Programming Forums > Application Development > C++

titaniumdecoy · Mar 22nd, 2006, 1:29 PM

I keep getting the error "'NULL' was not declared in this scope" in all my C++ programs. From what I understand, 0 is the equivalent of NULL, but I am wondering why NULL does not seem to work.

I am working on a LinkedList class, and am having the problem that when I try to access the last ListNode in the list, the loop does not stop cycling through memory after it has reached the last ListNode. I am trying to compare the nodes to 0 (empty nodes have been initialized to 0 because NULL does not work). What am I doing wrong? Thanks.

(Toggle Plain Text)

int LinkedList::dequeue() {
    ListNode *current = front;
    while (current->next != 0) { // What should my test condition be?
        current = current->next;
    }
    return current->value;
}

EDIT: I solved the problem, I had not initialized the front ListNode to 0.

titaniumdecoy · Mar 22nd, 2006, 1:45 PM

I still need help with using NULL instead of 0.

I have another question, as well: Why can I only initialize an object I have created with a 0 parameter constructor as follows:

(Toggle Plain Text)

LinkedList list;

And not

(Toggle Plain Text)

LinkedList list();

Since I have declared a 0 parameter constructor:

(Toggle Plain Text)

LinkedList::LinkedList()
  : front( 0 ) { }

jayme · Mar 22nd, 2006, 1:50 PM

Quote:

Originally Posted by titaniumdecoy

I have another question, as well: Why can I only initialize an object I have created with a 0 parameter constructor as follows:

(Toggle Plain Text)

LinkedList list;

And not

(Toggle Plain Text)

LinkedList list();

A constructor is automatically called on object initialization, that's why you don't have "list()". If a constructor wasn't automatically called, using the brackets would be necessary. If you require a constructor to get input, you could use brackets, like so:

(Toggle Plain Text)

LinkedList::LinkedList(int a)
{
    b = a; //b is a data member of LinkedList
}

LinkedList list(5); //set b to 5

bgeraghty · Mar 22nd, 2006, 1:55 PM

Well, in regards to your NULL situation, I can't tell you much without a little more info. Where were you trying to use NULL that caused this. One thing to keep in mind is that NULL is really a pointer to 0.

Paste the code where NULL was bugging out and maybe I can help.

Also, Which compiler do you use?

jayme · Mar 22nd, 2006, 2:04 PM

http://www.eskimo.com/~scs/cclass/notes/sx10d.html

Polyphemus_ · Mar 22nd, 2006, 2:06 PM

#include <cstdlib>

In cstdlib, NULL is defined.

EDIT: damn, jayme was 2 minutes faster

DaWei · Mar 22nd, 2006, 2:36 PM

Quote:

NULL is really a pointer to 0.

The definition of the NULL pointer is much more complicated. On your system, with your compiler, perhaps its value is zero. Perhaps not. Please be careful with the answers you give novices.

nnxion · Mar 22nd, 2006, 5:39 PM

Quote:

Originally Posted by DaWei

The definition of the NULL pointer is much more complicated. On your system, with your compiler, perhaps its value is zero. Perhaps not. Please be careful with the answers you give novices.

Ah yeah, remember the 7 page thread on this discussion on DS? I learned a lot from that, hope that grumpy makes an 'article' about that in the Contributor's Corner.

DaWei · Mar 22nd, 2006, 5:49 PM

The important thing to remember is that a pointer value of zero is required to be interpreted by the compiler as NULL, even if the actual value of NULL is not zero in some particular implementation, and that a dereferenced NULL pointer should not access valid memory. At least, that's what my feeble brain is dredging up, right now. We can hope that Grumpy weighs in here, and not the turkey that says, "pointers don't have values." The concept of the NULL pointer is so esoteric that I don't really cover it in great detail in my tutorial.

grumpy · Mar 24th, 2006, 3:00 AM

Hmmm.... I would have responded earlier, but I've been away due to a family emergency.

NULL is a macro that is specified in the C standard (declared in <stdlib.h>). In C++ it is supported in <cstdio> which might mean (i.e. I haven't checked the standard, so take this with a grain of salt) it is a constant in namespace std rather than a macro.

As Dawei said, if a pointer is initialised to zero the result is the same as if that pointer was assigned as the NULL pointer. Similarly, if a pointer is compared to zero (which includes tests like pointer == 0 or, more tersely, !pointer) the result is the same as if the pointer is compared to a NULL pointer of the same type (i.e. the test "pointer == 0" is the same as the test "pointer == NULL" or [more precisely] "pointer == (type_of_pointer *)NULL"). The NULL pointer is a special value of a pointer that effectively indicates "this pointer points at nothing".

