What is "null pointer"

n. (context computing English) A pointer that does not point to any data object.

In computing, a null pointer has a value reserved for indicating that the pointer does not refer to a valid 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 a valid object. However, depending on the language and implementation, an uninitialized pointer may not have any such guarantee. It might compare equal to other, valid pointers; or it might compare equal to null pointers. It might do both at different times.

Null pointers have different semantics than null values. A null pointer in most programming languages means "no value", while a null value in a relational database means "unknown value". This leads to important differences in practice: most programming languages will treat two null pointers as equal, but a relational database engine does not regard two null values as equal (since they represent unknown values, it is unknown whether they are equal).