Exception Error : Access violation reading location 0xDDDDDDDD

Question

I am trying to create a dynamic string array in c++. When trying to display the contents of my dynamic string array to the console I receive this error:

Exception thrown at 0x0FD670B6 (msvcp140d.dll) in Assignment4.exe: 0xC0000005: Access violation reading location 0xDDDDDDDD.

Here is my code:

DynamicStringArray.h

#pragma once
#include "stdafx.h"
#include <string>
#include <iostream>

using namespace std;

class DynamicStringArray
{
public:
    DynamicStringArray();
    DynamicStringArray(DynamicStringArray &array);
    ~DynamicStringArray();
    int getSize();
    void displayContents();
    void addEntry(const string &addElement);
    string getEntry(int index);
    int deleteEntry(const string &deleteElement);

private: 
    string *dynamicArray;
    int size;
};

DynamicStringArray.cpp

#include "stdafx.h"
#include "DynamicStringArray.h"
#include <string>
#include <iostream>

using namespace std;

DynamicStringArray::DynamicStringArray()
{
    dynamicArray = NULL;
    size = 0;
}

DynamicStringArray::DynamicStringArray(DynamicStringArray &array)
{
    if (dynamicArray != NULL)
    {
        size = 0;
        delete [] dynamicArray;
        dynamicArray = NULL;
    }

    size = array.getSize();
    dynamicArray = new string[size];
    for (int i = 0; i < size; i++)
        dynamicArray[i] = array.dynamicArray[i];
}

DynamicStringArray::~DynamicStringArray()
{
    cout << "In destructor." << endl;
    delete [] dynamicArray;
    dynamicArray = NULL;
}

int DynamicStringArray::getSize()
{
    return size;
}

void DynamicStringArray::displayContents()
{
    if (size != 0)
        for (int i = 0; i < size; i++)
            cout << "Item-" << i << ": " << dynamicArray[i] << endl;
    else
        cout << "Array is empty." << endl;
}

void DynamicStringArray::addEntry(const string &addElement)
{
    string *temp = new string[size + 1];
    for (int i = 0; i < size; i++)
            temp[i] = dynamicArray[i];
    temp[size] = addElement;
    size++;
    delete [] dynamicArray;
    dynamicArray = temp;
    delete[] temp;
}

string DynamicStringArray::getEntry(int index)
{
    if ((index >= 0) && (index < size))
    {
        return dynamicArray[index];
    }
    return NULL;
}

int DynamicStringArray::deleteEntry(const string &deleteElement)
{
    if(size == 0)
    {
        return false;
    }

    for (int i = 0; i < size; i++)
    {
        if (dynamicArray[i] == deleteElement)
        {
            string *temp = new string[size - 1];
            for (int x = 0; x < size - 1; ++x)
            {
                if (x < i)
                    temp[x] = dynamicArray[x];
                else
                    temp[x] = dynamicArray[x + 1];
            }

            delete[] dynamicArray;
            dynamicArray = temp;
            delete[] temp;
            --size;
            return true;
        }
    }

    return false;
}

main:

int main()
{
    DynamicStringArray dsArray1;
    cout << "dsArray1.displayContents():" << endl;
    dsArray1.displayContents();  // Should indicate array is empty
    cout << "Display dsArray1.getSize()= " << dsArray1.getSize() << endl;

    dsArray1.addEntry("Entry-A");
    dsArray1.displayContents();

    dsArray1.addEntry("Entry-B");
    dsArray1.displayContents();
    dsArray1.addEntry("Entry-C");
    dsArray1.displayContents();
    return 0;
}

Can anyone tell me what I am doing wrong. How can i fix this problem?

Solution

Please note that all of this is already available by utilizing std::vector<std::string>. The std::vector class is the dynamic array class that C++ provides, and there is little to no reason to make home-made versions of what is available to you.

---

Having said this, one glaring issue is that your copy constructor is incorrect. The dynamicArray is uninitialized, but you use it here:

if (dynamicArray != NULL)

There is no guarantee what value dynamicArray has. The fix is to remove this entire block of code in the copy constructor:

if (dynamicArray != NULL)
{
    size = 0;
    delete [] dynamicArray;
    dynamicArray = NULL;
}

Since the copy constructor constructs a brand new object, there is no reason to "pretest" for a NULL pointer and thus do unnecessary work. Remember that the object did not exist, so there is nothing preliminary to do.

---

The second issue is that you're issuing a delete [] temp; call in the addEntry and deleteEntry functions. Remove these lines, as you are deallocating the memory that you've just assigned to dynamicArray.

---

The third issue is that you're missing the user-defined assignment operator. The assignment operator has the following signature, and you need to provide the implementation:

DynamicStringArray& operator=(const DynamicStringArray& );

Without this function, assigning a DynamicStringArray to another DynamicStringArray will cause memory leaks and double deallocation of memory when both objects go out of scope.

One implementation can use the copy / swap idiom:

#include <algorithm>
//...
DynamicStringArray& DynamicStringArray::operator=(const DynamicStringArray& rhs)
{
    DynamicStringArray temp(rhs);
    std::swap(temp.dynamicArray, dynamicArray);
    std::swap(temp.size, size);
    return *this;
}

---

Another issue is this:

string DynamicStringArray::getEntry(int index)
{
    if ((index >= 0) && (index < size))
    {
        return dynamicArray[index];
    }
    return NULL;  // <-- Undefined behavior if this is done
}

It is undefined behavior to assign a std::string object with NULL. Either return an empty string, or throw an exception if the index is out of bounds.

---

In conclusion, I highly recommend that you read up on the Rule of 3 when it comes to designing classes that must implement correct copy semantics.

Comments

[Miles Budnek]

The assignments in addEntry are all std::strings, so they're fine (implementing an operator= would still probably be a good idea). The problem there is with the last line delete[] temp;

[PaulMcKenzie]

@MilesBudnek I updated the answer to point out the issue with both addEntry and deleteEntry.