A better memory managed MFC CArray

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Introduction

When I used MFC's CArray in one of my projects, I found it's not very heap-friendly.

It does use a pre-allocated memory pool but it doesn't use it clearly. For example, it doesn't have a constructor that allow us to explicitly declare the size of our pre-allocated memory pool. Furthermore, I think MFC CArray doesn't make clear between the actual memory pool size and the elements size of the array.

And if we really need to "pre-allocate", we has to make use of its grow-by variable, which is unclear and confusing.

How to use

Include the two files NewBValArray.h and NewBConfig.h in your project

Declare a variable:

#include "NewBValArray.h"
NewBValArray<MyClass,MyClass> rgMyArray(1000);

So rgMyArray will pre-allocate a memory pool of 1000 MyClass items.

NOTE: rgMyArray is still a blank array, i.e rgMyArray.GetSize() = 0; as long as you don't "add" items to it through Add(), or oprerator[].

Otherwise, it can be used the same as MFC's CArray, i.e you can safely replace all CArray with NewBArray.

Why don't I use MFC CArray?

After my posting, many ask me why I didn't use MFC's CArray?

I do know that MFC CArray can pre-allocate memory. But it really doesn't work as I expect.

What do I expect? I'd like to separate the holder (memory) and the meaning of array elements. That means I'll allocate a big chunk of memory to hold the elements' data. This memory should be allocated in the array's ctor and delete at array's dtor. During its lifetime, this part of memory shouldn't be changed

Now let's dive into the MFC CArray source code. It's contains 4 variables:

TYPE* m_pData;   // the actual array of data 
int m_nSize;     // # of elements (upperBound - 1)
int m_nMaxSize;  // max allocated
int m_nGrowBy;   // grow amount

Now, let see how can we force CArray to pre-allocate memory by SetSize(int nNewSize, int nGrowBy). nNewSise is the new desired size. nGrowBy I'll explain later. Then assume that we have

CArray<MyClass,MyClass> rgMFCArray;

What if I do this:

rgMFCArray.SetSize(1000,0);

Now I expect that rgMFCArray has 1000 pre-allocated spaces for MyClass, means I can Add to it 1000 MyClass variables. But actually rgMFCArray now contains 1000 MyClass "default" variables and if I call rgMFCArray.Add(...) then the m_pData will need to reallocate to a 1001 MyClass space : not what I expect.

Again, what if I do this:

rgMFCArray.SetSize(0,1000);

Now I make use of the grow-by stuff. At this time rgMFCArray will assign its m_nGrowBy = 1000. That's all! No allocation of memory. So if you do rgMFCArray[1], you'll get ASSERT false.

Then what if:

rgMFCArray.Add(someMyClassvariable);

Yeah, this time it's worked. Now CArray will allocate a memory chunk of size 1000*sizeof(MyClass). So if I really like to make use of pre-allocate memory on MFC's CArray, I have to use its m_nGrowBy variable. So confusing!

More! MFC's Array doesn't provide you any method to "remove" all CArray elements or change its memory allocation.

You can ask me about CArray::RemoveAll(). This method will remove all array elements plus deallocate its memory.

But really,we can remove all elements of MFC CArray with a long statement

for (int i = rgMFCArray.GetCount() - 1; i >= 0 ; i --)
   rgMFCArray.RemoveAt(i)

Again, it make me confused and uneasy.

So that's why I rewrite my own array class by just "hacking" MFC's CArray source, cause really MFC CArray is still a good boy, especially its making use of placement new and delete.

In my new class, I explicitly separate m_nSize (number of elements) and m_nMaxSize (the size of pre-allocated memory pool). I added a ctor that has one parameter to declare the size of pre-allocated memory, and this memory I can take from my own allocator as in many projects. And the method RemoveAll() just effectively "removes" the elements, i.e calls their dtor but leaves the memory pool untouched. This memory pool will be cleaned at NewBArray's dtor. Moreover, I still keep the ability to "grow" the memory pool to a larger one if the user adds more elements the array capacity. But this action shouldn't happen because it makes the heap fragment and cause memory allocation, and copy overhead.

This code is written by Nguyen Binh. I greatly appreciate any feedback.