Re: Can you fix this program? : C++ Dynamic Array Problems

Russell Mangel wrote:
I have written the following program using VS2005. The program is a
Dynamic Array similar to System.Collections.ArrayList in .NET. The
program works okay until I reach 65536, I can't seem to figure out
why, as it seems my logic is working okay. I am a .NET programmer so
I am not used to dealing with un-managed C++ code. Please criticize
my code if you think it is poorly written.

The main criticism would be why write it at all? This code is spelled
std::vector<T> in C++. There's simply no need to write code like this
except for the learning exercise of it. A few other criticisms inline
below.


This is the loop from main() that will blow up the program if you
change 65536 to a larger value.
for(int i = 0; i < 65536; i++)
{
folder.entryID = "Testing";
fc->Add(folder);
}

Thanks
Russell Mangel
Las Vegas, NV

// Begin Code


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

using namespace std;

struct Folder
{
char *entryID;
};

Should the above have a destructor? In your example, you're only assigning
character literals to entryID, so the answer is NO, but in the real world,
that might be a different story. If entryID needs to be delete[]'d, then
you should define a destructor, assignment operator and copy constructor for
this class.

class FoldersCollection
{
public:
FoldersCollection::FoldersCollection()
{
Count = 0;
Capacity = 0;
}
FoldersCollection::~FoldersCollection()
{
delete []m_Folders;
}
int Count;
int Capacity;

You might want to wrap these fields in accessors. As is, a client can
simply change your Count or Capacity, breaking your class from the outside.

void FoldersCollection::Add(Folder folder)
{
if(Capacity == 0)
{
m_Folders = new Folder[INITIAL_CAPACITY];
m_Folders[0].entryID = folder.entryID;
Count ++;
Capacity = INITIAL_CAPACITY;
}
else
{
if(Count < Capacity)
{
m_Folders[Count].entryID = folder.entryID;
Count ++;
}
else
{
printf("Resizing Array. Capacity: %d.\n", Capacity);
Resize();

m_Folders[Count].entryID = folder.entryID;
Count ++;
}
}
}

Rewrite the above:

void FoldersCollection::Add(Folder folder)
{
EnsureCapacity(Count+1);
m_Folders[Count++] = folder;
}

Folder* FoldersCollection::GetList()
{
return m_Folders;
}

If you're going to simply expose the inner array, there's little point in
even making a class since a client can simply run roughshod all over your
array once you've returned a pointer. I'd reconsider this design.

private:
Folder *m_Folders;
Folder *m_Temp;

Eliminiate the above member varialbe- m_Temp

static const int INITIAL_CAPACITY = 4;
void FoldersCollection::Resize()
{
// Double Capacity
int newCapacity = Capacity*=2;
// Create new Array
m_Temp = new Folder[newCapacity];
// Copy elements
for(int i = 0; i < Capacity; i++)
{
m_Temp[i].entryID = m_Folders[i].entryID;
}

delete []m_Folders;
m_Folders = m_Temp;
Capacity = newCapacity;
}

Rewrite this:

void FoldersCollection::EnusreCapacity(int requiredCapacity)
{
if (requiredCapacity <= Capacity)
return;

int newCapacity = Capacity*2;
if (newCapacity < INITIAL_CAPACITY)
newCapacity = INITIAL_CAPACITY;
if (newCapacity < requiredCapacity)
newCapacity = requiredCapacity;

Folder* temp = new Folder[newCapacity];

if (Count > 0)
{
for (int i = 0; i < Count; ++i)
temp[i] = m_Folders[i];
delete[] m_Folders;
}

m_Folders = temp;
}

};
void main(void)
{
FoldersCollection *fc = new FoldersCollection;
Folder folder;

// Works okay up to 65536, change to larger value to crash program...
// What I am doing wrong...
for(int i = 0; i < 65536; i++)
{
folder.entryID = "Testing";
fc->Add(folder);
}
printf("Finished... Count: %d Capacity: %d \n", fc->Count,
fc->Capacity); delete fc;
}


.

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