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Null pointer
In computing, a null pointer (sometimes shortened to nullptr or null) 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, 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 semantically incomplete, since they do not offer the possibility to express the difference between "not applicable", "not known", and "future" values.[citation needed]
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 to it as a "billion dollar mistake".
In C, two null pointers of any type are guaranteed to compare equal. Prior to C23, the preprocessor macro NULL was provided, defined as an implementation-defined null pointer constant in <stdlib.h>, which in C99 can be portably expressed with #define NULL ((void*)0), the integer value 0 converted to the type void* (see pointer to void type). Since C23, a null pointer is represented with nullptr which is of type nullptr_t (first introduced to C++11), providing a type safe null pointer.
The C standard does not say that the null pointer is the same as the pointer to memory address 0, though that may be the case in practice. Dereferencing a null pointer is undefined behavior in C, and a conforming implementation is allowed to assume that any pointer that is dereferenced is not null.
In practice, dereferencing a null pointer may result in an attempted read or write from memory that is not mapped, triggering a segmentation fault or memory access violation. This may manifest itself as a program crash, or be transformed into a software exception that can be caught by program code. There are, however, certain circumstances where this is not the case. For example, in x86 real mode, the address 0000:0000 is readable and also usually writable, and dereferencing a pointer to that address is a perfectly valid but typically unwanted action that may lead to undefined but non-crashing behavior in the application; if a null pointer is represented as a pointer to that address, dereferencing it will lead to that behavior. There are occasions when dereferencing a pointer to address zero is intentional and well-defined; for example, BIOS code written in C for 16-bit real-mode x86 devices may write the interrupt descriptor table (IDT) at physical address 0 of the machine by dereferencing a pointer with the same value as a null pointer for writing. It is also possible for the compiler to optimize away the null pointer dereference, avoiding a segmentation fault but causing other undesired behavior.
In C++, while the NULL macro was inherited from C, the integer literal for zero has been traditionally preferred to represent a null pointer constant. However, C++11 introduced the explicit null pointer constant nullptr and type nullptr_t to be used instead, providing a type safe null pointer. nullptr and type nullptr_t were later introduced to C in C23.
In languages 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.
Hub AI
Null pointer AI simulator
(@Null pointer_simulator)
Null pointer
In computing, a null pointer (sometimes shortened to nullptr or null) 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, 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 semantically incomplete, since they do not offer the possibility to express the difference between "not applicable", "not known", and "future" values.[citation needed]
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 to it as a "billion dollar mistake".
In C, two null pointers of any type are guaranteed to compare equal. Prior to C23, the preprocessor macro NULL was provided, defined as an implementation-defined null pointer constant in <stdlib.h>, which in C99 can be portably expressed with #define NULL ((void*)0), the integer value 0 converted to the type void* (see pointer to void type). Since C23, a null pointer is represented with nullptr which is of type nullptr_t (first introduced to C++11), providing a type safe null pointer.
The C standard does not say that the null pointer is the same as the pointer to memory address 0, though that may be the case in practice. Dereferencing a null pointer is undefined behavior in C, and a conforming implementation is allowed to assume that any pointer that is dereferenced is not null.
In practice, dereferencing a null pointer may result in an attempted read or write from memory that is not mapped, triggering a segmentation fault or memory access violation. This may manifest itself as a program crash, or be transformed into a software exception that can be caught by program code. There are, however, certain circumstances where this is not the case. For example, in x86 real mode, the address 0000:0000 is readable and also usually writable, and dereferencing a pointer to that address is a perfectly valid but typically unwanted action that may lead to undefined but non-crashing behavior in the application; if a null pointer is represented as a pointer to that address, dereferencing it will lead to that behavior. There are occasions when dereferencing a pointer to address zero is intentional and well-defined; for example, BIOS code written in C for 16-bit real-mode x86 devices may write the interrupt descriptor table (IDT) at physical address 0 of the machine by dereferencing a pointer with the same value as a null pointer for writing. It is also possible for the compiler to optimize away the null pointer dereference, avoiding a segmentation fault but causing other undesired behavior.
In C++, while the NULL macro was inherited from C, the integer literal for zero has been traditionally preferred to represent a null pointer constant. However, C++11 introduced the explicit null pointer constant nullptr and type nullptr_t to be used instead, providing a type safe null pointer. nullptr and type nullptr_t were later introduced to C in C23.
In languages 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.