'NULL' was not declared in this scope

'NULL' was not declared in this scope

Sebastian Mineur

Member #6,943

February 2006

Hey, guys!
Can anyone tell me why the compiler would give me this error with the following code:

VECTOR* MODEL::create_vertice(float fPosX, float fPosY)
{
    if(this->iNumberOfVertices < 100)
    {
        this->Vertices[this->iNumberOfVertices] = new VECTOR(fPosX, fPosY);
        return this->Vertices[this->iNumberOfVertices++];
    }
    return NULL;
}

NULL is a standard part of the C/C++ language, and shouldn't have to be declared, right? And it's wierd 'cause it compiles this code just fine:

MODEL* VGFX::create_model()
{
    if(this->iNumberOfModels < 100)
    {
        this->Models[this->iNumberOfModels] = new MODEL;
        return this->Models[this->iNumberOfModels++];
    }
    return NULL;
}

To me, the the two snippets are using the keyword in exactly the same way...

X-G

Member #856

December 2000

Quote:

NULL is a standard part of the C/C++ language, and shouldn't have to be declared, right?

Neg. Allegro #defines it though, which is probably why it works in the other file.

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Sebastian Mineur

Member #6,943

February 2006

Quote:

Neg. Allegro #defines it though, which is probably why it works in the other file.

Hmmm, I see. And you're absolutely right! VGFX included allegro.h, but MODEL did not.
Thanks for the help, and for the quick reply

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Speedo

Member #9,783

May 2008

You still have to include a definition of NULL somewhere. For example this won't compile:

int main( ) {
  int * foo = NULL;
  return 0;
}

but this will:

#include <stdlib.h>

int main( ) {
  int * foo = NULL;
  return 0;
}

Vanneto

Member #8,643

May 2007

Or use 0 and stop worrying about it.

P.S.
I'm aware that C's definition of NULL is (void*)0. But he said C/C++, which means he sometimes writes C code using a C++ compiler. His code also leads me to believe he uses C++. So 0 it is. \o/

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James Stanley

Member #7,275

May 2006

#ifndef NULL
#define NULL 0
#endif

GullRaDriel

Member #3,861

September 2003

Isn't NULL supposed to be 0 or (void *)0 ?

Edited

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James Stanley

Member #7,275

May 2006

Makes no difference to me. C doesn't care. And seeing as the OP has been instructed to use <stdlib.h> and not <cstdlib>, and there was no correction about C++ not C, I think it's fair to say he's using C.

GullRaDriel

Member #3,861

September 2003

I have just found this:

#define NULL 0;                   // A typical definition of NULL in C++ 
#define NULL  ((void*)0)          // C defines NULL this way

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James Stanley

Member #7,275

May 2006

You sure you haven't got those the wrong way round?
In my experience, C++ complains about not casting, while C doesn't care.

Vanneto

Member #8,643

May 2007

He is correct.

// C++ 
#ifndef NULL
    #define NULL 0
#endif

// C
#ifndef NULL
    #define NULL (void*)0
#endif

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James Stanley

Member #7,275

May 2006

Ha. Fair enough.
I tried to look in my computer, but stdlib.h said it gets it from stddef.h, and stddef.h isn't known to whereis.

Tobias Dammers

Member #2,604

August 2002

Stroustup himself recommends using 0 instead of NULL, which is completely counter-intuitive (seeing that 0 is an integer, not a pointer), but that's how He sees it. Last time I checked, that is.

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Speedo

Member #9,783

May 2008

Quote:

Well, I pointed him to stdlib.h because, at least in MSVC, that's where NULL is actually defined (cstdlib includes stdlib.h):

/* Define NULL pointer value */
#ifndef NULL
#ifdef __cplusplus
#define NULL    0
#else
#define NULL    ((void *)0)
#endif
#endif

Edit: I got myself absorbed in reading some of Stroustrup's C++ FAQs, and happened to come across what he says about NULL:

Quote:

In C++, the definition of NULL is 0, so there is only an aesthetic difference. I prefer to avoid macros, so I use 0. Another problem with NULL is that people sometimes mistakenly believe that it is different from 0 and/or not an integer. In pre-standard code, NULL was/is sometimes defined to something unsuitable and therefore had/has to be avoided. That's less common these days.

If you have to name the null pointer, call it nullptr; that's what it's going to be called in C++0x. Then, "nullptr" will be a keyword.

Jonny Cook

Member #4,055

November 2003

I usually use 0 instead of NULL. Makes my source files smaller.

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Thomas Fjellstrom

Member #476

June 2000

Well, that and 0 is what you're SUPPOSED to use

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Edgar Reynaldo

Major Reynaldo

May 2007

Except that using NULL reminds you that you're talking about a pointer and not just an integer (even though pointers are basically integers anyway).

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Thomas Fjellstrom

Member #476

June 2000

Except in C++ NULL may be declared as (void *)0 which is completely incompatible with any other type in the known C++ universe, and the compiler will barf all over you. Sure, some headers might "fix" that for you, it might not be fixed everywhere. Also, go ask C++'s creator if its a recommended thing to do.

Edgar Reynaldo

Major Reynaldo

May 2007

It's not like you can't #ifdef #undef #define it if you want to. Also , did you read the part of Stroustrup's FAQ that Speedo quoted above? Aesthetic difference is what he called it. FAQ #null

Vanneto

Member #8,643

May 2007

Just use 0.

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type568

Member #8,381

March 2007

Lend me a flamethrower somebody.

Arthur Kalliokoski

Second in Command

February 2005

Pointers are integers, although the default size of integer may be different from the default size of pointers. Kind of like an index for all of memory rather than just an array.

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BAF

Member #2,981

December 2002

Quote:

If you have to name the null pointer, call it nullptr; that's what it's going to be called in C++0x. Then, "nullptr" will be a keyword.

So C++0x isn't going to make null actually null (instead of 0 or some other crap)? That's lame.

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Ron Novy

Member #6,982

March 2006

Sorry... Just thought this was funny: www.nullptr.org

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bamccaig

Member #7,536

July 2006

BAF said:

So C++0x isn't going to make null actually null (instead of 0 or some other crap)? That's lame.

What's the difference? ::) And how do you expect to efficiently represent nullptr as anything other than 0? :-/

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