vba-queue

Generic Queue based on an encapsulated VB Collection

A lightweight, Collection-backed FIFO queue for VBA with:

• O(1) enqueue at the rear and dequeue from the front (position 1)
• 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

• Fast enqueue/dequeue (front is position 1)
• 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 (don't mutate during enumeration).

Error behavior

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

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🚀 Quick Start

Dim q As New Queue

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

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⏱️ Performance

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

#	Count	vba-queue	System.Collections.Queue
1	10	0.00045	0.00518
2	100	0.00045	0.00516
3	1,000	0.00045	0.00523
4	10,000	0.00045	0.00340
5	100,000	0.00045	0.00338

Performance is consistent regardless of queue size. System.Collections.Queue uses late binding, which explains its relatively poor performance.

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📄 License

MIT © 2025 Vincent van Geerestein