What does *& mean in C?

Question

I was new to the concepts of trees. I was studying Serialization and deSerialization. I got a example program from a link, copied it, and executed it. It ran, but when I tried to understand it, I could not understand one line - void deSerialize(Node *&root, FILE *fp)

Why is *& mean?

The whole code is:

#include <stdio.h>
#define MARKER -1

/* A binary tree Node has key, pointer to left and right children */
struct Node
{
int key;
struct Node* left, *right;
};

/* Helper function that allocates a new Node with the
given key and NULL left and right pointers. */
Node* newNode(int key)
{
Node* temp = new Node;
temp->key = key;
temp->left = temp->right = NULL;
return (temp);
}

// This function stores a tree in a file pointed by fp
void serialize(Node *root, FILE *fp)
{
// If current node is NULL, store marker
if (root == NULL)
{
    fprintf(fp, "%d ", MARKER);
    return;
}

// Else, store current node and recur for its children
fprintf(fp, "%d ", root->key);
serialize(root->left, fp);
serialize(root->right, fp);
}

// This function constructs a tree from a file pointed by 'fp'
void deSerialize(Node *&root, FILE *fp)
{
// Read next item from file. If theere are no more items or next
// item is marker, then return
int val;
if ( !fscanf(fp, "%d ", &val) || val == MARKER)
   return;

// Else create node with this item and recur for children
root = newNode(val);
deSerialize(root->left, fp);
deSerialize(root->right, fp);
}

// A simple inorder traversal used for testing the constructed tree
void inorder(Node *root)
{
if (root)
{
    inorder(root->left);
    printf("%d ", root->key);
    inorder(root->right);
}
}

/* Driver program to test above functions*/
int main()
{
// Let us construct a tree shown in the above figure
struct Node *root        = newNode(20);
root->left               = newNode(8);
root->right              = newNode(22);
root->left->left         = newNode(4);
root->left->right        = newNode(12);
root->left->right->left  = newNode(10);
root->left->right->right = newNode(14);

// Let us open a file and serialize the tree into the file
FILE *fp = fopen("tree.txt", "w");
if (fp == NULL)
{
    puts("Could not open file");
    return 0;
}
serialize(root, fp);
fclose(fp);

// Let us deserialize the storeed tree into root1
Node *root1 = NULL;
fp = fopen("tree.txt", "r");
deSerialize(root1, fp);

printf("Inorder Traversal of the tree constructed from file:\n");
inorder(root1);

return 0;
}

Any help appreciated.

Answer 1

The *& is not a single symbol. But a combination of two:

* for a pointer. & for reference.

So you have the function:

void deSerialize(Node *&root, FILE *fp)

This function first parameter is a reference to a Node pointer.

Meaning - when using it you send it a Node * object. And the function itself can change this pointer value - since you passed it by reference.

This allows you to allocate memory inside the function.

A different way to write this function would be:

Node *deSerialize(Node *root, FILE *fp)

And use it differently ofcourse:

root->left = deSerialize(root->left, fp)

See full solution here: http://ideone.com/5GzAyd. And relevant part:

Node *deSerialize(Node *root, FILE *fp)
{
    // Read next item from file. If theere are no more items or next
    // item is marker, then return
    int val;
    if ( !fscanf(fp, "%d ", &val) || val == MARKER)
       return NULL;

    // Else create node with this item and recur for children
    root = newNode(val);
    root->left = deSerialize(root->left, fp);
    root->right = deSerialize(root->right, fp);
    return root;
}