Null object: Difference between revisions

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Classic BASIC does have the concept of un-initialised or undefined variables.

=={{header|C}}==

C has the null pointer. By ~~convention~~, it ~~points~~ to ~~memory~~ ~~address~~ zero. It is (supposedly) garanteed to be pointing to nothing, so receiving one of those likely means you are not looking at an object--<i>but</i>, there are occasions where changing content of a null pointer actually does something (say, on DOS); and a function that's supposed to return a pointer on success doesn't always return a 0 otherwise (e.g. mmap returns -1 for failure).

C has the null pointer, written as "0", whose internal representation is often, [http://c-faq.com/null/varieties.html though not always], the same as integer zero. It is (supposedly) garanteed to be pointing to nothing, so receiving one of those likely means you are not looking at an object--<i>but</i>, there are occasions where changing content of a null pointer actually does something (say, on DOS); and a function that's supposed to return a pointer on success doesn't always return a 0 otherwise (e.g. mmap returns -1 for failure).

There is a very common macro, <code>NULL</code>, which evaluates to <code>(void*) 0</code> or an equivalent value. NULL is compatible with all pointer types, including both data pointers and function pointers.