类方法中的内存访问错误

问题如下：我在类中使用列表（禁止使用 STL）我至少编写了方法。此外，当我在 main 中描述实例并尝试调用 addElement 方法时，会发生内存访问错误。怎样成为？为什么会这样？

Main.cpp 列表

#include "stdafx.h"
#include "vset.h"
#include <iostream>
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
    vset a;
    a.AddElement(&a.ilast, 5);
    //  a.AddElement(&a.last, 7);
    a.PrintList(a.ihead);
    return 0;
}

列出vset.cpp类的实现

    #pragma once
    #include "vset.h"
    #include <iostream>
    using namespace std;

    vset::SingleList * vset::makeFirst(int d)
    {
        SingleList *cRec = new SingleList;
        cRec->Data = d; cRec->next = 0; cRec->prev = 0;
        return cRec;
    }

    void vset::AddElement(vset::SingleList **last, int d)
    {
        vset::SingleList *cRec = new SingleList;
        cRec->Data = d; cRec->next = 0; cRec->prev = *last;
        (*last)->next = cRec;
        *last = cRec;
    }

    vset::SingleList * vset::search(SingleList * const pbeg, int d)
    {
        SingleList *cRec = pbeg;
        while (cRec)
        {
            if (cRec->Data == d)break;
            cRec = cRec->next;
        }
        return cRec;
    }

    bool vset::remove(vset::SingleList **head, vset::SingleList **last, int key)
    {
        if (SingleList *pkey = search(*head, key))
        {
            if (pkey == *head)
            {
                *head = (*head)->next;
                (*head)->prev = 0;
            }
            else if (pkey == *last)
            {
                *last = (*last)->prev;
                (*last)->next = 0;
            }
            else {
                (pkey->prev)->next = pkey->next;
                (pkey->next)->prev = pkey->prev;
            }
            delete pkey;
            return true;
        }
        return false;
    }

    vset::SingleList * vset::insert(vset::SingleList * const head, vset::SingleList **last, int key, int d)
    {
        if (SingleList *pkey = search(head, key))
        {
            SingleList *cRec = new SingleList;
            cRec->Data = d; cRec->next = pkey->next;
            cRec->prev = pkey; pkey->next = cRec;
            if (pkey != *last) (cRec->next)->prev = cRec;
            else *last = cRec;
            return cRec;
        }
        return 0;
    }

    void vset::PrintList(vset::SingleList * const head)
    {
        SingleList *cRec = head;
        while (cRec)
        {
            cout << cRec->Data << " ";

            cRec = cRec->next;
        }
    }

    void getData()
    {

    }

    vset::vset()
    {
        SingleList *ihead = makeFirst(0);
        SingleList *ilast = ihead;
    //  getLast(&last);
    }

    vset::vset(int d)
    {
        SingleList *ihead = makeFirst(d);
        SingleList *ilast = ihead;
    }
    /*vset::SingleList getLast(vset::SingleList **last)
    {
        static vset::SingleList **olast;
        if (!last) olast = last;
        return **olast;
    }*/
    vset::~vset()
    {
    }

vset.h 接口

    #pragma once

    class vset
    {
    public:
        struct SingleList
        {
            int Data;

            SingleList *next;
            SingleList *prev;
        };
        SingleList *ihead;
        SingleList *ilast;
    public:
        vset();
        vset::vset(int d);
        ~vset();
        SingleList * makeFirst(int d);
        void AddElement(SingleList **last, int d);
        SingleList * search(SingleList * const pbeg, int d);
        bool remove(SingleList **head, SingleList **last, int key);
        SingleList * insert(SingleList * const head, SingleList **last, int key, int d);
        void PrintList(SingleList * const head);
        void getData();
        vset::SingleList getLast(vset::SingleList);
    };