[SOLVED] Q_INVOKABLE vs Q_PROPERTY (QML hanging...)

Erakis

Hi,

I don't understand why using Q_INVOKABLE instead of Q_PROPERTY to return a QObject derivated class pointer method made my application crashing/hanging. Moreover I see this error message in my console

Expression: _BLOCK_TYPE_IS_VALID(pHead->nBlockUse) Error

Like the memory is double free somewhere....

Here is how to reproduce :

class ChildrenClass : public QObject
{
    Q_OBJECT
public:
    explicit SubClass(QObject *parent = 0);
    Q_INVOKABLE QString getData() {return "Hello world !";};
}

class ParentClass : public QObject
{
    Q_OBJECT
public:
    explicit ParentClass (QObject *parent = 0);

    QObject* qmlInstance(QQmlEngine *engine, QJSEngine *scriptEngine)
    {
         return new ParentClass ();        
    }

    Q_INVOKABLE ChildrenClass * childrenClass() {
         return &GlobalChildrenClass; // Children class is 
    };
}

In the main.cpp, I registered the types

ChildrenClass   GlobalChildrenClass; // Static instance of a children class

int main(int argc, char *argv[])
{
      // Register ChildrenClass
    qmlRegisterType<ChildrenClass >("ChildrenClass", 1, 0, "ChildrenClass");

    // Create singleton for ParentClass
    qmlRegisterSingletonType<ParentClass>("ParentClass ", 1, 0, "ParentClass ", &ParentClass ::qmlInstance);
    ...
}

Now in the main.qml

import ChildrenClass 1.0

Window {
    id : mainWindow
    width :800
    height :480
    visible: true
    ...
 
    Component.onCompleted : {
        var g = ParentClass.childrenClass().getData(); // This is hanging the application
    }

But If I do :

class ParentClass : public QObject
{
    Q_OBJECT
    Q_PROPERTY(ChildrenClass * childrenClass READ childrenClass)
public:
    explicit ParentClass (QObject *parent = 0);
}

...
    Component.onCompleted : {
        var g = ParentClass.childrenClass.getData(); // This is well working :) !
    }

It is a bug or QML JS engine is not able to retrieved a subclass pointer and then calling a function of this pointer ? Or maybe there something wrong with my implementation ?

Best regards,

Erakis

I finally understand by myself why it why not working :)

The parent class is exporting a pointer to the QML side. As it is a pointer, the QML Engine take the ownership and then once he finished using it, he free memory.

So I have to add this to the ParentClass

explicit ParentClass (QObject *parent = 0)
{
      QQmlEngine::setObjectOwnership(&GlobalChildrenClass, QQmlEngine::CppOwnership);
}

So I deduce that a property do not free memory of a pointer, while a Q_INVOKABLE yes !.

Except also that a Q_PROPERTY can be notifiable, I don't know Finally I don't know any other difference between Q_PROPERTY vs _Q_INVOKABLE.

Hope it will help somebody else :)

Best regards,

SGaist

Hi,

Q_INVOKABLE is used to make functions accessible throughout Qt meta object system. Q_PROPERTY is much more complex and offers additional features