Problems with setCentralWidget

[Rockem]

What is the point of hiding Qt architecture if you are immediately passing the pressumably non-Qt component to a Qt-expecting method?

Comp* c = new Comp;

Comp* c = new Comp;

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The above works fine. Using CompView won't work because the object is aligned differently, so the compiler generates wrong offset for calling QWidget's methods.

first, this is just an example, using the whole factory does seperate QT classes
from the rest of the application.

does someone know of a way around it ?
can't I separate logic and gui when using Qt ?

[marcel]

I didn't read all the posts, just the last question.

can't I separate logic and gui when using Qt ?

Of course you can... Qt does it with the MVC architecture, for example. There are other ways too.

I'm not sure what CompView is supposed to do, but it doesn't look good. Having an object inheriting from a GUI class and a class that implements app logic is not called "separating logic".
Actually, there's a word for that: "mess". In the end you'll end up with an object that exposes both things anyway.

[Rockem]

I didn't read all the posts, just the last question.

Of course you can... Qt does it with the MVC architecture, for example. There are other ways too.

I'm not sure what CompView is supposed to do, but it doesn't look good. Having an object inheriting from a GUI class and a class that implements app logic is not called "separating logic".
Actually, there's a word for that: "mess". In the end you'll end up with an object that exposes both things anyway.

can you give me an example ? for the short program I have ?
how can I have a widget but hide all Qt related ?

I don't know what you call a mess, but in java this is what we do and
it separates perfectly so we can work with multiple views (swing, swt, web ..)

[marcel]

I can't(not yet) because I still don't see the point... Are you writing a library? Or what?

[wysota]

can you give me an example ? for the short program I have ?
how can I have a widget but hide all Qt related ?

Currently you are not hiding anything. You still need a QApplication object, you'll need to run QApplication::exec() eventually, etc. This is certainly not the way to go, regardless of the language used.

If you want to hide Qt, hide all of it and not a part of it - have a function(s) that will do everything starting from making sure there is a QApplication object available, through instantiating the object, showing it, manipulating it up to destroying it when it's not needed anymore. Currently you're just handling instantiation and what about the rest?

There are for instance Photoshop plugins written in Qt and they handle the situation very well, so obscuring the technology behind the interface is possible, you are just doing it the wrong way.

Here is a trivial example (not tested - and you still need QApplication::exec() somewhere):

class Iface {
public:
  virtual void initialize() = 0;
  virtual void show() = 0;
  virtual void destroy() = 0;
};
 
class Implementation : public Iface {
public:
  void initialize() {
    if(!QApplication::instance()){
      QApplication *app = new QApplication;
      widget = new QMainWindow;
    }
  }
  void show() { widget->show(); }
  void destroy() { delete widget; }
private:
  QMainWindow *widget;
};

class Iface {
public:
  virtual void initialize() = 0;
  virtual void show() = 0;
  virtual void destroy() = 0;
};

class Implementation : public Iface {
public:
  void initialize() {
    if(!QApplication::instance()){
      QApplication *app = new QApplication;
      widget = new QMainWindow;
    }
  }
  void show() { widget->show(); }
  void destroy() { delete widget; }
private:
  QMainWindow *widget;
};

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[elcuco]

Plain old casting and doubble inheritance do not work as expected. Look at this line:

mw.setCentralWidget((QWidget*)c);

mw.setCentralWidget((QWidget*)c);

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Try using qobject_cast or dynamic_cast:

mw.setCentralWidget( qobject_cast<QWidget*>(c) );
mw.setCentralWidget( dynamic_cast<QWidget*>(c) );

mw.setCentralWidget( qobject_cast<QWidget*>(c) );
mw.setCentralWidget( dynamic_cast<QWidget*>(c) );

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or just no cast at all:

mw.setCentralWidget( c );

mw.setCentralWidget( c );

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And do try to put the QObject first in the double inheritance.

[wysota]

qobject_cast won't work because CompView is not a QObject. dynamic_cast seems to work.

[Rockem]

yea, dynamic_cast works great !!!

thanx