Unable to change bools in C++ struct

fcarney

@mzimmers when I get strange stuff not working I do a clean, run qmake, build. Sometimes it is just a moc issue. Not sure how that works with cmake though.

mzimmers

@fcarney tried that -- same results. Good suggestion, though.

mzimmers

OK, so I discovered something, but I don't understand it. I stepped through this in the debugger, and confirmed the problem is in the C++ side: Screenshot 2023-10-18 142850.png

Then, for lack of anything else to try, I removed the initializer in my struct:

// bool m_maintenance = false; // before
bool m_maintenance; // now

and...
Screenshot 2023-10-18 143100.png
For another test, I added this line to my c'tor:

bool m_maintenance = false;

And got the original (bad) result.

I also "changed polarity" (initialize to true; try to set to false) with the same (bad) result.

So...this problem just got a little bigger. I'm not willing to release code to the field that creates structs with uninitialized values; I need to figure out what's going on here.

EDIT: this occurs on 6.6 as well as 6.5.3.

ANY ideas, no matter how far-fetched, are welcome. Thanks...

JoeCFD

Check this out.
https://stackoverflow.com/questions/1069621/are-members-of-a-c-struct-initialized-to-0-by-default
It may not be a C++ problem. Instead it is a C problem. Struct is very C style. I even avoid using it.

mzimmers

@JoeCFD interesting article, though my issue is with modification, not initialization.

Regarding the use of structs: I suppose I could use a class, but I'd be astonished if that made a difference. I'm deliberately avoiding using QObject here, too.

JoeCFD

@mzimmers bool m_maintenance; // now
is undefined.

can you try:
bool m_maintenance{ false };
I do not know if it will make a difference.

mzimmers

@JoeCFD said in Unable to change bools in C++ struct:

@mzimmers bool m_maintenance; // now
is undefined.

I realize that, and that's not acceptable. But currently I have the options of leaving it undefined, or initializing it and (somehow) making it QML-immutable in the process.

I can't even modify it if I initialize it in QML, like this:

    property schedule newSchedule: ({ "maintenance": false })

mzimmers

@JoeCFD said in Unable to change bools in C++ struct:

can you try:
bool m_maintenance{ false };

Same (bad) result. Thanks for the suggestion, though.

JoeCFD

@mzimmers
try
class Schedule

not struct Schedule
.

mzimmers

@JoeCFD no change in behavior.

GrecKo

@JoeCFD struct is just a class where everything is public by default.

GrecKo

@mzimmers what's the issue on this output? We don't know which field is which. Also why are you using m_maintenance in QML? That's not the name of your property.

mzimmers

@GrecKo I don't know how that got in there, but that wasn't how it was originally, and it's not how it is now. My screenshots above are correct.

mzimmers

OK, guys - here's a minimal repeatable example:

// schedule.h
#pragma once

#include <QObject>
#include <QMetaType>
#include <QtQml/qqmlregistration.h>
#include <QtQmlIntegration/QtQmlIntegration>

struct Schedule
{
    Q_GADGET
    QML_VALUE_TYPE(schedule)
    QML_STRUCTURED_VALUE
    Q_PROPERTY(bool maintenance MEMBER m_maintenance)
public:
    bool m_maintenance;
    Q_INVOKABLE Schedule() {}
    bool operator ==(const Schedule &rhs) const {return true;}
};

// Main.qml
import QtQuick
import QtQuick.Controls
import QtQuick.Layouts
import QtQuick.Window

Window {
    id: mainWindow
    width: 640
    height: 480
    visible: true

    property schedule newSchedule: ({  })

    Switch {
        onClicked: {
            console.log("switch is " + checked)
            newSchedule.maintenance = checked
            console.log("newSchedule.maintenance is " + newSchedule.maintenance)
        }
    }
}

Output:

qml: switch is true
qml: newSchedule.maintenance is false
qml: switch is false
qml: newSchedule.maintenance is false
qml: switch is true
qml: newSchedule.maintenance is false
qml: switch is false
qml: newSchedule.maintenance is false

I can provide the cmake file as well if desired, but it doesn't contain anything unusual.

GrecKo

@mzimmers said in Unable to change bools in C++ struct:

bool operator ==(const Schedule &rhs) const {return true;}

If you remove this non-sensical operator== your code works as expected.

My guess is that the engine modifies a temp Schedule internally, checks the equality and assign it only if it's different. As it's always equal it won't modify it.

mzimmers

@GrecKo bingo!

I'd just stubbed out that comparison function to satisfy the compiler, but you were absolutely right. In my app, I'd neglected to include the maintenance member in my comparison function. Seems to work fine now.

Kind of has me wondering -- if I set my compiler options to no optimization, I wonder if it would have worked...

Anyway, thanks to EVERYONE for the help on this.