vba-queue-array

Generic Queue Class based on a circular encapsulated VB Array

A lightweight, circular array-backed FIFO queue for VBA with:

• O(1) enqueue at the rear and dequeue from the front via circular buffer
• Safe enumeration (For Each) via a proper COM enumerator
• Clear error semantics (Peek / Dequeue on empty queue raise error 5)
• Zero dependencies (pure VBA)

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📁 Files

File	Description
Queue.cls	Source file with Rubberduck annotations ('@Description, '@DefaultMember, '@Enumerator)
Queue_WithAttributes.cls	Ready-to-import version with VB attributes baked in — no Rubberduck required

Both files are identical in behaviour. Import Queue_WithAttributes.cls if you are not using Rubberduck.

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📦 Features

• Circular buffer avoids data shifting on every Dequeue
• Dynamic resizing doubles on grow (GrowthFactor = 2); shrinks by half when occupancy drops below ShrinkTrigger (25%)
• Enumeration: For Each item In Queue (via hidden [_NewEnum])
• Utility export: Items([base]) returns a 0- or 1-based array copy
• Pure VBA, no external references, Rubberduck-friendly annotations

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⚙️ Public Interface

Member	Type	Description
Enqueue(Item)	Sub	Adds an item to the rear of the queue.
Dequeue()	Function	Returns and removes the front item. Raises error 5 if empty.
Peek(Default)	Property	Returns the front item without removing it. Raises error 5 if empty.
Count	Property	Number of items.
IsEmpty	Property	True if empty, else False.
Clear	Sub	Removes all items.
Items([base])	Function	Returns all items as a Variant() array; base default = 0.
For Each	Enumerator	Iterates front → rear on a snapshot. (Mutation during iteration is safe but iterates the old snapshot.)

Error behavior

• Empty queue on Peek / Dequeue raises vbErrorInvalidProcedureCall (=5) with source "Queue.Peek" or "Queue.Dequeue".

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🚀 Quick Start

DimqAsNewQueueq.Enqueue"alpha"q.Enqueue"beta"Debug.Printq.Peek' -> alpha (front)Debug.Printq.Dequeue' -> alpha (removed)Debug.Printq.Dequeue' -> beta (removed)Debug.Printq.IsEmpty' -> True

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⏱️ Performance

Timings (ms) for one Enqueue + one Dequeue, measured on Windows x64:

#	Count	vba-queue-array	vba-queue (Collection)
1	10	0.00073	0.00045
2	100	0.00072	0.00045
3	1,000	0.00034	0.00045
4	10,000	0.00035	0.00045
5	100,000	0.00040	0.00045

The array-based version is more memory-efficient and converges to faster performance at larger sizes. The Collection-based version has lower overhead at small counts. The circular buffer avoids data shifting on every Dequeue.

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📄 License

MIT © 2025 Vincent van Geerestein