CArray not de-allocating memory

Question

I have a very simple problem and I seem to be stumped at whatever is happening. Look at the following code:

CArray<double, double&> arr;
arr.SetSize(50000);

for(int i =0; i< 50000; i++)
{
    arr[i] = (i+2)*3.14f;
}

arr.RemoveAll();

I would assume that after RemoveAll(), the memory would be freed but it seems like it is not happening. To check memory footprint, open Taskmanager and watch your exe's Memory. It increases on arr.SetSize() call but it never decreases even when this arr goes out of scope. Can somebody shed some light on nthis?

Answer 1

First thing:

Task Manager != Memory Profiler

The operating system (or more specifically the C runtime system) would certainly cache some of the memory you allocate.

Note that the operating system usually isn't dumb - if it's memory that's not being used by the application and the OS needs more memory to satisfy some other program's memory needs, it will allocate accordingly. But if that's not the case (it isn't most of the time), then keeping it for the application to reuse when is a winning strategy.

Because of these sort of optimizations, you really can't use Task Manager to get an accurate view of an application's memory usage.

Second thing:

Like all non-stupid dynamic array classes, CArray doesn't release the memory backing the array even when you remove all the elements in case you need to use the memory buffer again. It would be an awful waste of processor cycles to delete the underlying memory buffer only to find that you need to waste even more processor cycles to reallocate another buffer to handle your next CArray::Append() call that might come right after a CArray::RemoveAll().

Should you really want to get rid of that extra space, use CArray::FreeExtra(). Note that the function may involve allocating a new buffer and copying the elements over to the new one, then deleting the old buffer.