[Qt-interest] Calling parent Widget slot

[Ian Thomson]

Hi,

Donal O'Connor wrote:
> Now this works fine, until I try and change a member variable in the 
> setStatus(const QString & statusText) method.

In case you are wondering why this happens, it's because static_cast 
only does compile-time type checking. If you give it a QWidget pointer 
and static cast it to ParentWidgetType pointer, it will only check that 
such a cast can possibly make sense. Of course it is possible because 
ParentWidgetType derives QWidget, so the static cast will succeed 
regardless of whether you really give it a ParentWidgetType or any other 
QWidget type.

If you do such a cast on a QWidget which isn't a ParentWidgetType and 
call a functions on it, they may or may not succeed. If a function 
doesn't actually do anything specific to ParentWidgetType, then it might 
well succeed, but if it starts look at member functions then it's likely 
to crash horribly. I suspect that this is what you did. Your 
parentWidget() was probably a different widget from what you expected.

The safe way to do such a cast it like this:
ParentWidgetType* parent = dynamic_cast<ParentWidgetType*>(parentWidget());
if (parent)
	parent->setStatus(tr("Update Status"));

dynamic_cast does a runtime type check which will return a null pointer 
if the cast is not possible. However dynamic_cast is quite slow and Qt 
provides an alternative for QObject types called qobject_cast which is 
used in the same way as dynamic_cast, but has added benefits. See:
http://doc.trolltech.com/4.5/qobject.html#qobject_cast

However the signal/slot way is the best approach because it keeps 
objects self-contained.

Cheers,
Ian.