Add pointer of object to linked list c++

Question

I'm looking for a help to understand linked list. I have such a task: aclass DNAList:

1. This class is a linked list of nodes that have pointers to (not copies of) DNA objects and at a minimum should contain:
   • Appropriate constructor(s) and destructor
   • Data members to store: Head pointer to list
2. A DNANode struct or class that holds a pointer to a DNA object and a "next" pointer to a DNANode object (and a "prev" pointer if you're using a doubly-linked list).
3. A push_back(DNA* newDNA) method that adds a node to the end of the list
4. A find(int id) method that returns a DNA* if a DNA object with id exists in the list; otherwise it returns NULL
5. An obliterate(int id) method that deletes the DNA entry with an accession number id and removes the corresponding node
6. An int size() method that returns the number of elements in the list

First of all, I try to do push_back(DNA* newDNA) method. Any help, please?

Thank you.

DNAList.h

#ifndef DNALIST_H
#define DNALIST_H
#include <iostream>
#include <string>
#include "DNA.h"


class DNAList{
    //data members
    private:
        DNA* headPtr;
    public:
        DNAList();
        ~DNAList();
        struct DNANode;
        void push_back(DNA* newDNA);
        DNA* find(int id);
        void obliterate(int id);
        int size();
        DNA* getHeadPtr(){
            return headPtr;
        }

       void setHeadPtr(DNA* head){
            headPtr= head;
       }

};

#endif

DNAList.cpp

#include <iostream>
#include <string>
#include "DNAList.h"

//constrictor
    DNAList::DNAList(){}
//destructor
    DNAList::~DNAList(){
        delete headPtr;
        headPtr = NULL;
    }
//struct that holds pointer to a DNA object  and a "next" pointer to a
DNANode object
    struct DNANode{
        DNA* dnaPtr;
        DNANode* next;
    };
//
    void push_back(DNA* newDNA){

        //dnaPtr = new DNANode;

    }

    DNA* find(int id){

    }
    void obliterate(int id){

    }
    int size(){
        return 0;
    }

Answer 1

The simplest way of having a linked list of DNA* , would be to use the standard <list> container and use list<DNA*> . And to avoid memory leaks, even list<shared_ptr<DNA>> .

But your task seems to be an exercise to learn about linked lists. So here some tips for your own push_back() :

void push_back(DNA* newDNA)
{
    DNANode *element = new DNANode;// create a new node
    element->dnaPtr = newDNA;      // set it up  
    element->next = nullptr;       // it has no next element now

    if (headPtr==nullptr)          // hoping you have initalized the head at construction
         headPtr = element;        // either it's first element of empty list
    else {                         // or you need to find the last node
        DNANode *last = headPtr;   // starting at head
        while (last->next)         // and going from node to node
            last = last->next;  
        last->next = element;      // here you are
    }
}

You can then inspire yourself from this to write your find() and size() (if you don't maintain the size in a data element), and even obliterate() .