MainView provides the following signals and slots. We need another class, MainView, which ultimately derives from QObject. The default constructor is only required for queued connections. We implement them explicitly to set breakpoints or to print debugging messages. The copy constructor and the assignment operator simply perform a member-wise copy – like the compiler generated versions would do. The class – let’s call it Copy – looks as follows. Setting the Stageįor our experiments, we need a copyable class that we will pass by const reference or by value to signals and slots. So, let’s have a systematic and detailed look at how arguments are passed to signals and slots. There is a good discussion on stackoverflow, which unfortunately leaves it to the reader to pick the right answer from all the answers and comments. The Qt documentation doesn’t say a word about it. Nearly every customer asks this question at some point in a project. How often is a an object copied, if it is emitted by a signal as a const reference and received by a slot as a const reference? How does the behaviour differ for direct and queued signal-slot connections? What changes if we emit the object by value or receive it by value?

Qt returning a value from a slot