QWidget & inheritance

[shahriar25]

Hi
I have a QWidget (pointer) and I want to change it in runtime to different widgets that inherit QWidget. I want to be able to use the new widget's functions but I can't do it. can anyone help?
Thanks in advance.

QWidget *widget;

widget = new QProgressBar();

widget->setRange(0,100); //Error (the function not recognized)

[thEClaw]

To make this work, you will have to convert the pointer to your widget first, e.g.

QProgressBar *p = qobject_cast<QProgressBar *>(widget);
if(p)
  p->setRange(...);

If you want to do something similar with other types of widgets (QLabel, QSlider, ...), you would have to convert the pointer every time you want to call a function that is not virtual in QWidget itself. I.e.

QWidget *widget;
widget = new QLabel();

widget->sizeHint();//works, since QWidgets implementation has been overridden in QLabel
widget->setText("...");//does *not* work since you are calling this method on a QWidget!
QLabel *l = qobject_cast<QLabel *>(widget);
if(l)
  l->setText("...");//works, since you are explicitely calling QLabels implementation of the method

[shahriar25]

Hi @thEClaw
what if I want to delete the widget that is a QProgressbar after a while? if I delete the widget, the new pointer that is a QProgressBar is deleted too?

[jsulm]

If you delete the object your pointer is pointing to then the object will be deleted even if it is QProgressBar:

QWidget *widget;
widget = new QProgressBar();
delete widget; // Will delete QProgressBar

As far as I know Qt uses virtual destructors for base classes, so even if the pointer is of base class type the correct destructor will be called.

[kshegunov]

@jsulm said:

As far as I know Qt uses virtual destructors for base classes

Yes. QObject's destructor is virtual. Classes that derive from other bases might not have a virtual destructor, though (e.g. QStringList, not that you would want to create that in the heap).

@shahriar25

I have a QWidget (pointer) and I want to change it in runtime to different widgets that inherit QWidget. I want to be able to use the new widget's functions but I can't do it. can anyone help?

You can, as @thEClaw has show, however the rationale behind that somewhat escapes me. Why do you want to do that?

[shahriar25]

Hi @kshegunov
I want the user to enter the widget's name an the program to show that widget and change it by entering sth.

[kshegunov]

@shahriar25
This is a very unusual requirement. I doubt qobject_cast will help you for this, though. You probably want to create objects by class name, which is possible through the meta-object system, however it's a bit hackish. Something along the lines of:

QWidget * parent; // The parent of the widget we are creating

QByteArray className = QByteArrayLiteral("QDialog*");
int typeId = QMetaType::type(className);
if (typeId == 0)
    ; // Error - class is not registered

//< Here is needed an additional check if that class actually derives from QObject

const QMetaObject * metaObject = QMetaType::metaObjectForType(typeId);
QObject * widget = metaObject->newInstance(Q_ARG(QWidget *, parent));

But why do you need to do that in the first place?

[shahriar25]

@kshegunov
wow your code really creates an object by it's name. this is an interesting thing. but this is not what I want. my program get predefined commands from user or an external program and does something specific. the commands are limited and I want to create about 10 widgets one at a time. so thEClaw's answer is enough here. thank you all

[kshegunov]

@shahriar25 said:

wow your code really creates an object by it's name.

It should, but I can't claim merit for it. I think I read about that approach some time ago in the mailing list.

but this is not what I want. my program get predefined commands from user or an external program and does something specific.

I see. Then I must have misunderstood you. But in any case, I'm glad it worked out in the end.

[shahriar25]

Yes of course but only with your help this becomes possible