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Type: Bug
Resolution: Fixed
Priority: P1: Critical
Fix Version/s: 6.8.5, 6.8, 6.9.3, 6.10.0 Beta4, 6.10, 6.11.0 FF
Affects Version/s: 6.8.4, 6.9.1, 6.10.0 Beta2
Component/s: QML: Declarative and Javascript Engine
Labels:
None
Environment:
Windows 10 22H2, MSVC 2022 x64

Commits:
b1c48db75 (dev), 6efdecd36 (6.10), 28cb8f65b (6.9), e603a5eff (tqtc/lts-6.8)

See the attached example. CppObj is a QML_ELEMENT while CppBridge is a QML_SINGLETON.

It creates 3 CppObjs with JavaScriptOwnership and no parent:

Identifier	Comment
-1	Instantiated declaratively in QML
0	Instantiated from C++, not stored
1	Instantiated from C++, stored in QML property

Code

import QtQuick
import QtQuick.Controls.Basic
import JSOwnershipStudy

Window {
    width: 400
    height: 300
    visible: true

    property CppObj objFromCpp: CppBridge.singleObj(1) // identifier == 1
    CppObj { id: objFromQml } // identifier == -1

    Component.onCompleted: {
        const orphanedObj = CppBridge.singleObj(0) // identifier == 0
    }

    Button {
        text: "Collect Garbage"
        onClicked: gc()
    }
}

Steps to reproduce

Run the attached example
Click the "Collect Garbage" button
Close the window to quit

Outcomes

(All versions) CppObj IDs -1 and 1 are destroyed upon quitting (Expected)
(Qt 6.5) CppObj ID 0 is destroyed by the garbage collector (Expected)
(Qt 6.8+) CppObj ID 0 is never destroyed (Not Expected)

Setting QV4_GC_TIMELIMIT=0 and QV4_MM_AGGRESSIVE_GC=1 does not help.

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JSOwnershipStudy-SingleObjects.zip
05 Aug '25 07:57
2 kB
Sze Howe Koh

relates to

QTBUG-139025 [Reg 6.5.9 -> 6.8.4] Lists of objects with JavaScriptOwnership, stored in var properties, are no longer destroyed

Closed

QTBUG-139059 Generated code does not track objects on JavaScript stack

Closed

QTBUG-138944 List of objects with JavaScriptOwnership are not ref-counted correctly when assigned to ListView.model

Closed

resulted in

QTBUG-139053 Re-design transfer of QObject ownership to JavaScript

Open

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For Gerrit Dashboard: QTBUG-138919
#	Subject	Branch	Project	Status	CR	V
666948,19	QmlCompiler: Ensure QObjects returned to AOT-compiled code are wrapped	dev	qt/qtdeclarative	Status: MERGED	+2	0
672768,2	QmlCompiler: Ensure QObjects returned to AOT-compiled code are wrapped	6.10	qt/qtdeclarative	Status: MERGED	+2	0
673156,2	QmlCompiler: Ensure QObjects returned to AOT-compiled code are wrapped	6.9	qt/qtdeclarative	Status: MERGED	+2	0
673208,4	QmlCompiler: Ensure QObjects returned to AOT-compiled code are wrapped	tqtc/lts-6.8	qt/tqtc-qtdeclarative	Status: MERGED	+2	0