Does redim use the heap or the stack memory in VBA?

When you use ReDim in VBA (Visual Basic for Applications), you are indeed performing a dynamic memory allocation at runtime, which involves reallocating memory.
Here's a detailed explanation of the process and its implications:

Static vs Dynamic Allocation

1- Static Allocation:

Dim tempArray(0 To 2) As Integer

Memory for tempArray is allocated on the stack at compile time.
The size of the array is fixed and determined at compile time.
Stack allocation is typically faster and more efficient since it involves a simple pointer adjustment.

2- Dynamic Allocation:

Dim tempArray() As Integer
Dim arraySize As Integer
Dim inputData As String

inputData = InputBox("Please give me the size of your array")
arraySize = Val(inputData)
ReDim tempArray(0 To arraySize)  

Memory for tempArray is allocated on the heap at runtime.
The size of the array is determined at runtime based on user input.
Heap allocation is slower compared to stack allocation because it involves more complex memory management operations, such as finding a suitable block of memory and potentially moving existing data.

The ReDim Process

When you use ReDim in VBA:

1- Memory Allocation:

VBA allocates a new block of memory on the heap to accommodate the new size of the array.
The old memory block is deallocated (freed), which involves updating internal memory management structures to mark it as available.

2- Data Movement (if ReDim Preserve is used):
If you use ReDim Preserve, VBA needs to copy the existing elements of the array to the newly allocated memory block.
This adds overhead as it involves iterating through the existing elements and copying them one by one to the new memory location.

Performance Considerations:

Speed:

Dynamic allocation (ReDim) is generally slower than static allocation due to the overhead of heap memory management and, potentially, data movement.
The difference in speed may be negligible for small arrays but can become significant for larger arrays or in performance-critical applications.

Memory Usage:

Heap allocation allows for more flexible memory usage, enabling dynamic resizing of arrays based on runtime requirements.
However, frequent resizing with ReDim can lead to memory fragmentation and increased memory usage.

Best Practices:

Minimize the Use of ReDim:
Try to minimize the number of ReDim operations, especially inside performance-critical loops.
If you need to resize an array multiple times, consider resizing in larger increments to reduce the number of allocations.

Avoid Unnecessary ReDim Preserve:
Use ReDim Preserve only when you need to retain existing data.
If the data does not need to be preserved, a simple ReDim without Preserve can be more efficient.

Conclusion:

In summary, using ReDim in VBA involves dynamic memory allocation on the heap, which is slower compared to static allocation on the stack due to the overhead of memory management and potential data movement.
While VBA abstracts much of this complexity, understanding these underlying processes can help you write more efficient code, especially when working with large datasets or in performance-critical scenarios.