Null pointer

In computing, a null pointer (sometimes shortened to nullptr or null) or null reference is a value indicating that the pointer or reference does not refer to an object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown length or the failure to perform some action; this use of null pointers can be compared to nullable types and to the Nothing value in an option type. A null pointer should not be confused with an uninitialized pointer: a null pointer is guaranteed to compare unequal to any pointer that points to an object. However, in general, most languages do not offer such a 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 semantically incomplete, since they do not offer the possibility to express the difference between "not applicable", "not currently known", and "not yet determined" values. In systems with a tagged architecture, a possibly null pointer can be replaced with a tagged union which enforces explicit handling of the exceptional case; in fact, a possibly null pointer can be seen as a tagged pointer with a computed tag. 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, or null pointer exception. It is one of the most common types of software weaknesses, and Tony Hoare, who introduced the concept, has referred...