NULL Pointer in C++

A NULL pointer in C++ represents a pointer that does not refer to any valid memory address.

• It indicates that the pointer is intentionally set to point to nothing and holds the value NULL (or nullptr in C++11 and later).
• It is commonly used during variable declaration to show that a pointer is not yet associated with any memory, and it is also returned by many built-in functions to indicate a failure condition.
• Attempting to dereference a NULL pointer results in undefined behavior and may cause the program to crash.

Pointer is currently null.
Pointer is not null.
Value at the memory location pointed to by the pointer: 5

Explanation:

• First, the pointer is set to a NULL value.
• The program checks whether the pointer is NULL before dereferencing it to avoid runtime errors.
• Then a valid memory address is assigned to the pointer.
• The pointer is checked again before dereferencing.
• Since the pointer is no longer NULL, the else part is executed.

Syntax

We can create a NULL pointer of any type by simply assigning the value NULL to the pointer as shown:

int* ptrName = NULL; // before C++11
int* ptrName = nullptr;
int* ptrName = 0; // using the null pointer constant 0

A null pointer can be initialized using the null pointer constants 0, NULL, or (since C++11) nullptr. In modern C++, nullptr is the preferred way to represent a null pointer because it is type-safe.

Checking NULL Pointer

We can check whether a pointer is a NULL pointer by using the equality comparison operator.

ptrName == NULL
or
ptrName == nullptr

The above expression will return true if the pointer is a NULL pointer. False otherwise.

Applications of Null Pointer in C++

Null pointers serve several important purposes in C++, from safe initialization of pointers to error handling and resource management.

• Initialization: It is a good practice to Initialize pointers to a null value as it helps avoid undefined behavior by explicitly indicating they are not pointing to valid memory locations.
• Default Values: Null pointers act as default or initial values for pointers when no valid address is assigned to the pointers.
• Error Handling: They are useful in error conditions or to signify the absence of data that enables better handling of exceptional cases.
• Resource Release: To release the resources, like the destructor of a class, or to set pointers to NULL after deletion we can use a null pointer to avoid accidentally using or accessing the released memory.
• Sentinel Values: A null pointer can be used to indicate the end of a data structure or a list like in the linked list last node has a null pointer as the next field.

Issues with NULL

NULL pointer makes it possible to check for pointer errors but it also has its limitations:

• Dereferencing a NULL pointer causes undefined behavior that may lead to runtime errors like segmentation faults.
• We need to check explicitly for NULL pointers before dereferencing it to avoid undefined behavior.