set head to NULL ('NULL' : undeclared identifier)

Question

I defined a linked list in C++. I am trying to set a NULL value to the variable head (in the constructor of Movie_LinkedList), but I got:

movie.h(40): error C2065: 'NULL' : undeclared identifier

please note that I can't include any library except of iostream

#define NAME_LEN 100

class Movie_DataBase
{
private:
    long int m_Code;
    char m_Name[NAME_LEN];
    bool belongs_to_netwroks[2];    
public:
    Movie_DataBase();
    Movie_DataBase(char movie_name[],long int movie_code);
};


class Movie_LinkedList
{
private:
    class Movie_Node
    {
    public:
        Movie_DataBase* data;
        Movie_Node* prev;
        Movie_Node* next;
    public:
        Movie_Node();

    };
    Movie_Node* head; //pointer to the first node in the linkedlist
    int a;
public:
    void set_movie();
    Movie_LinkedList() { head = NULL; }

};

Any help appreciated!

Brainless

That's because NULL hasn't been defined. Either define it or set head = 0 – Equerry

Why did you delete your code? – Candiot

because there are students that can copy my code.. – Brainless

They still can; it's in the edit history. – Ting

Woe to those students who copy broken code! :-P – Glassworks

Answer 1

As written, NULL isn't defined in your program. Usually, that's defined in a standard header file -- specifically <cstddef> or <stddef.h>. Since you're restricted to iostream, if yours doesn't get NULL implicitly from that header, you can use 0 or, in C++11, nullptr, which is a keyword and doesn't require a header. (It is not recommended to define NULL yourself. It might work sometimes, but it is technically illegal.)

Glassworks

including a header file for a macro is unnecessary. I am not the downvoter though – Lenhard

OP is not allowed to include headers! – Lenhard

@Aniket The only way to get NULL is by including one of the standard headers which define it. – Lent

that's certainly not true @JamesKanze the other way is posted below as my answer – Lenhard

@Aniket Your answer is wrong. It's illegal to define NULL in your code (at least if you include any standard headers). – Lent

Answer 2

You should include <stddef.h> or <cstddef>.

However you can use 0 or nullptr too.

Perihelion

I think nullptr is only for c++11 – Equerry

Op is not allowed to include headers. and assigning a pointer to an integer will cause compiler error. – Lenhard

No it won't. Try int* p = 0; Very common. – Glassworks

hmm @Glassworks try this: int *p = 0x1 - however the compiler says it is ok to assign 0 but not 1 – Lenhard

Thus spake the Creator: stroustrup.com/C++11FAQ.html#nullptr Note also the references. – Glassworks

@Aniket That's because a constant integral expression evaluating to 0 is a null pointer constant, and converts implicitly to a null pointer of any type. How did you think NULL was defined? – Lent

Answer 3

use the following include:

#include <stddef.h>

Datum

@Aniket He has to, otherwise he cannot use NULL. – Lent

Answer 4

No libraries needed!

on the top of the header file,

do this:

#ifndef NULL
#define NULL (0)
#endif

Lenhard

This is forbidden by the standard, and results in undefined behavior. – Lent

Worse: the definition you give isn't even legal. – Lent

@JamesKanze it is legal now – Lenhard

The problem was the difference between (void *)0 being NULL in C and 0 in C++ because of a certain incompatibilities. – Lenhard

Provided it is in one of the standard headers. You're not allowed to provide definitions of things that are in the standard headers. – Lent

but he is not including standard headers. And you can always undefine any macro(even in standard libraries) – Lenhard

There's no real difference between traditional C and C++ here. NULL has always been 0, since the days of Kernighan and Richie. For some unknown reason, the C standards committee decided to allow an additional form, but it wasn't in K&R C, and certainly isn't traditional. – Lent

let us continue this discussion in chat – Lenhard

<iostream> is a standard header. And while it is legal to undefine NULL, it's not legal for you to define it. – Lent

See §17.6.4.3.1/1: "A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header." – Lent

A proscription routinely violated, alas, when it comes to the min/max macros. – Glassworks

Answer 5

NULL isn't actually part of the core C or C++ language; it's defined to be 0 in stddef.h

Since you're using C++, prefer nullptr, which is a keyword, and typed. (Assuming it's available. It's part of C++11, so technically not all compilers will support it; practically, you'd be hard-pressed to find a compiler that doesn't)

Ting

Answer 6

I also encountered this. So need to add <string.h> headers.

#include<string.h>

typedef struct{
 node *head;
 node *current;
 int size
}list;

void appendList(list *l, int n){
  if(l->head == NULL){
    
  }
}

int main(){
 return 0;
}

I tried to create a linkedlist, and I encountered this.

Stylite