Null Pointers

Null Pointers

A pointer usually points to something real, but sometimes it should point to nothing. C++ calls that a null pointer.

Declaring a null pointer

#include <iostream>
using namespace std;

int* ptr = nullptr;   // points at nothing

nullptr is the modern keyword meaning "no address" (introduced in C++11). Old code may use 0 or NULL instead, but nullptr is the type-safe choice.

Check before dereference

int* ptr = nullptr;
if (ptr != nullptr) {
    cout << *ptr << endl;   // safe
}

The guard ptr != nullptr makes sure we do not touch memory when a pointer has no target.

Dereferencing null is undefined behavior

int* ptr = nullptr;
cout << *ptr << endl;   // danger!

A null pointer holds address 0, which is owned by the operating system, not your program. Reading through it is undefined behavior - the program crashes or prints garbage.

Dangling pointers

A closely related trap is the dangling pointer: the address is still stored, but the memory was already freed.

int* ptr = new int(42);
delete ptr;               // memory is released
cout << *ptr << endl;     // dangling!

Using a dangling pointer is undefined behavior too. If you manage memory by hand, set the pointer back to nullptr after a delete.

Safe habits

• Start a pointer at nullptr when it has no object yet.
• Check ptr != nullptr before dereferencing.
• After delete, point the pointer at nullptr.

TL;DR

• nullptr means "points at no address".
• Check ptr != nullptr before dereferencing.
• Dereferencing null is undefined behavior (a crash).
• A pointer to freed memory is called dangling.
• After a manual delete, set the pointer back to nullptr.