Despite all this, there is no requirement that NULL be a zero value. The compiler is required to do a conversion of a zero to a NULL value behind the scenes when working with pointers. But the value of "0 == (int)NULL" is not required to give a true (or non-zero integer in C) result.

The declarations "LinkList list;" and "LinkedList list();" are equivalent in C++ if LinkedList has a default constructor (a constructor that accepts no arguments). In C++, naming a variable named list may confuse people (but not the compiler) as std::list is a template type in the standard library.

Mar 22nd, 2006, 1:29 PM	#1
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 843 Rep Power: 3	'NULL' was not declared in this scope I keep getting the error "'NULL' was not declared in this scope" in all my C++ programs. From what I understand, 0 is the equivalent of NULL, but I am wondering why NULL does not seem to work. I am working on a LinkedList class, and am having the problem that when I try to access the last ListNode in the list, the loop does not stop cycling through memory after it has reached the last ListNode. I am trying to compare the nodes to 0 (empty nodes have been initialized to 0 because NULL does not work). What am I doing wrong? Thanks. (Toggle Plain Text) int LinkedList::dequeue() { ListNode current = front; while (current->next != 0) { // What should my test condition be? current = current->next; } return current->value; } int LinkedList::dequeue() { ListNode current = front; while (current->next != 0) { // What should my test condition be? current = current->next; } return current->value; } EDIT: I solved the problem, I had not initialized the front ListNode to 0. Last edited by titaniumdecoy; Mar 22nd, 2006 at 1:42 PM.

Mar 22nd, 2006, 1:45 PM	#2
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 843 Rep Power: 3	I still need help with using NULL instead of 0. I have another question, as well: Why can I only initialize an object I have created with a 0 parameter constructor as follows: (Toggle Plain Text) LinkedList list; LinkedList list; And not (Toggle Plain Text) LinkedList list(); LinkedList list(); Since I have declared a 0 parameter constructor: (Toggle Plain Text) LinkedList::LinkedList() : front( 0 ) { } LinkedList::LinkedList() : front( 0 ) { }

Mar 22nd, 2006, 5:49 PM	#9
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	The important thing to remember is that a pointer value of zero is required to be interpreted by the compiler as NULL, even if the actual value of NULL is not zero in some particular implementation, and that a dereferenced NULL pointer should not access valid memory. At least, that's what my feeble brain is dredging up, right now. We can hope that Grumpy weighs in here, and not the turkey that says, "pointers don't have values." The concept of the NULL pointer is so esoteric that I don't really cover it in great detail in my tutorial. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Mar 22nd, 2006, 1:55 PM	#4
bgeraghty Newbie Join Date: Mar 2006 Posts: 15 Rep Power: 0	Well, in regards to your NULL situation, I can't tell you much without a little more info. Where were you trying to use NULL that caused this. One thing to keep in mind is that NULL is really a pointer to 0. Paste the code where NULL was bugging out and maybe I can help. Also, Which compiler do you use?

Mar 22nd, 2006, 2:06 PM	#6
Polyphemus_ Expert Programmer Join Date: Aug 2005 Location: Rotterdam, the Netherlands Posts: 942 Rep Power: 4	#include <cstdlib> In cstdlib, NULL is defined. EDIT: damn, jayme was 2 minutes faster

Mar 24th, 2006, 3:00 AM	#10
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,209 Rep Power: 5	Hmmm.... I would have responded earlier, but I've been away due to a family emergency. NULL is a macro that is specified in the C standard (declared in <stdlib.h>). In C++ it is supported in <cstdio> which might mean (i.e. I haven't checked the standard, so take this with a grain of salt) it is a constant in namespace std rather than a macro. As Dawei said, if a pointer is initialised to zero the result is the same as if that pointer was assigned as the NULL pointer. Similarly, if a pointer is compared to zero (which includes tests like pointer == 0 or, more tersely, !pointer) the result is the same as if the pointer is compared to a NULL pointer of the same type (i.e. the test "pointer == 0" is the same as the test "pointer == NULL" or [more precisely] "pointer == (type_of_pointer *)NULL"). The NULL pointer is a special value of a pointer that effectively indicates "this pointer points at nothing". Despite all this, there is no requirement that NULL be a zero value. The compiler is required to do a conversion of a zero to a NULL value behind the scenes when working with pointers. But the value of "0 == (int)NULL" is not required to give a true (or non-zero integer in C) result. The declarations "LinkList list;" and "LinkedList list();" are equivalent in C++ if LinkedList has a default constructor (a constructor that accepts no arguments). In C++, naming a variable named list may confuse people (but not the compiler) as std::list is a template type in the standard library.

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