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See also Null References - The Billion Dollar Mistake by Tony Hoare, Tony Hoare, Null (SQL), Null reference, Null pointer, Null pointer exception, Nullable type

Snippet from Wikipedia: Null pointer

In computing, a null pointer or null reference is a value saved for indicating that the pointer or reference 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; or the comparison might be undefined behaviour.

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, and Tony Hoare, who introduced the concept, has referred to it as a "billion dollar mistake".

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