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POTrying to understand pointers and double pointers
 

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  1. POTrying to understand pointers and double pointers
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    <p>I'm trying to implement a simple linked list in C++. I can create nodes and they seem to link themselves correctly. My question involves the listIterate() function, but I've attached the entire code here in case it is needed. </p> <pre><code>#include &lt;iostream&gt; using namespace std; //Wrapper class which allows for easy list management class LinkedList { //Basic node struct struct node { int data; node *link; }; node *head; //Pointer to the head (also referred to as root) node, or the first node created. node *current; //Pointer to the /latest/ node, or the node currently being operated on. node *tail; //Pointer to the tail node, or the last node in the list. public: //Default constructor. Creates an empty list. LinkedList() { head = NULL; current = NULL; tail = NULL; cout &lt;&lt; "*** Linked list created. Head is NULL. ***\n"; } //Default destructor. Use to remove the entire list from memory. ~LinkedList() { while(head != NULL) { node *n = head-&gt;link; delete head; head = n; } } /* appendNode() Appends a new node to the end of the linked list. Set the end flag to true to set the last node to null, ending the list. */ void appendNode(int i) { //If there are no nodes in the list, create a new node and point head to this new node. if (head == NULL) { node *n = new node; n-&gt;data = i; n-&gt;link = NULL; head = n; //head node initialized, and since it is ALSO the current and tail node (at this point), we must update our pointers current = n; tail = n; cout &lt;&lt; "New node with data (" &lt;&lt; i &lt;&lt; ") created. \n---\n"; } else { //If there are nodes in the list, create a new node with inputted value. node *n = new node; n-&gt;data = i; cout &lt;&lt; "New node with data (" &lt;&lt; i &lt;&lt; ") created. \n"; //Now, link the previous node to this node. current-&gt;link = n; cout &lt;&lt; "Node with value (" &lt;&lt; current-&gt;data &lt;&lt; ") linked to this node with value (" &lt;&lt; i &lt;&lt; "). \n---\n"; //Finally, set our "current" pointer to this newly created node. current = n; } } /* listIterate() Iterates through the entire list and prints every element. */ void listIterate() { //cursor node *p; //Start by printing the head of the list. cout &lt;&lt; "Head - Value: (" &lt;&lt; head-&gt;data &lt;&lt; ") | Linked to: (" &lt;&lt; head-&gt;link &lt;&lt; ") \n"; p = head-&gt;link; cout &lt;&lt; *p; } }; int main() { LinkedList List; List.appendNode(0); List.appendNode(10); List.appendNode(20); List.appendNode(30); List.listIterate(); </code></pre> <p>}</p> <p><b>Now, </b> I'll refer to this method, listIterate().</p> <pre><code>void listIterate() { //cursor node *p; //Start by printing the head of the list. cout &lt;&lt; "Head - Value: (" &lt;&lt; head-&gt;data &lt;&lt; ") | Linked to: (" &lt;&lt; head-&gt;link &lt;&lt; ") \n"; p = head-&gt;link; cout &lt;&lt; *p; } </code></pre> <p>The command <code>cout &lt;&lt; *p;</code> throws an error and I believe this is why: At this point, p is pointing to <code>head-&gt;link</code>, which is another pointer which points to the link field of my head node. Now, I understand if I dereference p at this point in the program, there would be no actual value in <code>head-&gt;link</code> as it points to a variable. </p> <p>To me, if I dereference p twice (<code>**p</code>), it should follow the pointer twice (<code>p</code> -> <code>head-&gt;link</code> -> The value of the second node in the linked list (<code>10</code>). However, dereferencing p twice throws this error.</p> <pre>LinkedListADT.cc:89: error: no match for ‘operator*’ in ‘** p’</pre> <p>Can anyone help me understand why this is the case? Is this an illegal operation? Is it performed in another way that I'm not familiar with?</p>
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    1. COWhat's the first error? The question is incomplete without it. Note that prefixing lines with four spaces marks them as code, which isn't appropriate markup for error messages. Until the SE platform [supports `<samp>`](http://meta.stackexchange.com/q/42692/), the most appropriate way of marking output is with `<blockquote><pre>`.
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    2. COThis has been fixed. Thanks for the heads up. The first error is the result of trying to deref a pointer, which is invalid. The original wording of the question gave the impression that I didn't know why this was happening, which is incorrect. More interested in the second point.
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