Private slot implementation in pimpl pattern by Qt-way.

Introduction.

Some time before I wrote about pimp pattern by Qt-way. In that post I was trying to show you the good way of using pimpl pattern, and use it at all. Especially when you implement your own library. Binary compatibility is good idea and d-pointers help to make it easily. Now I’ll try to extend you knowledges about this pattern, and show you new useful tool for making d-pointers better and more flexible. The next post is finally will close the pimpl topic: it will be about shared d-pointers and implicit sharing. But this topic is about private slots, the mechanism, that common used by Qt classes.

What is it? And why to use it?

Private slot is extension of d-pointers. Private slots give you way to realize slots in private class, even if private class is not QObject ( it’s not QObject almast in all classes ). But public class have to be QObject. Really the slot is not Private’s class slot, it is part of public class.
Why we need slot in private class? Let’s look at some example. There is exists QAbstractScrollArea class.It’s simply displays it’s view
port widget, and move it, if it has large geometry. QAbstractScrollAreaPrivate has two QScrollBars to make opportunity to user move viewport’s widget. Ok, and what to do when user changed scrollbar’s value ? In common way we need to connect appropriate signal of scrollbar to slot of our class, that implement necessary behaviour (scroll in our case). But this slots will became available outside our class in this case. It’s not good idea to show private implementation outside. The other possible solution is to make private class derived from QObject. Don’t do this (you have very heavy reasons to do it, and private slots is not one of them)!!!
Don’t worry, Trolls made a grade solution – private slots! This is very simple and flexible way to resolve this issue.

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