c++ memory access violation

Question

I am learning C++ and I am having quite a lot of trouble with my current assignment. I have completed a good amount of it so far. However I have been making very slow progress of late due to what I think is my poor understanding of what is going on behind the scenes.

What I am trying to do in the following code is:

• Get two separate values (Bullet damage). Done.
• Create a dynamic array. Done.
• Fill a part (that is the size of a modulus of a random number between 1 and 10) of said dynamic array with one value and the rest with the other in a random order. Here I am having trouble.
• Clean up the memory used by said dynamic array. Done.

The error I get is as follows:

Unhandled exception at 0x00a323e3 in Class 3.exe: 0xC0000005: Access violation reading location 0xcdcdcdcd.

I'm pretty sure that the error occurs when I try to set ammoArray[i] to a value. But I don't know why it's giving it to me, my code compiles fine. I played around with it a bit and in one case I got it to store the memory addresses of bDamage and sDamage and then print out the memory addresses of each element of the array. What I want it to do is store the values held by bDamage and sDamage .

Now for my question:

Why won't ammoArray store the values of bDamage and sDamage instead of the memory addresses of the array's elements? And How do I get it to store them?

Here is my Main.cpp:

#include <cstdlib>
#include "Ammunition.h"
#include "AmmunitionManager.h"
#include "Bullet.h"
#include "Game.h"
#include "Pistol.h"
#include "Player.h"
#include "Point.h"
#include "Shell.h"
#include "Shotgun.h"
#include "WeaponManager.h"
#include "Weapons.h"

using namespace std;

void main()
{
    Ammunition amVar;
    AmmunitionManager *var = new AmmunitionManager();

    amVar.setBDamage(6);
    amVar.setSDamage(2);
    var->FillAmmoArray(amVar.getSDamage(),amVar.getBDamage());

    system("PAUSE");
}

Here is the .h file of the class in question:

#ifndef AMMUNITIONMANAGER_H
#define AMMUNITIONMANAGER_H
#include "Point.h"
#include "Ammunition.h"

class AmmunitionManager
{
public:
    AmmunitionManager();
    AmmunitionManager(int,int);
    ~AmmunitionManager();

    void FillAmmoArray(int,int);
private:
    Ammunition Ammo;
    int **ammoArray;
};

#endif

Here is the .cpp file of the class in question:

#include <iostream>
#include <cstdlib>
#include <ctime>
#include "AmmunitionManager.h"
#include "Point.h"
#include "Ammunition.h"

using namespace std;

AmmunitionManager::AmmunitionManager()
{
}


AmmunitionManager::AmmunitionManager(int sDamage,int bDamage)
    :Ammo(sDamage,bDamage)
{
    cout << "Filling ammo reservoir." << endl;
    ammoArray = new int* [10];
}

void AmmunitionManager::FillAmmoArray(int sDamage,int bDamage)
{
    srand(time(NULL));
    int *holdS = &sDamage;
    int *holdB = &bDamage;
    if(ammoArray)
    {
        for(int i = 0;i < 9;i++)
        {
            int randC = rand() % 2 + 1;
            if(randC == 1)
            {
                cout << "Was: " << ammoArray[i] << endl;//I am getting the error here.
                ammoArray[i] = holdS;
                cout << "Is: " << ammoArray[i] << endl;
            }
            if(randC == 2)
            {
                cout << "Was: " << ammoArray[i] << endl;//I am getting the error here.
                ammoArray[i] = holdB;
                cout << "Is: " << ammoArray[i] << endl;
            }
        }
    }
}

AmmunitionManager::~AmmunitionManager()
{
    *ammoArray = 0;
    if(ammoArray)
    {
        delete [] ammoArray;
    }
}

Answer 1

The problem is that you initialize AmmunitionManager with the default constructor:

AmmunitionManager *var = new AmmunitionManager();

In you default constructor you do nothing so ammoArray may contain any value. It is better to initialize all the data to their default values:

AmmunitionManager::AmmunitionManager() : Ammo(), ammoArray(NULL/* or nullptr for C++11 */)
{
}

Now if you call for

var->FillAmmoArray(amVar.getSDamage(),amVar.getBDamage());

It will exit immediately since ammoArray is NULL. Or probably you want to initialize ammoArray anyway, so the default constructor should have its initialization as well:

AmmunitionManager::AmmunitionManager() : Ammo()
{
    ammoArray = new int* [10];
}

Also srand should be called only once, so better to place this code

srand(time(NULL));

in the main() or any other module which is guaranteed to be executed only once.

In the destructor, there is no need to zero *ammoArray=0 , it actually puts 0 at the first element of that array (and that's it), you anyway delete it. And imagine that ammoArray is NULL, accessing *ammoArray would cause another segmentation fault.

Also there is no need to check for ammoArray beibg NULL before deleting it. The standard allows to ' delete ' NULL pointers. delete will just return without doing nothing.

General note

It is better to use (safer and easier to maintain) std::vector instead of (dynamic) arrays and smart pointers instead of flat ones.

Answer 2

I'm not getting any errors (VS2013). But the values stored are the addresses of sDamage and bDamage.

Did you properly use AmmunitionManager(int sDamage,int bDamage) as a constructor for creating the AmmunitionManager object? From what I'm seeing, you're not.

Apart from that, may I ask why you're using exotic constructs such as **ammoArray instead of eg a simple vector<int> ? I'm guessing it's part of your assignment, but I'm asking just to make sure I'm not missing anything.

I called the object like this:

int _tmain(int argc, _TCHAR* argv[])
{
    AmmunitionManager* tc = new AmmunitionManager(5,10);
    tc->FillAmmoArray(10,10);

    return 0;
}

Answer 3

Why won't ammoArray store the values of bDamage and sDamage instead of the memory addresses of the array's elements?

Because you said it should store addresses.
Here is a pointer to a pointer:

int **ammoArray;

and here is an array of pointers:

ammoArray = new int* [10];

And How do I get it to store them?

By doing this instead:

int *ammoArray;

and this:

ammoArray = new int [10];

and adjusting FillAmmoArray accordingly.

The default constructor should look like this:

AmmunitionManager::AmmunitionManager()
  : ammoArray(nullptr)
{
}

The destructor should look like this:

AmmunitionManager::~AmmunitionManager()
{
    delete [] ammoArray;
}

And you should only call srand once.
It's usually done at the beginning of main .

Answer 4

It's a bit tricky answering without building and debugging, but the first thing that strikes me are: Why are you using pointers ( * ) to int throughout?

Why don't you just have the array as a pointer:

int *ammoArray;

and make the other int -instances (remove the pointers - * and the address-of 's ( & ))?

Regards