Thread: C++ pointer issue

I am using a binary tree whereby the search item is return using pass by reference i.e. "searchElement" store the result

Code:

int BSTree::retrieve ( String searchKey, Car *searchElement ) 
{
	return ( retrieveSub (root, searchKey, searchElement));
}

//recursion partner fucntion of retrieve
int BSTree::retrieveSub ( BSTreeNode *ptr, String key, Car *element ) 
{	
	if ( ptr == 0 )
		return 0;
	else if ( key < ptr->element.key() )
		return retrieveSub (ptr->left, key, element);
	else if ( key > ptr->element.key() )
		return retrieveSub (ptr->right, key, element);
	else
	{
		//at this point, the item address is correctly assign to element
                //however, when return to the retrieve function, the searchElement is NULL
                element = &ptr->element;
		return 1;
	}
}

During debugging, the watch value of element is correct, but when return to parent function, the searchElement is NULL. (both element and searchElement should point to the same address.) What am I missing here? Thanks

element is a local variable of retrieveSub. modifying it is useless, it won't affect anything else

maybe you wanted to modify what it pointed to? or something?

Thanks for the reply. Maybe my concept was wrong, I tot passing a pointer to the retrieveSub can do the trick. vart, here's my 2nd attempt to pass the pointer by reference, correct me if i m wrong, thanks.

Code:

int BSTree::retrieve ( String searchKey, Car &searchElement ) 
{
	return ( retrieveSub (root, searchKey, searchElement));
}

//recursion partner fucntion of retrieve
int BSTree::retrieveSub ( BSTreeNode *ptr, String key, Car &element ) 
{	
	if ( ptr == 0 )
		return 0;
	else if ( key < ptr->element.key() )
		return retrieveSub (ptr->left, key, element);
	else if ( key > ptr->element.key() )
		return retrieveSub (ptr->right, key, element);
	else
	{
                element = ptr->element;
		return 1;
	}
}

However, the compiler complains error for this version. Perhaps due to the varying size of the Car object.

class Car
{
private:
..
public:
Passenger *head, *tail; //a dynamic linked list
}

My last resort is to create a global Car* to store the retrieve element.

Sorry for the wrong implementation. I have change to reference to pointer, but it complains the same error during execution:

doexit(int code = CXX0065: Error: unable to obtain expression value, int quick = 221716, retcaller=1376944)

where the destructors are called and the application exits.

Code:

int BSTree::retrieve ( String searchKey, Car *&searchElement ) 
{
	return ( retrieveSub (root, searchKey, searchElement));
}

//recursion partner fucntion of retrieve
int BSTree::retrieveSub ( BSTreeNode *ptr, String key, Car *&element ) 
{	
	if ( ptr == 0 )
		return 0;
	else if ( key < ptr->element.key() )
		return retrieveSub (ptr->left, key, element);
	else if ( key > ptr->element.key() )
		return retrieveSub (ptr->right, key, element);
	else
	{
                *element = ptr->element;
		return 1;
	}
}

Thanks for ur advice

Thanks again vart, I made a silly mistake juz now. You are correct, I want the second way but did it in the wrong way. Now it works brilliantly