c++ - Incomplete Type

Question

I'm getting a incomplete type error for the 'next' and 'previous' variables. I'm not sure what I am doing wrong because I am very rusty on writing classes in C++. Any help would be appreciated! Thanks.

#include<iostream>

using namespace std;

class LinearNode
{
    public:
        //Constructor for the LinearNode class that takes no arguments 
        LinearNode();
        //Constructor for the LinearNode class that takes the element as an argument
        LinearNode(int el);
        //returns the next node in the set.
        LinearNode getNext();
        //returns the previous node in the set
        LinearNode getPrevious();
        //sets the next element in the set
        void setNext(LinearNode node);
        //sets the previous element in the set
        void setPrevious(LinearNode node);
        //sets the element of the node
        void setElement(int el);
        //gets the element of the node
        int getElement();

    private: 
        LinearNode next;
        LinearNode previous;
        int element;        
};//ends the LinearNode class

Implementation file:

#include<iostream>
#include"LinearNode.h"

using namespace std;

//Constructor for LinearNode, sets next and element to initialized states
LinearNode::LinearNode()
{
    next = NULL;
    element = 0;
}//ends LinearNode default constructor

//Constructor for LinearNode takes an element as argument.
LinearNode::LinearNode(int el)
{
    next = NULL;
    previous = NULL;
    element = 0;
}//ends LinearNode constructor

//returns the next element in the structure
LinearNode::getNext()
{
    return next;
}//ends getNext function

//returns previous element in structure
LinearNode::getPrevious()
{
    return previous;
}//ends getPrevious function

//sets the next variable for the node
LinearNode::setNext(LinearNode node)
{
    next = node
}//ends the setNext function

//sets previous for the node
LinearNode::setPrevious(LinearNode node)
{
    previous = node;
}//ends the setPrevious function

//returns element of the node
LinearNode::getElement()
{
    return element;
}//ends the getelement function

//sets the element of the node
LinearNode::setElement(int el)
{
    element = el;
}//ends the setElement function

test file:

    #include<iostream>
#include"LinearNode.h"

using namespace std;

int main()
{
    LinearNode node1, node2, move;
    node1.setElement(1);
    node2.setElement(2);

    node2.setNext(node1);
    node1.setPrevious(node2);

    move = node2;

    while(move.getNext() != NULL)
        cout << move.getElement() << endl;

}

Answer 1

Your type has a recursive definition, which is prohibited.

class LinearNode
{
private: 
    LinearNode next;
    LinearNode previous;
};

The data members next and previous are instances (not references or pointers) of the LinearNode class, which is not completely defined yet.

You probably want this:

class LinearNode
{
private: 
    LinearNode * next;
    LinearNode * previous;
};