无法将一个指针分配给另一个指针

Question

I need some help with the troubleshooting the error i'm getting.

Error- C2440- 'initializing':cannot convert from 'DATAHash<ItemType>' to 'DATAHash<ItemType> *'

I'm using Visual Studio.

template<typename ItemType>
class Hash
{
private:
    int tablesize;
    /// Creating a Hashtable of pointer of the (class of data type)
    DATAHash<ItemType>*        Hashtable;

and for Hash class, my default constructor is

template<typename ItemType>
Hash<ItemType> ::Hash(int size)
{
    tablesize = size;
    Hashtable[size]; // make array of item type

    for (int i = 0; i< size; i++)
        Hashtable[i] = NULL;

    loadfactor = 0;
}

This is where my error is

/// This is add function
/// It adds the data that is passed as a parameter
/// into the Hash table
template<typename ItemType>
void Hash<ItemType>::additem(ItemType data)
{
    DATAHash<ItemType> * newdata = new DATAHash<ItemType>(data);

    /// gets the data as a parameter and calls Hash function to create an address
    int index = Hashfunction(data->getCrn());

    /// Checking if there if there is already a data in that index or no.
    DATAHash<ItemType> * tempptr = Hashtable[index];  <------- Error line

    // there is something at that index
    // update the pointer on the item that is at that index
    if (tempptr != nullptr)
    {

        // walk to the end of the list and put insert it there

        DATAHash<ItemType> * current = tempptr;
        while (current->next != nullptr)
            current = current->next;

        current->next = newdata;

        cout << "collision index: " << index << "\n";

        return;
    }

This is my first time posting a question so, if there's something else i need to post, Let me know.

Thanks for the help.

-Rez

Answer 1

You need to get the pointer like this:

DATAHash<ItemType> * tempptr = &Hashtable[index];

However I am not really sure this is what you should be doing. You are invoking the [] operator on that pointer but you never allocate any memory for it.

How the Hashtable member is initialized?

Answer 2

So when you take the index of something as you are Hashtable[index] , you get an actual value back. DATAHash<ItemType>* is of type pointer, so it holds an address.

I would guess the solution you would be looking for is taking the address of Hashtable[index] , so you would need to fix that line with:

DATAHash<ItemType> * tempptr = &Hashtable[index];