This line uses the Qt5 new-style connect() and a C++11 lambda function in place of a named slot function.
Qt Code: 
  1. QObject::connect(
  2.   cb,    
  3.   //^^^ sender object
  4.   static_cast<void (QComboBox::*)(int)>(&QComboBox::currentIndexChanged),   
  5.   //^^^ a PointerToMemberFunction for the currentIndexChanged(int) signal function and not the currentIndexChanged(QString) one
  6.   [&](int idx) { view->setRootIndex(model.index(idx, 0)); }  
  7.   //^^^ a C++11 lambda function accepting an int argument (Functor)
  8. );
  9.  
  10. // equivalent to the traditional Qt
  11.  
  12. connect(
  13.   cb,
  14.   SIGNAL(QComboBox::currentIndexChanged(int)),
  15.   SLOT(someSlot(int))
  16. );
  17.  
  18. // with a slot function
  19. void someSlot(int idx) {
  20.   view->setRootIndex(model.index(idx, 0)); 
  21. }
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