_block_type_is_valid(phead- nblockuse) line 52

[Lesterberne]

This is my code. Apparently the error is line 52 and i have no idea how to solve it :( I read a bunch of posts that's it's due to deleting a pointer twice or something but i have no idea how does that relate to my code. any help would be appreciated :)

Classes:

#include <iostream>
#include <string>
using namespace std;

int DefaultListSize = 20;

class Student
{
private:
    int id;
    string major;
public:
    Student(int a = 0, string b = " ")
    {
        id = a;
        major = b;
    }
    int GetId()
    {
        return id;
    }
    string GetMajor()
    {
        return major;
    }
    void SetStudent()
    {
        int a;
        string b;
        cout << "\nPlease enter the id of the student:\n";
        cin >> a;
        cout << "\nPlease enter the major of the student:\n";
        cin >> b;
        id = a;
        major = b;
    }
    void SetStudent(int a, string b)
    {
        id = a;
        major = b;
    }
    void print()
    {
        cout << "\nThis student has the following id: " << id << " and the following major: " << major << ".\n";
    }
};

class Major
{
private:
    string code;
    string description;
public:
    Major(string a = " ", string b = " ")
    {
        code = a;
        description = b;
    }
    string GetCode()
    {
        return code;
    }
    string GetDescription()
    {
        return description;
    }
    void SetMajor()
    {
        string a;
        string b;
        cout << "\nPlease enter the code of the major:\n";
        cin >> a;
        cout << "\nPlease enter the description of the major:\n";
        cin >> b;
        code = a;
        description = b;
    }
    void SetMajor(string a, string b)
    {
        code = a;
        description = b;
    }
    void print()
    {
        cout << "\nThis major has the following code: " << code << " and the following description: " << description << ".\n";
    }
};

template <class Elem>
class List
{
public:
    virtual void clear() = 0;
    virtual bool insert(const Elem&) = 0;
    virtual bool append(const Elem&) = 0;
    virtual bool remove(Elem&) = 0;
    virtual void setStart() = 0;
    virtual void setEnd() = 0;
    virtual void prev() = 0;
    virtual void next() = 0;
    virtual int leftLength() const = 0;
    virtual int rightLength() const = 0;
    virtual bool setPos(int pos) = 0;
    virtual bool getValue(Elem&) const = 0;
    virtual void print() const = 0;
};

template <class Elem> // Array-based list
class AList : public List<Elem>
{
private:
    int maxSize;     // Maximum size of list
    int listSize;    // Actual elem count
    int fence;       // Position of fence
    Elem* listArray; // Array holding list
public:
    AList(int size = DefaultListSize)
    {
        maxSize = size;
        listSize = fence = 0;
        listArray = new Elem[maxSize];
    }
    AList(const AList &obj) //copy constructor
    {
        maxSize = obj.maxSize;
        listSize = fence = 0;
        listArray = new Elem[maxSize];
        *listArray = *obj.listArray;
    }
    ~AList() { delete[] listArray; }
    void clear()
    {
        delete[] listArray;
        listSize = fence = 0;
        listArray = new Elem[maxSize];
    }
    void setStart() { fence = 0; }
    void setEnd() { fence = listSize - 1; }
    void prev()   { if (fence != 0) fence--; }
    void next()   {
        if (fence < listSize)
            fence++;
    }
    int leftLength() const  { return fence + 1; }
    int rightLength() const
    {
        return listSize - fence - 1;
    }
    bool setPos(int pos)
    {
        if ((pos >= 0) && (pos < listSize))
            fence = pos;
        return (pos >= 0) && (pos < listSize);
    }

    bool getValue(Elem& it) const
    {
        if (rightLength() == 0) return false;
        else {
            it = listArray[fence + 1];
            return true;
        }
    }
    bool insert(const Elem& item)
    {
        if (listSize == maxSize) return false;

        for (int i = listSize; i>fence; i--)
            // Shift Elems up to make room
            listArray[i] = listArray[i - 1];

        listArray[fence + 1] = item;
        listSize++; // Increment list size
        return true;
    }
    bool append(const Elem& item)
    {
        if (listSize == maxSize) return false;
        listArray[listSize++] = item;

        return true;
    }
    bool remove(Elem& it)
    {
        if (rightLength() == 0) return false;

        it = listArray[fence]; // Copy Elem

        for (int i = fence; i<listSize - 1; i++)
            // Shift them down
            listArray[i] = listArray[i + 1];

        // Decrement size
        listSize--;
        return true;
    }
    virtual void print() const
    {
        Elem a;
        if (getValue(a)) a.print();
    }
};

Main:

int main()
{
    //AList<Major> ListM;
    AList<AList<Student>> ListA;

    cout << "Hello dear user,\nDuring this program you will be able to do the following at any time:\n";
    cout << "1- Add a new student\n2- Print students in any given major\n3- Print students in all majors\n4- Remove any student from any major\n";

    cout << "\nFirst how about adding your first student?\n";

    AList<Student> A;
    Student a;
    a.SetStudent();
    if (A.insert(a)) cout << "yes\n";
    if (ListA.insert(A)) cout << "yes\n";
    system("pause");
    return 0;
}

[Lesterberne]

I have used debugging and my program gets the error when deleting the listarray of AList<Student> A.
How can i fix this?

[Lesterberne]

I figured what happened and i guess ListA is being deleted by the destructor and then the destructor is trying to delete A which has already been deleted the first time it got called.
How can i prevent this from happening?