Writing a container class

[SCB3]

I Keep getting this error:

Unhandled exception thrown: write access violation.
this->**data** was 0x1110112. occurred

but unsure why,

Header File:

#pragma once
#include <string>
#include <cassert> //Needed for assert, see report for more details on why I'm using this

class myMap
{
private:
	int myLength; //Number of Array Member/Length of array
	int *data; //Pointer to it's posistion in the array

public:
	//Constructors 
	myMap() : myLength(0), data(nullptr)//Empty Array
	{
	}

	myMap(int myLength) : myLength(myLength)
	{
		data = new int[myLength];
	}

	//Decontructor

	~myMap()
	{
		delete[] data;
		//Don't need to set to Null or 0 here as the object is destroyed after this function
	}


	//Delete Function
	void erase()
	{
		delete[] data;
		data = nullptr;//set to Null or otherwise will be pointing at deallocated memory
		myLength = 0;
	}

	int& operator[](int index) //Overloading the [] Operator to access elements of the array, checking the bounds of the index to ensure it's a valid array using the assert function and returning the length of the array
	{
		assert(index >= 0 && index < myLength);
		return data[index];
	}
	
//Resizes the aray destroying all elements in the process
void reallocate(int newLength)
{
	erase();//delete any exsisting elements

	//If empty, return here
	if (newLength <= 0)
		return;

	//Allocate new elements for the array
	data = new int[newLength];
	myLength = newLength;
}

//Resize the array, keeping the exsisting elements (Useful for modifying or editing arrays)
void resize(int newLength)
{
	erase();
	//if Array is already the right length we needed
	if (newLength == myLength)
		return;

	//Resiziing to an empty array
	if (newLength <= 0)
	{
		erase();
		return;
	}

	//Allocate a new array, copy the elements one by one to the new array, destroy the old array and make data point to the new array
	//New Array
	int *myData = new int[newLength];
	//Copy the elements to the new array
	if (myLength > 0)
	{
		int copying = (newLength > myLength) ? myLength : newLength;
		for (int index = 0; index < copying; index++)
		{
			myData[index] = data[index];
		}

		//destroy the old array, clearing memory
		delete[] myData;

		//Point at the new array just created
		data = myData;
		myLength = newLength;

	}
}

//Insert

void insert(int value, int index)
{
	//Create an array larger than the old array
	int *myData = new int[myLength + 1];

	//copy all to the new array
	for (int i = 0; i < index; i++)
	{
		myData[i] = data[i];
	}

	//insert new value into the array
	myData[index] = value;
	
	delete[] data;
	data = myData;
	myLength++;
}

//Delete Function
void remove(int index)
{
	//If this is the last elemnt in the array, set to empty and return, preventing a memory leak and/or crash
	if (myLength == 1)
	{
		erase();
		return;
	}

	//Create Array smaller than the original
	int *myData = new int[myLength - 1];

	//Copy all from the old array
	for (int i = 0; i < index; i++)
	{
		myData[i] = data[i];
	}

	//Copy all values afer the removed one

	for (int j = index + 1; j < myLength; j++)
	{
		myData[j + 1] = data[j];
	}

	delete[] data;
	data = myData;
	myLength--;
}


int getLength()
{
	return myLength;
}
};

It's on this line:

//copy all to the new array
	for (int i = 0; i < index; i++)
	{
		myData[i] = data[i];
	}

But as far as I can see, it should be working fine

[coder777]

The problem is on line 87/90. You delete myData and then set the now invalid pointer to the member data which becomes invalid as well. So line 87 should be delete[] data;

[SCB3]

Thanks, I changed it now, but still getting the same error

[ne555]

¿main()?

you are missing a proper copy constructor and you access out of bounds in .remove()

		int *myData = new int[myLength - 1];
		//...
		// Copy all values afer the removed one

		for(int j = index + 1; j < myLength; j++) {
			myData[j + 1] = data[j];
		}

suppose that myLength=5, then myData may hold 5-1=4 elements
in the last iteration of the loop, j reaches myLength-1, that is 4
so you are accessing myData[5], but the last valid index is 3

[SCB3]

This is Main

#include "stdafx.h"
#include "Map.h"
#include <iostream>
using namespace std;


int main()
{
	//testing - array with 10 elements
	myMap array(10);

	for (int i = 0; i < 10; i++)
	{
		array[i] = i + 1;
	}
	//Starting Array
	cout << "Starting Array: " << endl;
	for (int j = 0; j < array.getLength(); j++)
	{
		cout << array[j] << " " << endl;
	}
	cout << "Resizing the Array: " << endl;
	//Resize the array
	array.resize(8);
	for (int j = 0; j < array.getLength(); j++)
	{
		cout << array[j] << " " << endl;
	}

	//Insert into array
	cout << "Inserting into the Array: " << endl;
	array.insert(20, 5);

	for (int j = 0; j < array.getLength(); j++)
	{
		cout << array[j] << " " << endl;
	}

	//Delete from the Array
	cout << "Deleting from the array: " << endl;
	array.remove(4);
	for (int j = 0; j < array.getLength(); j++)
	{
		cout << array[j] << " " << endl;
	}

    return 0;
}

Where am I going out od bounds btw, I cant see where I fix that part

Nevermind, fixed that part, was obvious when I looked back at it

[Chervil]

Line 62

     erase();

does not seem necessary - it gets rid of all the data before it can be copied.

[SCB3]

Ah yes, I completely forgot to delete that line, that's worked somewhat, thanks :)

[ne555]

> Where am I going out od bounds btw, I cant see where I fix that part
in remove() function, line 140.


another problem in .resize()

/* my comments are written in this style */
// Resize the array, keeping the exsisting elements (Useful for modifying or
	// editing arrays)
	void resize(int newLength) {
		erase(); /* you won't keep the existing elements */

		//...

		// Allocate a new array, copy the elements one by one to the new array,
		// destroy the old array and make data point to the new array  New Array
		int *myData = new int[newLength];
		// Copy the elements to the new array
		if(myLength > 0) { /* because you called .erase(), at this point myLenght is 0 */
			int copying = (newLength > myLength) ? myLength : newLength;
			for(int index = 0; index < copying; index++) {
				myData[index] = data[index]; /* because you called .erase(), here data is nullptr */
			}

			// destroy the old array, clearing memory
			delete[] myData; /* another time the same error, you invalidate `myData' */

			// Point at the new array just created
			data = myData; /* and then make `data' invalid */
			myLength = newLength;
		}
	}

dont edit the OP, make a new post with your updated code.