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Details

Type: Bug
Resolution: Fixed
Priority: P1: Critical
Fix Version/s: 6.5.2, 6.6.0 Beta1, 6.7.0 FF
Affects Version/s: 6.5.1
Component/s: QML: Declarative and Javascript Engine
Labels:
- Reported_by_support_standard
Environment:
Windows 10 22H2, MSVC 2019 x64

Commits:
4fdaf22a5 (dev), 544f50597 (6.5), 585ad82fa (6.6), 3c0b8076e (dev), f7f9aa00a (6.5), 0be66b13b (6.6)

Description

Related to https://doc.qt.io/qt-6/qtqml-cppintegration-exposecppattributes.html#c-methods-and-the-this-object

Code
cppobj.h

class CppObj : public QObject
{
    Q_OBJECT
    QML_ELEMENT

public:
    CppObj(QObject* parent = nullptr) : QObject(parent) {}

    Q_INVOKABLE void sum(int a, int b)
    {
        qDebug().noquote() << this << QString("says %1 + %2 = %3").arg(a).arg(b).arg(a+b);
    }
};

Main.qml

import QtQuick.Controls.Basic
import InvokableProperty

ApplicationWindow {
    width: 640
    height: 480
    visible: true

    property var qmlMethod: function(a, b) { console.log(`${this} says ${a} + ${b} = ${a+b}`) }
    property var cppMethod: objA.sum

    CppObj { id: objA; objectName: "objA" }
    CppObj { id: objB; objectName: "objB" }

    Button {
        text: "Calculate"
        onClicked: {
            qmlMethod(1, 2)
            qmlMethod.call(objB, 2, 1)

            objA.sum(3, 4)
            objA.sum.call(objB, 4, 3)

            cppMethod(5, 6)
            cppMethod.call(objB, 6, 5)
        }
    }
}

Steps to reproduce

Build and run the test code
Click the button 3 times

Note: The enviroment variable QT_LOGGING_RULES="qt.qml.method.behavior=false;" is set to disable the note that is printed repeatedly in Qt 6.5:

Calling C++ methods with 'this' objects different from the one they were retrieved from is broken, due to historical reasons. The original object is used as 'this' object. You can allow the given 'this' object to be used by setting 'pragma NativeMethodBehavior: AcceptThisObject'

Qt 6.4.3 output, for every click (expected)
This shows the expected (but erroneous) historical C++ output. The QML output is correct.

qml: Main_QMLTYPE_3(0x2243e73ceb0) says 1 + 2 = 3
qml: CppObj(0x2243b840b40, "objB") says 2 + 1 = 3
CppObj(0x2243b840d60, name = objA) says 3 + 4 = 7
CppObj(0x2243b840d60, name = objA) says 4 + 3 = 7
CppObj(0x2243b840d60, name = objA) says 5 + 6 = 11
CppObj(0x2243b840d60, name = objA) says 6 + 5 = 11

Qt 6.5.1 output, for the 1st click (expected)
Without pragma NativeMethodBehavior: AcceptThisObject, the historical behaviour is preserved as expected.

qml: Main_QMLTYPE_3(0x1a177582a30) says 1 + 2 = 3
qml: CppObj(0x1a1745d30f0, "objB") says 2 + 1 = 3
CppObj(0x1a1745d2870, name = objA) says 3 + 4 = 7
CppObj(0x1a1745d2870, name = objA) says 4 + 3 = 7
CppObj(0x1a1745d2870, name = objA) says 5 + 6 = 11
CppObj(0x1a1745d2870, name = objA) says 6 + 5 = 11

Qt 6.5.1 output, for the 2nd and subsequent clicks (unexpected)
The outputs of the 1st click are not reproduced. Instead, we get "Too many arguments, ignoring 1" when invoking `cppMethod` (in contrast with `objA.sum.call(objB, 4, 3)` which produces the expected historical output)

qml: Main_QMLTYPE_3(0x1a177582a30) says 1 + 2 = 3
qml: CppObj(0x1a1745d30f0, "objB") says 2 + 1 = 3
CppObj(0x1a1745d2870, name = objA) says 3 + 4 = 7
CppObj(0x1a1745d2870, name = objA) says 4 + 3 = 7
expression for onClicked@qrc:/InvokableProperty/Main.qml:24
Too many arguments, ignoring 1
"Could not convert argument 0 at"
     "expression for onClicked@qrc:/InvokableProperty/Main.qml:24"
Passing incomatible arguments to signals is not supported.
expression for onClicked@qrc:/InvokableProperty/Main.qml:25
Too many arguments, ignoring 1
"Could not convert argument 0 at"
     "expression for onClicked@qrc:/InvokableProperty/Main.qml:25"
Passing incomatible arguments to signals is not supported.

Use case
A customer has existing code where many Q_INVOKABLE methods are stored in properties and invoked directly, like 'cppMethod(5, 6)'

Other notes

If using the test code from ~~QTBUG-114090~~ instead of the one attached to this ticket, the 2nd click will crash at cppMethod.call(objB) on Qt 6.5.1
There is a typo in the warnings above: "incomatible"