Disappearing of object inside variable of another object - c++

Question

I got strange program behaviour and crashing.
I created class 'List', with array of pointers to objects of class 'Student'. I observe that I succesfully called 'Student' object in 'List' constructor. But I'm unable to call 'Student' object from any other 'List' methods.
I even checked the same line for testing inside 'List' constructor and 'push' method, resulting in program crash.
Here is testing line: cout << (studentBox[numb] -> getRef()) << endl;
Here is the problematic part of code:
#include <iostream>
using namespace std;
class Student
{
private:
int referenceNumb;
int markNumb;
public:
Student();
Student(int, int);
~Student();
int getRef();
int getMark();
};
class List
{
private:
int numb;
Student* studentBox[1000];
public:
List();
~List();
void push(int, int);
int getAVG();
};
int main()
{
List* base = NULL;
int x, y;
char mod;
do
{
cout << "Waiting for orders." << endl;
cout << "1 - Quit," << endl;
cout << "2 - Add new student," << endl;
cout << "3 - Calculate average mark," << endl;
cout << "0 - Create new list." << endl;
cin >> mod;
switch(mod)
{
case '0': base = new List(); break;
case '1': cout << "Bye."; break;
case '2':
if(base != NULL)
{
cout << "Specify student's reference number: "; cin >> x;
cout << "Specify student's mark: "; cin >> y;
base->push(x, y);
}
else cout << "List does not exist!" << endl;
break;
case '3':
if(base != NULL)
{
cout << "Average mark is equal: " << base->getAVG();
}
else cout << "List does not exist!";
cout << endl;
break;
default: cout << "Correct command required!" << endl; break;
}
}
while(mod!='1');
return 0;
}
Student::Student()
{
referenceNumb = NULL;
markNumb = NULL;
}
Student::Student(int r, int m)
{
referenceNumb = r;
markNumb = m;
}
Student::~Student()
{
referenceNumb = NULL;
markNumb = NULL;
cout << "pusto." << endl;
}
int Student::getRef()
{
return referenceNumb;
}
int Student::getMark()
{
return markNumb;
}
List::List()
{
int numb = 0;
studentBox[numb] = new Student();
cout << (studentBox[numb] -> getRef()) << endl;
}
List::~List()
{
}
void List::push(int x, int y)
{
cout << (studentBox[numb] -> getRef()) << endl;
if(studentBox[numb] != NULL)
{
studentBox[numb] = new Student();
cout << (studentBox[numb] -> getRef()) << endl;
}
else cout << "Hujnia" << endl;
}
int List::getAVG()
{
int temp = 0;
for(int i=0; i<numb; i++)
{
temp += studentBox[i]->getMark();
}
return (temp / numb);
}

Answer 1

There are a couple of major problems that I see. The first is that the numb member of the List isn't initialized.
List::List()
{
int numb = 0; // this creates a new local variable called numb, it doesn't
// initialize the numb member.
studentBox[numb] = new Student();
cout << (studentBox[numb] -> getRef()) << endl;
}
Do this instead:
List::List()
: numb(0) // initialize the numb member to zero.
{
studentBox[numb] = new Student();
cout << (studentBox[numb] -> getRef()) << endl;
}
Another problem is that numb appears to be meant to indicate the size of the list, but it is never changed when a new student is added.