Problem with checking if class is NULL

[TwinkiE_HunteR-G]

Hello all,

Ok, I am taking a C++ class at a local community college, and I'm
working on a small program for my own learning needs and I ran into a
problem.

The class I'm building includes a pointer to another object of the
same class, so that I can loop through the objects, using the objects
Next_Object() member function which returns the adress of the next
object in the list.

In my loop I am setting a pointer to the class to what Next_Object()
returns, and I need to check if that is NULL in order to exit the loop
(i.e tmp_object != NULL). Here is where my problem is. The compiler
says that I need to overload the != operator to do this, but if the
object is NULL, how would it call it's member function?

Any help would be appreciated.


- TwinkiE_HunteR-G strikes again!
: (e-mail address removed)
: HARD-WiRE Web Design
: http://www.hard-wire.net

 

[Jack Klein]

Hello all,

Ok, I am taking a C++ class at a local community college, and I'm
working on a small program for my own learning needs and I ran into a
problem.

The class I'm building includes a pointer to another object of the
same class, so that I can loop through the objects, using the objects
Next_Object() member function which returns the adress of the next
object in the list.

In my loop I am setting a pointer to the class to what Next_Object()
returns, and I need to check if that is NULL in order to exit the loop
(i.e tmp_object != NULL). Here is where my problem is. The compiler
says that I need to overload the != operator to do this, but if the
object is NULL, how would it call it's member function?

Any help would be appreciated.

There must be a difference between the description that you posted and
the code that you didn't. Any pointer to any object type, and even
pointers to functions can be compared to NULL. If your compiler does
not accept this, what you are trying to compare is NOT a pointer.

Post the smallest possible code sample that show problem, with the
definitions of the data types.

 

[Jon Willeke]

In my loop I am setting a pointer to the class to what Next_Object()
returns, and I need to check if that is NULL in order to exit the loop
(i.e tmp_object != NULL). Here is where my problem is. The compiler
says that I need to overload the != operator to do this, but if the
object is NULL, how would it call it's member function?

Any help would be appreciated.

Are you sure tmp_object is a pointer?

 

[Gernot Frisch]

Are you sure tmp_object is a pointer?

And if it's not:
class A
{
public:
A(){}
~A(){}

A m_A;
};

int main(int, char**)
{
A myA;
}

This will get you some trouble, won't it? My compiler doesn't allow
it, does yours?
-Gernot

 

[TwinkiE_HunteR-G]

And if it's not:
class A
{
public:
A(){}
~A(){}

A m_A;
};

int main(int, char**)
{
A myA;
}

This will get you some trouble, won't it? My compiler doesn't allow
it, does yours?
-Gernot

OK, Im sorry for the trouble, I have been up for a couple of days when
I wrote this and I missed the single character to declare tmp_object
as a pointer. My apologizes and thanks for the quick and helpful
responses.


- TwinkiE_HunteR-G strikes again!
: (e-mail address removed)
: HARD-WiRE Web Design
: http://www.hard-wire.net

 

[Bill Seurer]

Are you sure tmp_object is a pointer?

And just what is "NULL"? Use the proper 0.

 

[Christopher Benson-Manica]

And just what is "NULL"? Use the proper 0.

NULL is equally proper.

 

[Buster]

I think NULL is defined in stdio.h

I think you're right, but stdio.h is deprecated. NULL is defined in the
standard headers clocale, cstddef, cstdio, cstdlib, cstring, ctime and
cwchar. I posted this information earlier on without attribution. I got
it from the marvellous "C++ in a Nutshell" by Ray Lischner.

 

[Anil Mamede]

NULL is equally proper.

I think NULL is defined in stdio.h

Anil Mamede

 

[Bill Seurer]

NULL is equally proper.

The only mention of "NULL" in the standard is in the description of
cstddef. Also see section 5.1.1 of _The C++ Programming Language_.

 

[Kevin Goodsell]

The only mention of "NULL" in the standard is in the description of
cstddef. Also see section 5.1.1 of _The C++ Programming Language_.

I see many mentions of NULL scattered throughout the Nov. '97 draft that
I have. In particular, C.2.2.3 (non-normative) lists <clocale>,
<cstddef>, <cstdio>, <cstdlib>, <cstring>, <ctime>, and <cwchar> as
containing a definition for NULL.

It probably defers to the C standard to formally give the headers where
NULL is defined.

-Kevin