Lists, the first step. Create the 'head node'?

This is a discussion on Lists, the first step. Create the 'head node'? within the C++ Programming forums, part of the General Programming Boards category; Hi, I am trying to store pointers to some inherited classes within a linked list, but am stuck with the ...

  1. #1
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    Lists, the first step. Create the 'head node'?

    Hi,

    I am trying to store pointers to some inherited classes within a linked list, but am stuck with the first part.

    I understand that each node contains two pointers to the next and previous, but how should I create the first 'head node' that points to the first object that will be added?

    Here is my code:

    header file :

    Code:
    class Vehicle
    {
    friend class listclass;
    
    public:	
    	Vehicle(int publicmpg = 0); 
    
    protected:
    	Vehicle *llink, *rlink;
    
    	int protectedmpg;  
    };
    
    Vehicle::Vehicle(int publicmpg) 
    {
    	protectedmpg = publicmpg; 	
    }
    
    //-------------------------
    //
    //-------------------------
    
    class Listclass
    {
    public:
    	
    	//addvehicle()
    	Listclass(int publicfirst = 0);
    
    protected:
    
    	int protectedfirst; 
    };
    
    Listclass::Listclass(int publicfirst)
    {
    	protectedfirst = publicfirst;
    }
    
    //-------------------------
    //
    //-------------------------
    
    class SportsCar : public Vehicle
    {
    public:
    	SportsCar(int publicsixspeed = 0, int publicmpg = 0);
    
    protected:
    	int protectedsixspeed;
    };
    
    SportsCar::SportsCar(int publicsixspeed, int publicmpg) : Vehicle(publicmpg) 
    {
    	protectedsixspeed = publicsixspeed;
    }


    main.cpp :
    Code:
    #include <iostream>
    #include <list>
    #include "Vehicle.h"
    
    using namespace std;
    
    int main()
    {
    	list<Vehicle*> listname;
    
    	Listclass headnode;
    
    	//     Should I create the 'head node' here before
    	//     going on to creating actual objects?
    
    	int publicmpg = 11;
    	int publicsixspeed = 22;
    
    	SportsCar node(publicmpg, publicsixspeed);
    
    	SportsCar *ptrToSportsCar;
    
    	ptrToSportsCar = &node;
    
    	listname.push_back( ptrToSportsCar );
    
    	cout << "past push" << endl;
    
    	system ("PAUSE");
    	return 0;
    How/should I create the 'head node' first, and then add the first items to the list?

    I've read up and understand the 'head node' will only contain a pointer to the first node in the list, but how do I create it?.

    I thought it might be easiest to create a single link list first, then try modify it into a doubly linked list.


    Once I have the list established I think I'll be fine adding new items to the list, but I'm stuck 'getting it all setup' if you like.

    Appreciate anyone's advice.

    Thanks!
    Last edited by Swerve; 05-20-2009 at 07:50 PM.

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Usually the variable called "head" is not a node itself, just a pointer to whatever happens to be the first in line. If the list is empty it points to NULL. That becomes a special case in your insert function.

  3. #3
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    Thanks.

    By 'insert' you mean use push_back to add a null pointer to the list, yes?

    should this be hard-coded into the program where I wrote the //comments in main.cpp?

    I know it's simple, but I just don't seem to get it right.

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    You would never add a null pointer to the list under any circumstances. When you construct the list you should set a pointer to NULL, and this pointer will be NULL when and only when the list is empty. At any other time it will point to the front of the list.

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