Thread: Method returning NULL fails?

Hi,
I have the following method and I would like to know why this makes my program to crash?
I come from an Objective-C background where nothing occurs when an instance that is NULL or zero is sent a method.

Code:

blob LLAHTracking::ClosestBlobToPoint2D(CvPoint point){
	
	int minDist = 999999;
	int dist;
	blob *closestBlob = NULL;
	MyBlobList::blobs *blobs = m_bloblist.ExtractedBlobs();

	for(MyBlobList::blobs::iterator iblob = (*blobs).begin(); iblob != (*blobs).end(); iblob++){
		dist = (*iblob)->distance(point.x, point.y);
		if( dist < minDist ){
			minDist = dist;
			closestBlob = (*iblob);
		}
	}
	return *closestBlob;
}

As you can see, when there is no blobs to iterate in the list the method will return NULL and my program just crashes.
How can I solve this kind of errors? Or maybe is any other reason I might be missing?

Thanks in Advance

Originally Posted by nacho4d

I come from an Objective-C background where nothing occurs when an instance that is NULL or zero is sent a method.

Okay, but if data that is essential to some process is missing, there is no possible language where that will NOT result in a (probably fatal) error.

In this case, if it is "point" you are referring to:

Code:

		dist = (*iblob)->distance(point.x, point.y);

If point is NULL it has no members x and y.

If you mean blobs, the same logic applies. If that is a possible outcome (m_bloblist.ExtractedBlobs() could return NULL), you need to check the return value.

Originally Posted by EVOEx

You don't return NULL. You return NULL, dereferenced. Returning NULL (being a pointer) is fine.

However, how it works now, is that you return something that is copied (blob), as you return not the pointer but the object itself. Now when this object is copied, it's copied from the memory address 0 to another place where the return value is stored. But this address 0 can't be read, in your case, crashing the program.


To return NULL, return a POINTER. The pointer will be NULL; that's fine. But don't -ever- dereference NULL.

Thanks! that was the problem, I change my method from this
blob LLAHTracking::ClosestBlobToPoint2D(CvPoint point);
to this
blob * LLAHTracking::ClosestBlobToPoint2D(CvPoint point);
also I am checking blobs is not NULL before using it. And now It runs fine.


MK27:

Okay, but if data that is essential to some process is missing, there is no possible language where that will NOT result in a (probably fatal) error.

Yeah, you are right, if you pass a NULL or equivalent as the argument of a method it might make a program to crash, but if you do for example:

Code:

Object *obj = nil;  //equivalent of NULL in Objective-C 
obj.something;

in Objective-C it will run without problems (it does nothing ) and this is a big difference with C++ that causes me trouble sometimes
Anyways. Thanks for your help guys.

You saved my day