Can't initialize an array of pointers in C++

Question

I have an array of pointers to a structure called "table" (the structure is called Node).

I declare the array as so in the class:

Node * table;

Then, in another method, I initalize the table:

this->table = new Node [this->length];

And everything works fine. this->length is a valid entry, this->table is pointing to the right array, and etc. However, then I try to change the value of the elements:

for(int i = 0; i < this->length; i++) {
    this->table[i] = new Node;
}

Or even

for(int i = 0; i < this->length; i++) {
    this->table[i] = 0;
}

And everything starts bugging out. Why can't I set these pointers to anything?

This is the error I get: enter image description here

(Where line 15 is the line of "this->table[i] = new Node;").

I hate to post long segments of code, so here's a shortened version of the code itself:

template <class T, class S>
class HashMap {
    public:
        HashMap();
    private:
        struct Node;
        Node * table;
        static const unsigned length = 100;
};

template <class T, class S>
HashMap<T, S>::HashMap() {
    this->table = new Node [HashMap::length];
    for(int i = 0; i < HashMap::length; i++) {
        this->table[i] = new Node;
    }
}

template <class T, class S>
struct HashMap<T, S>::Node {
    T value;
    S key;
    Node * next;
};

No research I'm doing is telling me what the error is; any help is appreciated!

Benjamin Lindley · Accepted Answer · 2013-08-14 04:42:44Z

4

You don't have an array of pointers. You have an array of Nodes. Apparently, what you want is something like this:

Node ** table;
...
this->table = new Node*[this->length];

Or maybe you don't actually need an array of pointers, but simply an array of nodes. In that case, no extra initialization is needed beyond:

this->table = new Node[this->length];

Beyond that, unless this is a learning exercise, take a look at the standard library, which has dynamic arrays and hash maps all ready for you.

edited Aug 14, 2013 at 4:42

answered Aug 14, 2013 at 4:37

Benjamin Lindley

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Comments

Igor Tandetnik · Accepted Answer · 2013-08-14 04:38:52Z

1

table is not an array of pointers. It's an array of Nodes (or rather, it points to an array of Nodes). The type of table is Node*; the type of table[i] is Node, not Node*.

If you actually do want an array of pointers, then you need

Node** table;
table = new Node*[length];

Or better still, something like

vector<unique_ptr<Node>> table;
table.resize(length);

answered Aug 14, 2013 at 4:38

Igor Tandetnik

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Comments

Ericzhang88120 · Accepted Answer · 2013-08-14 05:01:09Z

0

You do not have declared an array of pointers.
Node *table(point to a node)
Node **table(point to an array Nodes)

Node** table;
table =new Node*[this->length];
for(int i=0;i<this->length;i++)
{
   table[i]=new Node;
}

answered Aug 14, 2013 at 5:01

Ericzhang88120

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Comments

Krishan Subudhi · Accepted Answer · 2013-08-14 09:08:26Z

this->table = new Node [HashMap::length];

this->table is of type Node* and new Node [HashMap::length] also returns a Node* , i.e. an array of Node of lenght HashMap::length is created and the array address is stored in this->table pointer.

this->table[i] = new Node;

As an example, we can define:

int* arr = new int[10];

Here arr is of type int* but arr[0] will be of type int.

similarly, this->table[i] is of type Node and new Node returns Node*. Hence incompatible types. Correct line would be

this->table[i] = *new Node;

But, this line is unnecessary as the array of Nodes is already created and the memory is allocated. Using this line in the code will lead to a memory leak.

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