Minimize UEFN Verse Memory Allocations and Cell Usage

What you'll learn

• Why allocating new objects inside loops multiplies your memory cell usage.
• How to hoist a single reusable device reference out of the loop and drive it every tick.
• How to safely handle fallible array indexing and player access with if (X := ...).
• How to toggle a device's state in place instead of reconstructing objects.

How it works

UEFN reserves memory cells for the objects your Verse code creates. When you construct a widget, data structure, or device instance inside a for loop, the compiler must budget for a fresh allocation on every iteration. A tight loop across many players (or a wide numeric range) can balloon your cell count and trip the memory limit, which surfaces as compile warnings, editor stutter, or a blocked publish.

The cure is reuse. Instead of building new objects each pass, obtain your references once — via @editable device fields set in the editor — store any changing state in a mutable var, and mutate that state in place. The same cell is used repeatedly, so total usage stays flat and predictable regardless of loop length.

Just as important: many Verse calls are fallible. Reading GetPlayers()[0] or indexing any array can fail, so those calls use [] and must live inside a failure context like if (P := Players[0]):. Enabling/disabling a device is a side effect, not a fallible call, so it uses () and runs as a plain statement.

Let's build it

The device below grabs its player list and a button_device reference (assigned in the editor) before the loop. Each tick it simply toggles the button's enabled state using a mutable logic flag — no new objects are ever allocated inside the loop.

using { /Fortnite.com/Devices }
using { /Verse.org/Simulation }
using { /UnrealEngine.com/Temporary/Diagnostics }

# A device that does real per-tick work without allocating inside its loop.
memory_optimized_device := class<concrete>(creative_device):

    # Reference assigned in the editor — allocated ONCE, never rebuilt in a loop.
    @editable
    MyButton : button_device = button_device{}

    OnBegin<override>()<suspends>: void =
        # Hoist state OUT of the loop. This mutable flag is the only thing we
        # change each tick, so the memory footprint stays perfectly flat.
        var Enabled : logic = true

        # Snapshot the player list once. GetPlayers() returns an array; we do
        # NOT rebuild it every iteration.
        Players := GetPlayspace().GetPlayers()

        # Array indexing is fallible (uses []), so bind it in an if-context.
        # This runs ONCE, before the loop, to confirm we have at least one player.
        if (FirstPlayer := Players[0]):
            Print("Optimizer online. First player found among {Players.Length} players.")

        # Enable the button once up front (a side effect — plain () call).
        MyButton.Enable()

        # Drive the same button reference across many iterations. No allocations
        # happen here: we only Sleep, toggle a logic flag, and call side effects.
        for (I := 0..10):
            Sleep(1.0)
            # Toggle in place — Verse has no `not` assignment, so branch explicitly.
            if (Enabled?):
                MyButton.Disable()  # side effect, () call
                set Enabled = false
                Print("Tick {I}: button disabled, reusing the same cell.")
            else:
                MyButton.Enable()
                set Enabled = true
                Print("Tick {I}: button enabled, still zero new allocations.")

Try it yourself

1. Introduce a spike (in a scratch copy): try declaring a brand-new data structure inside the for (I := 0..10): loop. Notice how each iteration would demand its own cell.
2. Measure it: open the UEFN memory tools and compare cell usage when references are hoisted versus allocated per iteration.
3. Scale the loop: change the range to 0..100. With everything hoisted, your cell count is unchanged — only the flag and side-effect calls run more often.
4. Wire the button: drop a Button Device in your level and assign it to MyButton in the device's Details panel so Enable/Disable visibly fire.

Recap

• Allocate once: obtain widgets, data, and device refs before loops — use @editable fields for device references.
• Mutate in place: keep changing state in a var and update it with set; toggle a logic by branching, not set X = not X.
• Fallible vs. side effect: array/player indexing uses [] inside if/for; enabling a device is a () statement.
• Stay under the limit: flat allocations mean predictable cell usage and a smooth publish.