Why Set a Pointer to nullptr After Deleting It?

What is a Dangling Pointer?

A dangling pointer is a pointer that continues to hold the memory address of an object or variable after the memory has been deallocated (freed). This occurs when you delete a pointer, releasing the memory it points to, but the pointer itself still holds the address of the now-invalid memory. Accessing a dangling pointer can lead to undefined behavior, which might cause your program to crash, corrupt data, or produce unpredictable results.

Why is a Dangling Pointer Dangerous?

Using a dangling pointer is hazardous because it points to no longer valid memory. Since the memory has been freed, it may be reused for other purposes by the system. If your program attempts to access or modify this memory through a dangling pointer, it can result in:

Crashes: Your program might try to access memory reallocated for another purpose, leading to segmentation faults or access violations.
Corrupted Data: If the memory is reused and modified elsewhere in your program, accessing it through a dangling pointer can corrupt the data, leading to hard-to-diagnose bugs.
Undefined Behavior: The most dangerous aspect of dangling pointers is their unpredictable behavior. The program might work fine in some cases and fail in others, making the issue difficult to debug.

How to Prevent Dangling Pointers?

The simplest and most effective way to prevent dangling pointers is to set a pointer to nullptr immediately after deleting it. When a pointer is set to nullptr, it clearly indicates that the pointer is no longer pointing to any valid memory. Attempting to dereference a nullptr results in a predictable and immediate error, making it easier to identify and fix issues.

Example of a Dangling Pointer

int* ptr = new int(10);  // Allocate memory
delete ptr;              // Free the allocated memory

// ptr is now a dangling pointer
if (ptr != nullptr) {
    // This check might pass even though ptr is dangling
    *ptr = 20;  // Undefined behavior! The memory is no longer valid
}

Explanation:

After calling delete, the pointer ptr still holds the memory address of the now-freed memory. Even though the if check might pass, dereferencing ptr (*ptr = 20) leads to undefined behavior because the memory has been deallocated and may be used by something else.

Example of Setting a Pointer to `nullptr`

int* ptr = new int(10);  // Allocate memory
delete ptr;              // Free the allocated memory
ptr = nullptr;           // Set ptr to nullptr

if (ptr != nullptr) {
    // This check will fail, preventing undefined behavior
    *ptr = 20;  // Safe: this line won't be executed because ptr is nullptr
}

Explanation:

After deleting the memory, ptr is set to nullptr. The subsequent if check will fail, preventing any attempt to dereference a pointer that no longer points to valid memory. This makes the program safer and easier to debug.

Benefits of Setting a Pointer to `nullptr`

Safety:
- Accessing a nullptr is safer than accessing a dangling pointer. Dereferencing a nullptr results in a well-defined runtime error (such as a segmentation fault), which is easier to detect and fix compared to the unpredictable consequences of accessing a dangling pointer.
Easier Debugging:
- Setting a pointer to nullptr after deletion makes it immediately clear that the pointer no longer points to valid memory. This clarity helps in identifying and debugging issues more efficiently, as you can easily check whether a pointer is valid before using it.
Clear Intent:
- By explicitly setting a pointer to nullptr, you clearly indicate your intent that the pointer should no longer be used. This practice improves the readability and maintainability of your code, as it communicates to anyone reading the code that the pointer is intentionally invalidated.

Things to think about

Dangling Pointers: After you delete a pointer, it becomes a dangling pointer if it still holds the memory address of the deallocated memory. Accessing a dangling pointer can lead to undefined behavior, crashes, and data corruption.
Setting to nullptr: Setting the pointer to nullptr after deletion prevents accidental use of invalid memory, making your code safer. It provides a clear and reliable way to check whether a pointer is still valid.
Code Clarity: Setting a pointer to nullptr after deletion clearly indicates that the pointer is no longer valid, improving code readability and maintainability.

By adopting the practice of setting pointers to nullptr after deletion, you can write more robust, reliable, and maintainable C++ programs. This simple step can significantly reduce the risk of bugs and make your code easier to understand and debug.