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

c++ pointers null

49,028

Solution 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.)

Solution 2

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

However you can use 0 or nullptr too.

Solution 3

use the following include:

#include <stddef.h>

Solution 4

No libraries needed!

on the top of the header file,

do this:

#ifndef NULL
#define NULL (0)
#endif

Solution 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)

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49,028

Alon Shmiel

Updated on July 09, 2022

Comments

Alon Shmiel almost 2 years

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

Any help appreciated!
- Nick about 11 years
  
  That's because NULL hasn't been defined. Either define it or set head = 0
- CassOnMars about 11 years
  
  Why did you delete your code?
- Alon Shmiel about 11 years
  
  because there are students that can copy my code..
- Chowlett about 11 years
  
  They still can; it's in the edit history.
- metal about 11 years
  
  Woe to those students who copy broken code! :-P
Nick about 11 years

I think nullptr is only for c++11
Aniket Inge about 11 years

including a header file for a macro is unnecessary. I am not the downvoter though
Aniket Inge about 11 years

OP is not allowed to include headers!
Aniket Inge about 11 years

Op is not allowed to include headers. and assigning a pointer to an integer will cause compiler error.
metal about 11 years

No it won't. Try int* p = 0; Very common.
Aniket Inge about 11 years

hmm @metal try this: int *p = 0x1 - however the compiler says it is ok to assign 0 but not 1
James Kanze about 11 years

@Aniket The only way to get NULL is by including one of the standard headers which define it.
Aniket Inge about 11 years

that's certainly not true @JamesKanze the other way is posted below as my answer
metal about 11 years

Thus spake the Creator: stroustrup.com/C++11FAQ.html#nullptr Note also the references.
James Kanze about 11 years

@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?
James Kanze about 11 years

@Aniket He has to, otherwise he cannot use NULL.
James Kanze about 11 years

This is forbidden by the standard, and results in undefined behavior.
James Kanze about 11 years

@Aniket Your answer is wrong. It's illegal to define NULL in your code (at least if you include any standard headers).
James Kanze about 11 years

Worse: the definition you give isn't even legal.
Aniket Inge about 11 years

@JamesKanze it is legal now
Aniket Inge about 11 years

The problem was the difference between (void *)0 being NULL in C and 0 in C++ because of a certain incompatibilities.
James Kanze about 11 years

Provided it is in one of the standard headers. You're not allowed to provide definitions of things that are in the standard headers.
Aniket Inge about 11 years

but he is not including standard headers. And you can always undefine any macro(even in standard libraries)
James Kanze about 11 years

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.
Aniket Inge about 11 years

let us continue this discussion in chat
James Kanze about 11 years

<iostream> is a standard header. And while it is legal to undefine NULL, it's not legal for you to define it.
James Kanze about 11 years

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."
metal about 11 years

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