has such a type. In which sense it were "null"? That is unclear because even the root type usually has some methods (like sizeof, ==, etc). An object that '''has''' methods, which incidentally propagate exceptions when called, does not qualify as "null", does it? Otherwise, to have a "null object" would be enough to create a type with one method that always raise some exception.
::* Like above, but a value. null is such thing for reference types.
::* Unbound name. That could be a forward declaration, a deferred constant, external object (to be linked to) etc, which is left unbound until program crashes attempting to access the corresponding object (= a language design fault). Not very promising.
::* Anything else?
:: --[[User:Dmitry-kazakov|Dmitry-kazakov]] 08:05, 6 August 2008 (UTC)
== Recent change ==
This task used to be about definedness -- which is a property of references. A recent change has stated that it's asking about a property of "null-like values" instead. That (the shift of emphasis from references to values) can be a significant change in scope, depending on the language. And, at the moment, the task actually makes both kinds of scope claims -- one part still says it's about objects (which implies references, since we have the ability to modify objects without the objects losing their identity). --[[User:Rdm|Rdm]] 21:38, 10 November 2011 (UTC)
:Actually, the task has always been about "null". In fact, the original name of the task was [[Null]]. Most of the solutions were added during this time. It was renamed to "Undefined values/Check if a variable is defined" in a collective renaming which caused a ton of confusion. The task description has always talked about "null". --[[Special:Contributions/208.80.119.67|208.80.119.67]] 22:21, 10 November 2011 (UTC)
::For a long time, the task included [http://rosettacode.org/mw/index.php?title=Null_object&oldid=125487 this sentence]: ''Note: the title of this article does not appropriately represent the task. The task is about "null"-like values in various languages, which may or may not be related to the defined-ness of variables in your language.'' This sentence implied that the title, "Undefined values/Check if a variable is defined", was wrong. I renamed the page to "Null object", a phrase from the task description. This helps languages where the [[null object]] is a ''defined'' value, like <code>nil</code> in Common Lisp, <code>f</code> in Factor, <code>None</code> in Python, or <code>nil</code> in Ruby. --[[User:Kernigh|Kernigh]] 01:13, 11 November 2011 (UTC)
::: Please stop changing task names and descriptions; we want to avoid accidentally making implementations of tasks wrong. Well, unless you're prepared to go through all the implementations and ''fix'' anything invalidated yourself. (Not all languages have a type system that admits nulls — i.e., a bottom to the pointer/reference type system — but where that's the case, there's typically other mechanisms for saying the broader statement of “there's nothing here”.) –[[User:Dkf|Donal Fellows]] 09:30, 11 November 2011 (UTC)
:::: as has been said above, the title was wrong, the solutions are right. i went over them, and below is a listing of the issues i found. i'd appreciate if someone could verify those findings.--[[User:EMBee|eMBee]] 13:54, 11 November 2011 (UTC)
:::::In a language where the only references are names, the name change seems insignificant (if it was wrong before, for such a language, it would have to also be wrong after). (But it's also true that many language support references and names as independent concepts.) --[[User:Rdm|Rdm]] 17:04, 11 November 2011 (UTC)
== solutions that need review ==
* Basic: missing actual code
* Javascript: testing for defined
* Maxscript: test for undefined
* MUMPS: test for defined
* PARI/GP: can't tell if this example is about null or not.
* Perl: explains undef as a value but doesn't show how to tell the difference between the variable having a value and the variable actually being undefined. if that difference is not possible then this should be explained. i suppose it's the same as Perl 6, there it is explicitly stated that null objects can not be tested for.
== Quip? ==
This task almost demands a reference to Shakespeare's [[wp:Much_Ado_About_Nothing|Much Ado About Nothing]]. --[[User:Rdm|Rdm]] ([[User talk:Rdm|talk]]) 04:44, 16 April 2014 (UTC)
== Delphi vs. Pascal ==
The entries of [[Delphi]] and [[Pascal]] in this article should be exchanged, since Pascal is the language, Delphi is one implementation, (one among others)
And since the null-object is no Delphi-specific feature.
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