Take a look at the code statement below which assigns `NULL` to an integer pointer (`n`):

using namespace std;

int main() {
  // your code goes here
  int* n = NULL;
  return 0;
}

Executing the above code gives the `NULL undeclared error` at `main.cpp:5:12`.

This happens because `NULL` is *not* a built-in constant in the C or C++ languages. In fact, in C++, it's more or less obsolete, instead, just a plain $0$ is used. Depending on the context, the compiler will do the right thing.

# Solution

Since `NULL` is not a built-in constant, external libraries, headers, or certain definitions are required to be able to use it. Here are a few:

- `#include <stdio.h>`

- `#include <stddef.h>`: Add this line to use the pre-defined `NULL` constant.

- `#include <iostream>`: Add this line to use the pre-defined `NULL` constant.

- `#define NULL 0`: Add this line to define the `NULL` constant as an equivalent of $0$.

- Simply use a $0$ inst​ead of `NULL`. They are the same thing when it comes to pointers.

- In newer C++ (C++11 and higher), use `nullptr`.



What is the NULL undeclared error in C/C++?

NULL undeclared error occurs due to the lack of a built-in NULL constant in C/C++. Use headers, define NULL, use 0, or nullptr in newer C++.