Editing Null pointer (section)

{{Short description|Value indicating that a referenced dataset is invalid or doesn't exist}}

In [[computing]], a '''null pointer''' or '''null reference''' is a value saved for indicating that the [[Pointer (computer programming)|pointer]] or [[reference (computer science)|reference]] does not refer to a valid [[Object (computer science)|object]]. Programs routinely use null pointers to represent conditions such as the end of a [[List (computing)|list]] of unknown length or the failure to perform some action; this use of null pointers can be compared to [[nullable type]]s and to the ''Nothing'' value in an [[option type]].

A null pointer should not be confused with an [[uninitialized variable|uninitialized pointer]]: a null pointer is guaranteed to compare unequal to any pointer that points to a valid object. However, in general, most languages do not offer such guarantee for uninitialized pointers. It might compare equal to other, valid pointers; or it might compare equal to null pointers. It might do both at different times; or the comparison might be [[undefined behavior]]. Also, in languages offering such support, the correct use depends on the individual experience of each developer and linter tools. Even when used properly, null pointers are [[semantic completeness|semantically incomplete]], since they do not offer the possibility to express the difference between "not applicable", "not known", and "future" values.{{cn|date=March 2025}}

Because a null pointer does not point to a meaningful object, an attempt to access the data stored at that (invalid) memory location may cause a run-time error or immediate program crash. This is the '''null pointer error'''. It is one of the most common types of software weaknesses,<ref>{{cite web |url=https://cwe.mitre.org/data/definitions/476.html |title=CWE-476: NULL Pointer Dereference |website=MITRE}}</ref> and [[Tony Hoare]], who introduced the concept, has referred to it as a "billion dollar mistake".<ref name=":0" />