1. I've created a class PushButton - successor of QPushButton. Implementation of this widget was made in plugin for Qt Designer.
2. In Qt Designer i've selected QDockWidget like a container-form.
3. PushButton was placed on the form.
4. Form was saved in "widget.ui" file.
5. In main application dock widget is loading by
   @QWidget *myWidget = loader.load(&file, 0);@

with a qobject_cast 'ing.

6. Then i've tried to find my custom widget like a child of QDockWidget:
   @ myWidget->findChild<PushButton*>();@

but this method returned a NULL-pointer. Nevertheless, i can get target pointer by:

@QPushButton ancestor = myWidget->findChild<QPushButton>();@

Object's casting from QPushButton to PushButton returned NULL also.

My solution is:
@if(ancestor->inherits("PushButton"))
PushButton btn = reinterpret_cast<PushButton>(ancestor);
@