[Qt-interest] moc / multiple inheritance issue

[Enrico Ros]

Hello Jimmy,
  I looked at the code too. As Stephen Jackson said, there are 2 "problems".
You have this kind of graph:

                         .-> QLineEdit -> QWidget -> QObject
SpecialLineEdit 
                         '-> DeclareYourself

If you use:  (DeclareYourself*)   or  reinterpret_cast<DeclareYourself*>
you are "messing with the multiple inheritance chain" (see the path on the 
graph above).

You can't even do the C++ static cast for traversing the graph like that, if 
you use for example:
  static_cast<DeclareYourself*>(specialLineEdit)->declareYourself();
the compiler tells you:
  invalid static_cast from type 'QWidget*' to type 'DeclareYourself*'

There are 2 ways to do what you want, either:
  static_cast<SpecialLineEdit*>(specialLineEdit)->declareYourself();
or 
  dynamic_cast<DeclareYourself*>(specialLineEdit)->declareYourself();

- static_cast goes up-only in the graph, so it's good
- dynamic_cast knows The Good Way to handle the conversion (it may even fail 
to cast (for example Apples to Elephants), but this is not the case ;-).


Oh, and don't use the virtual inheritance with QObject descendants.

Enrico
  

On Monday 14 September 2009 02:21:52 Jimmy Cooksey wrote:
> Hello Everyone,
> 
> I'm experiencing a strange problem where moc appears to be hijacking a
> method call to my object that is multiply inheriting from some widget
> and another base class.
> 
> Here is some sample code:
> 
> // This is the base class
> class DeclareYourself
> {
> 	public:
> 	DeclareYourself() { };
> 	virtual ~DeclareYourself() { };
> 	virtual void declareYourself() = 0;
> };
> 
> class SpecialLineEdit : public QLineEdit, public DeclareYourself
> {
> 
> 	Q_OBJECT
> 
> 	public:
> 	SpecialLineEdit(QWidget* parent) : QLineEdit(parent), DeclareYourself();
> 	~SpecialLineEdit() { }
> 	void declareYourself()
> 	{
> 		setText("I'm a line edit!");
> 		return;
> 	}
> };
> 
> So when I create this object, I'm using something like...
> QWidget* specialLineEdit = new SpecialLineEdit(this);
> 
> The interesting thing is that this call...
> ((*DeclareYourself) specialLineEdit)->declareYourself();
> ... never calls SpecialLineEdit::declareYourself()
> It immediately calls this:
> SpecialLineEdit::qt_metacall(QMetaObject::Call _c, int _id, void **_a)
> and of course, I have no idea why.
> 
> The code itself compiles just fine, and the application does not crash.
> 
> I have created a sample project demonstrating my problem:
> http://www.jimster.org/QtTestCase.zip
> 
> Any help would be greatly appreciated!
> 
> Thanks,
> Jimmy
> _______________________________________________
> Qt-interest mailing list
> Qt-interest at trolltech.com
> http://lists.trolltech.com/mailman/listinfo/qt-interest
> 
 
 
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