How do we check if a pointer is NULL pointer?

In C or C++, checking if a pointer is NULL (or typically nullptr in C++11 and later versions) is a critical step, primarily to ensure program stability and prevent runtime errors, such as crashes caused by dereferencing a null pointer.

Checking Methods

Typically, you should check if the pointer is null before using it. This can be achieved with a simple if statement:

cpp
int *ptr = /* some initialization logic that might result in ptr being nullptr */;
if (ptr != nullptr) {
    // Pointer is not null; safely use ptr
    std::cout << *ptr << std::endl;
} else {
    // Pointer is null; handle error or appropriate logic
    std::cout << "Pointer is null and cannot be dereferenced." << std::endl;
}

Why Check for Null Pointers

1. Prevent crashes: When attempting to access memory pointed to by nullptr, the program triggers an access violation, typically causing a crash.
2. Ensure correct logical flow: Sometimes a null pointer may be part of the program logic, and checking ensures the correct execution path.

Practical Example

Consider a function to print array contents, but sometimes the array might not be created successfully, and a null pointer is passed. Here's an example of safe handling:

cpp
void printArray(int *arr, size_t size) {
    if (arr == nullptr) {
        std::cout << "The passed array is empty; nothing to print." << std::endl;
    } else {
        for (size_t i = 0; i < size; ++i) {
            std::cout << arr[i] << " ";
        }
        std::cout << std::endl;
    }
}

In this example, we first check if the array pointer arr is null. If it is, we skip iteration and element access, thus avoiding potential runtime errors.

Conclusion

Checking if a pointer is null is a common and important safety practice that helps us write more stable and robust software systems. In practical development, this check should become a standard programming habit.