Why is NULL undeclared?

Question

I have a problem with this struct contructor when I try to compile this code:

typedef struct Node
{
    Node( int data ) //
    {
        this->data = data;
        previous = NULL; // Compiler indicates here
        next = NULL;
    }

    int data;
    Node* previous;
    Node* next;
} NODE;

when I come this error occurs:

\linkedlist\linkedlist.h||In constructor `Node::Node(int)':|
\linkedlist\linkedlist.h|9|error: `NULL' was not declared in this scope|
    ||=== Build finished: 1 errors, 0 warnings ===|

Last problem was the struct, but it worked fine when it was in my main.cpp, this time it's in a header file and is giving me this problem. I am using Code::Blocks to compile this code

Answer 1

Are you including "stdlib.h" or "cstdlib" in this file? NULL is defined in stdlib.h/cstdlib

#include <stdlib.h>

or

#include <cstdlib>  // This is preferrable for c++

Answer 2

NULL is not a built-in constant in the C or C++ languages. In fact, in C++ it's more or less obsolete, just use a plain literal 0 instead, the compiler will do the right thing depending on the context.

Otherwise, add

#include <stddef.h>

to get the NULL definition.

UPDATE: I had the wrong header, corrected now. Thanks!

Answer 3

NULL isn't a native part of the core C++ language, but it is part of the standard library. You need to include one of the standard header files that include its definition. #include <cstddef> or #include <stddef.h> should be sufficient.

The definition of NULL is guaranteed to be available if you include cstddef or stddef.h. It's not guaranteed, but you are very likely to get its definition included if you include many of the other standard headers instead.

Answer 4

Don't use NULL, C++ allows you to use the unadorned 0 instead:

previous = 0;
next = 0;

Answer 5

Do use NULL. It is just #defined as 0 anyway and it is very useful to semantically distinguish it from the integer 0.

There are problems with using 0 (and hence NULL). For example:

void f(int); void f(void*);

f(0); // Ambiguous. Calls f(int).

The next version of C++ (C++0x) includes 'nullprt' to fix this.

f(nullptr); // Calls f(void*).