NULL Undeclared Error in C++

1. the NULL Keyword in C+
2. Include Relevant Predefined Header
3. Define NULL as a Constant
4. Use 0 Instead of NULL
5. Use nullptr Instead of NULL

This article will discuss the NULL keyword and the error of NULL undeclared in C++.

the NULL Keyword in C+

The NULL is constant in C++, which is used to initialize a pointer variable with a value of 0. We can use NULL or 0 interchangeably.

It’s a good practice to assign a NULL when you declare a pointer and don’t have an exact address to save in that pointer. So, it is called a null pointer until and unless it is pointed to some other value.

Syntax:

DataType *PointerName = NULL;

There are cases when you get the error when using the NULL keyword like this:

int main() {
  int* p = NULL;
  return 0;
}

Output:

Now, let’s discuss how to solve this error.

Include Relevant Predefined Header

The NULL keyword is declared a constant in different header files like iostream, stdio, or cstddef. You can include any of these to resolve this error.

#include <iostream>
int main() {
  int* p = NULL;
  return 0;
}

The above code executes without an error because NULL is also defined in iostream.

Define NULL as a Constant

You can define a constant named NULL in your code.

#define NULL 0
int main() {
  int* p = NULL;
  return 0;
}

Use 0 Instead of NULL

Use 0 instead of NULL. Both have the same meaning.

int main() {
  int* p = NULL;
  return 0;
}

Use nullptr Instead of NULL

In the modern versions of C++ like C++ 11, nullptr can be used as an alternative to the NULL keyword.

int main() {
  int* p = nullptr;
  return 0;
}