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'NULL' : undeclared identifier

Started by emileej Jul 15, 2003 at 4:31 AM 12 replies 2.1k views

Original Post

Jul 15, 2003 at 4:31 AM

//...

	int *ival;
//...

	ival=NULL;
//...

	bool IsInt(){ return (ival!=NULL); } //error C2065: ''NULL'' : undeclared identifier

//...

Now why - oh - why is this so? ------------- E-)mil "I feel like... I feel like... Like taking over the world!" - Day of the Tentacle

foofightr

Jul 15, 2003 at 4:45 AM

NULL is a #define in some headers, it is not built into any compiler. The most lightweight way is to copy it from a standard header into your own header. For example:


// the almighty NULL, shamelessly ripped from windef.h
#ifndef NULL
#  ifdef __cplusplus
#    define NULL    0
#  else
#    define NULL    ((void *)0)
#  endif
#endif

[edited by - foofightr on July 15, 2003 5:46:10 AM]

Jul 15, 2003 at 4:45 AM

At a guess, because you don''t have the header file included that defines NULL?

NULL is a preprocessor definition which is #define''d to 0.

#define NULL 0

emileej

Author

emileej

Jul 15, 2003 at 4:55 AM

The thing is: If I make a very simple program:


int main(int argc, char argv[]){
	void *ptr;
	ptr=NULL;
	return 0;
}

...all is ok. Wierd!

Qw3r7yU10p!

Jul 15, 2003 at 5:00 AM

are you sure that example has no headers included?

also what compiler and libraries are you using?

emileej

Author

emileej

Jul 15, 2003 at 5:06 AM

I am using MS VC++ 6 - just created an empty workspace and added main.cpp with the code you see - none else.

Fruny

203

Fruny

Jul 15, 2003 at 5:13 AM

#include <cstddef> or just use 0 instead.

Qw3r7yU10p!

Jul 15, 2003 at 5:27 AM

i tried exactly that and it came up with:

Compiling...
null.cpp
c:\work\sand\xor\null.cpp(4) : error C2065: ''NULL'' : undeclared identifier

I have VC6.0 SP5

are you sure there are no includes (such as stdafx.h) or anything else?

emileej

Author

emileej

Jul 15, 2003 at 5:42 AM

Positive, but ill just use
0 in stead - it seems simpler.

Qw3r7yU10p!

Jul 15, 2003 at 6:16 AM

I can''t believe that code is compiling.

Try compiling using ctrl-f7

Is it included in the project? Can you run it? If so place a breakpoint in the code. Rebuild it with browse information, right click on NULL and choose go to definition - what does it give you?

foofightr

Jul 15, 2003 at 12:13 PM

Don''t use 0 instead of NULL just because you''re lazy. The whole point of NULL is to help others (and yourself in a few months) be able to distinguish integers from pointers at a quick glance. 0 is for integers, NULL is for pointers. That''s the whole point.

Trienco

146

Trienco

Jul 15, 2003 at 3:13 PM

or use neither.. if (ptr) instead of if (ptr!=NULL) or if (!ptr) instead of if (ptr==0)... wont make it any more readable but you can be even more lazy. btw. stroustrup himself recommends to use 0 instead of NULL for fewer problems or at least defining it as const int and not via #define.

someone looking at the code should be able to recognize a pointer even without having any NULL around if you name your variables correctly.

Kurioes

Jul 15, 2003 at 3:30 PM

I just prefix my variable names with "p" for pointers (Hungarian notation).

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