Question

I get a code for Fibonacci Heap. This code uses the follow function to compare two keys:

int
cmp(void *x, void *y)
{
    int a, b;
    a = (int)x;
    b = (int)y;

    if (a < b)
        return -1;
if (a == b)
        return 0;
    return 1;
}

This works because the KEY is currently a INT number.

I would like to adapt this code to use my class called “Node”. This class have implemented the operator <. >, <=, == and >=.

My adaptation in the code was:

Node a, b   // instead int a, b
a = (Node)x;
b = (Node)y;

But I get the error:

dijkstra.cpp:168: error: no matching function for call to 'Node::Node(void*&)'
graph.h:39: note: candidates are: Node::Node()
graph.h:39: note:                 Node::Node(const Node&)

I tried also:

Node a, b   // instead int a, b
a = (Node*)x;
b = (Node*)y;

and get the error:

dijkstra.cpp:168: error: no match for 'operator=' in 'a = (Node*)x'
graph.h:39: note: candidates are: Node& Node::operator=(const Node&)

I give up to try set the value and solve the question as a follow:

int cmp(void *x, void *y)
{ 
    if ((Node*)x < (Node*)y)
        return -1;
    if ((Node*)x == (Node*)x)
        return 0;
    return 1;
}

I would like to understand what I am doing wrong in the previous examples.

Thanks in advance.

Answer 1

Change the following lines

a = (Node*)x;
b = (Node*)y;

to

a = *( (Node*) x); 
// type cast from void* to Node* and then fetch the contents by dereferencing the pointer.

b = *( (Node*) y);

You were merely type casting the pointers from void* to Node* and trying to assign the pointer to a variable of type Node.