How to cast QObject to custom QWidget in abstract factory pattern?

Hi all, Have a nice day.

I am new to the forum and is my first post, I reviewed most of this forum and the web and not get anything definitive to help me understand the problem I am waiting here can help me as I often they have done in other post for other people.

I am trying to apply the abstract factory pattern to create a series of modules that require to an application I'm developing.

I have taken an example from the book An Introduction to Design Patterns in C ++ With Qt (Prentice Hall-2011-2nd Edition) page 483, paragraph 16.1.1 Abstract Factory
Chapter 16, as well as, I read on the website of some other examples and
I have the following code to do what I need.

Code:

---

typedef QHash<QString, QMetaObject> T_Modules;
 
T_Modules modulesTypesList () {
 
    T_Modules hash;
 
    hash [ "LibraryGui" ] = LibraryGui::staticMetaObject;
    hash [ "ConvertGui" ] = ConvertGui::staticMetaObject;
    hash [ "BurnGui" ] = BurnGui::staticMetaObject;
    hash [ "PlaylistGui" ] = PlaylistGui::staticMetaObject;
    hash [ "PlayingGui" ] = PlayingGui::staticMetaObject;
 
    return hash;
}
 
Module *ModulesFactory::createModule ( QString className, QWidget *parent ) {
 
    T_Modules modulesType = modulesTypesList ();
 
    Module *moduleCreated = 0;
 
    if ( modulesType.contains ( className ) ) {
 
        const QMetaObject& metaObjectClass = modulesType [ className ];
        moduleCreated = qobject_cast<Module *> ( metaObjectClass.newInstance () );
 
        if ( moduleCreated == 0 ) {
 
            qDebug () << "Error creating " << className;
            abort ();
 
        } else if ( parent != 0 ) {
 
            moduleCreated->setParent ( parent );
        }
    } else {
 
        qDebug () << "Error creating " << className;
        abort ();
    }
    return moduleCreated;
}

---

In theory everything is fine (compiles complete), but when running, it generates the
following error:

Code:

---

Starting /home/remizero/Proyectos/Qt/rockola/build-rockola-Desktop-Debug/rockola...
Error creating  "LibraryGui"
The program has unexpectedly finished.
/home/remizero/Proyectos/Qt/rockola/build-rockola-Desktop-Debug/rockola crashed

---

Of course, not a bug properly, the detail is that I did not create the object (send to abort to avoid having to close the application and to debug faster; p)

My question is, What am I doing wrong trying to create object of type Module? on line 25

Code:

---

moduleCreated = qobject_cast<Module *> ( metaObjectClass.newInstance () );

---

I'm working with QtCreator 3.2.1, Qt 5.3.2, gcc 4.9.1 32bit

If you need a compilable example, I'll have to pass on the project with a minimum of 20 files and tell the file in question to go directly.

Sorry for my bad English.

Thanks in advance.

Does your class inherit Module? Does Module have a Q_OBJECT macro? Does newInstance() return a valid pointer?

Goodnight wysota. Thanks for responding so quickly.

Yes, my class inherits from Module and also has the Q_OBJECT macro. newInstance() returned a pointer to 0, which was driving me crazy lol.

Reviewing some structure in my class LibraryGui, the error had it in the constructor of the class, because I was missing the macro Q_INVOKABLE, which did not recognize the constructor, but still was still presenting another error is that the constructor required parameters and that I solved using the macro Q_ARG (MyObject * myArg), leaving my code section as follows:

Code:

---

typedef QHash<QString, QMetaObject> T_Modules;
 
T_Modules modulesTypesList () {
 
    T_Modules hash;
 
    hash [ "LibraryGui" ] = LibraryGui::staticMetaObject;
    hash [ "ConvertGui" ] = ConvertGui::staticMetaObject;
    hash [ "BurnGui" ] = BurnGui::staticMetaObject;
    hash [ "PlaylistGui" ] = PlaylistGui::staticMetaObject;
    hash [ "PlayingGui" ] = PlayingGui::staticMetaObject;
 
    return hash;
}
 
Module *ModulesFactory::createModule ( QString className, QWidget *parent ) {
 
    T_Modules modulesType = modulesTypesList ();
 
    Module *moduleCreated = 0;
 
    if ( modulesType.contains ( className ) ) {
 
        const QMetaObject& metaObjectClass = modulesType [ className ];
        [B][I][U]moduleCreated = qobject_cast<QAction *> ( metaObjectClass.newInstance ( Q_ARG ( QWidget*, parent ) ) );[/U][/I][/B]
 
        if ( moduleCreated == 0 ) {
 
            qDebug () << "Error creating " << className;
            abort ();
 
        } else if ( parent != 0 ) {
 
            moduleCreated->setParent ( parent );
        }
    } else {
 
        qDebug () << "Error creating " << className;
        abort ();
    }
    return moduleCreated;
}

---

And in my class LibraryGui, which inherits from Module, is as follows:

Code:

---

class LibraryGui : public Module {
 
        Q_OBJECT
 
    public:
        Q_INVOKABLE explicit LibraryGui ( QWidget *parent = 0 );
        ...
}

---

Thank you very much for your questions because I did check with my code more carefully.

So I close this thread.

Thank you very much again and hope that this error is a reminder to other programmers colleagues.

PS: I do not get as placing "solved" in the title of the post :(