Bug hunt!

[ETAkoG1T]

I made a program that can use a function to manipulate data from a list class. The program is very basic and I think the error makers may be obvious to some of you. I just wanted to find it without having to scrap my program. It doesn't generate a compile error but it announces a memory error while it is running. I use visual studio 2012, the program is an exercise from a c++ book.
list.h

#ifndef LIST_H_
#define LIST_H_
typedef double Item;
const int MAX = 10;
class List
{
private:
	Item arr[MAX];
	int elements;
public:
	List(Item ar[] = 0, int max = MAX);
	void AddToList(Item a);
	bool isempty();
	bool isfull();
	void visit(void (*pf) (Item &));
	void show();

};

#endif; 

main.cpp

// Ch10_8.cpp : Defines the entry point for the console application.
//
#include <iostream>
#include "stdafx.h"
#include "list.h"
void divide(Item &x);
int main()
{
	using std::cout;
	using std::cin;
	using std::endl;
	Item ages[4] = {0, 4, 8, 2};
	List age(ages, 4);
	cout << "The string is empty "<< age.isempty() << endl;
	cout << "The string is full " << age.isfull() << endl;
	cout << "The list, ";
	age.show(); 
	cout << endl;
	age.AddToList(23.5);
	age.visit(divide);
	List age2;
	cout << "The string is empty "<< age2.isempty() << endl;
	cout << "The string is full " << age2.isfull() << endl;
	cout << "The list, ";
	age2.show(); 
	cout << endl;
	age2 = age;
	cout << "The string is empty "<< age2.isempty() << endl;
	cout << "The string is full " << age2.isfull() << endl;
	cout << "The list, ";
	age2.show(); 
	cout << endl;
	while(!age2.isfull())
	{
		age2.AddToList(2.1);
	}
	cout << "The string is empty "<< age2.isempty() << endl;
	cout << "The string is full " << age2.isfull() << endl;
	cout << "The list, ";
	age2.show(); 
	cout << endl;
	cin.get();
	cin.get();
	return 0;
}

void divide(Item &x)
{
	x = x/2;
}

//function definitions for class
list.cpp

// Ch10_8.cpp : Defines the entry point for the console application.
//
#include <iostream>
#include "stdafx.h"
#include "list.h"
void divide(Item &x);
int main()
{
	using std::cout;
	using std::cin;
	using std::endl;
	Item ages[4] = {0, 4, 8, 2};
	List age(ages, 4);
	cout << "The string is empty "<< age.isempty() << endl;
	cout << "The string is full " << age.isfull() << endl;
	cout << "The list, ";
	age.show(); 
	cout << endl;
	age.AddToList(23.5);
	age.visit(divide);
	List age2;
	cout << "The string is empty "<< age2.isempty() << endl;
	cout << "The string is full " << age2.isfull() << endl;
	cout << "The list, ";
	age2.show(); 
	cout << endl;
	age2 = age;
	cout << "The string is empty "<< age2.isempty() << endl;
	cout << "The string is full " << age2.isfull() << endl;
	cout << "The list, ";
	age2.show(); 
	cout << endl;
	while(!age2.isfull())
	{
		age2.AddToList(2.1);
	}
	cout << "The string is empty "<< age2.isempty() << endl;
	cout << "The string is full " << age2.isfull() << endl;
	cout << "The list, ";
	age2.show(); 
	cout << endl;
	cin.get();
	cin.get();
	return 0;
}

void divide(Item &x)
{
	x = x/2;
}

I know the design of the program is flawed, I don't want to totally change my approach, just find the bug and kill it.

[guestgulkan]

You have posted main.cpp twice.

[ETAkoG1T]

lol, my mistake. That however is not the reason for the error :P
list.cpp

#include "list.h"
#include <iostream>

List::List(Item ar[], int max)
{
	int i = 0;
	while(i < max)
	{
		arr[i] = ar[i];
		i++;
	}
	elements = i;
}

void List::AddToList(Item a)
{
	if(elements < MAX)
		arr[elements] = a;
	++elements;
}

bool List::isempty()
{
	if(elements == 0)
		return true;
	else
		return false;
}

bool List::isfull()
{
	if(!elements < MAX)
		return true;
	else
		return false;
}

void List::visit(void (*pf) (Item &))
{
	for(int i = 0;i<elements;i++)
	{
		pf(arr[i]);
	}
}

void List::show()
{
	for(int i = 0;i<elements;i++)
	{
		std::cout << arr[i] << ' ';
	}
}

[guestgulkan]

You need to rethink your List constructor(s);

Your List constructor looks like this:

List(Item ar[] = 0, int max = MAX); //not good default parameters for the constructor

The implementation looks like this:

List::List(Item ar[], int max)
{
	int i = 0;
	while(i < max)
	{
		arr[i] = ar[i];
		i++;
	}
	elements = i;
}

In main you create a List Like this:
List age2; //Uses default parameters for constructor - which will use a NULL pointer!!!!!


Now you can see why you have the problem.

[ETAkoG1T]

Wow, that wasn't so hard after all :) I overloaded the constructor so that it would work with empty lists!