Programming Forums > Application Development > C++

kruptof · Dec 27th, 2006, 9:57 AM

When testing if something is NULL does it mean to see if something exists in that memory location, if that is (if not then please do explain) so then why is that am getting an error when i try to see if a struct*[] != NULL. here is the error that i have received.

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error C2676: binary '!=' : 'HashTable' does not define this operator or a conversion to a type acceptable to the predefined operator

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struct HashTable
{
	int age;
	static HashTable* table[5];
	HashTable* next;
	int createHash(char*, int);
	void insert(int,int);
};
HashTable* HashTable::table[5];

void HashTable::insert(int position, int data)
{
	if((*table)[position] != NULL)//error is on this line
	{
		HashTable* current = new HashTable;
	}

}

DaWei · Dec 27th, 2006, 10:28 AM

You're getting the error because you haven't defined a != operator for a HashTable (which is what your message says, right?). table [position] is a pointer, but if you dereference it, you're referring to a HashTable. You should test the pointer for NULL before you dereference it.

As for NULL, it's implementation defined. There's a good chance it's zero. Every location in memory has something in it. It's a piece of hardware that has bit cells that can only contain the states, 0 and 1 (except for a short period of time when it's transitioning between the two). There is no "nothing"; there's just an agreement that some "something" will represent "nothing".

kruptof · Dec 27th, 2006, 10:46 AM

Thanks a bunch Dawei, it worked.

As for the NULL: so instead of checking if its = NULL i can check if it is equal to 0, if so whats the difference between testing if its 0 or NULL;

DaWei · Dec 27th, 2006, 10:49 AM

Your compiler is required to treat zero as if it's NULL, in a pointer context.

Dec 27th, 2006, 10:28 AM	#2
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	You're getting the error because you haven't defined a != operator for a HashTable (which is what your message says, right?). table [position] is a pointer, but if you dereference it, you're referring to a HashTable. You should test the pointer for NULL before you dereference it. As for NULL, it's implementation defined. There's a good chance it's zero. Every location in memory has something in it. It's a piece of hardware that has bit cells that can only contain the states, 0 and 1 (except for a short period of time when it's transitioning between the two). There is no "nothing"; there's just an agreement that some "something" will represent "nothing". __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Dec 27th, 2006, 10:49 AM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Your compiler is required to treat zero as if it's NULL, in a pointer context. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Programming Forums > Application Development > C++

checking str*[] against NULL