PRB: Add() Member Function Consuming Large Amount of Memory

1.00 1.50 1.51   | 1.00 2.00 4.00
WINDOWS          | WINDOWS NT

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with:

      - Microsoft Visual C++ for Windows, versions 1.0, 1.50, and 1.51
      - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, and 4.0
        on the following platform(s): x86
  

SYMPTOMS

After calling CStringArray::Add() many times, unusually large amounts of memory become allocated to the application and not freed until the application terminates. This behaviour may be seen in the other array collection classes such as CObArray and CPtrArray as well.

CAUSE

Each time a call to Add() is made, the array may grow. Memory needs to be allocated to accommodate the new size of the array. The array is copied to the new block of memory and the old memory is freed for re-use. The Visual C++ run-time heap allocator doesn't return freed memory to the system but instead marks the memory block as unused so that it can re-use the memory later.

For each call to Add() that causes the array to be re-allocated, the old block is marked as unused. The sections marked as unused won't get re-used by the next call to Add() because the array is too large to fit into the unused section. Other allocations (such as the CString) prevent adjacent free unused blocks of memory from being combined into a bigger block of unused memory.

RESOLUTION

A developer using an array data structure typically knows the size of the array before allocating memory for it. However, if you don't know the size, you must re-allocate the array as it grows. This means that a new memory block must be allocated and the data from the old memory block must be copied to the new block before the old memory block is freed. This can cost time.

To work around the problem, call the member function SetSize() with the approximate size of the array. If you are uncertain about the size of the array, minimize the amount of memory reallocations by specifying a larger size to allow room to grow. The first parameter to SetSize() allows you to change the size of the array and the second parameter allows you to change the grow size. For example:

   CStringArray array;
      array.SetSize(100,100);

Alternatively, SetSize(0, 100) would cause the initial size for the array to be zero and an Add() would have to be done before any elements could be filled. This first Add() in this scenario would cause 100 elements to be allocated for use by the array.