Private Slots Qt 5

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Disclaimer: this article descrbes techiques that are not part of the public Qt API, using them may result in non-portable or version specific code. The example below was tested with version 4.3.3 on 64bit Linux.
Ok so, now that we have the disclaimer out of the way... If you are a Qt developer you are probably familiar with the concept of private implementation, or ’pimpl’ classes. If not, well, I won’t get into that here, but if you look it up on Google or Wikipedia you should get the idea fairly easily.
QTTRFUNCTIONS is a macro that defines all the ’tr’ functions used for multi-language support. Qtmetacast performs a dynamic cast, given the class name or the name of one of its base classes (Qt doesn’t rely on RTTI, obviously). Qtmetacall calls an internal signal or slot by its index. The button click (signal) is connected to the action (slot). In this example, the method slotmethod will be called if the signal emits. This principle of connecting slots methods or function to a widget, applies to all widgets. Qt5 Tutorial: QOBJECT Macro. The QOBJECT Macro is probably one of the weirdest things to whoever beginning to use Qt. Qt QObject Class says: The QOBJECT macro must appear in the private section of a class definition that declares its own signals and slots or that uses other services provided by Qt’s meta-object system. Signals and Slots. In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt’s widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. There’s a slots game Private Slots Qt5 that will appeal to virtually everyone. Finding your favourites will be an exciting, fun-filled journey of exploration. If you’re looking for somewhere to start, you might want to check some of our most popular Online Slots games in the Wheel of Fortune family of games.
Now, say you have a private class and you want it to have slots . One way is to add a slot to your public class then have it call the method you want in the private class, but this means you have to add a method to public API which means you break binary compatibility. Another is to have your private class extend QObject, but this adds overhead. Another way is to use the Q_PRIVATE_SLOT macro, which I will explain here.
First lets look at how to set up the private class.
file: slotTest_p.h

The Q_DECLARE_PUBLIC(slotTest) macro definition is:
#define Q_DECLARE_PUBLIC(Class) inline Class* q_func() { return static_cast(q_ptr); } inline const Class* q_func() const { return static_cast(q_ptr); } friend class Class;
The main purpose of this is to define a q_func() method to make sure we can accesses it with the correct const-ness and ensure its cast to the correct type. In practice, when implementing methods of the private class you should virtually never use q_func() or q_ptr directly; instead you should place the Q_Q(Class) macro, which is defined as: #define Q_Q(Class) Class * const q = q_func(), at the beginning of the method implementation. From then on in the method you can use the pointer q to refer to the public class. For example:

Ok, so now we have our private class, lets create the public class.
file: slotTest.h

It’s important that you don’t include the actual private file here, otherwise you will get errors like:

We have to include the Q_OBJECT macro here even though this example class doesn’t actually have any signals or slots of it’s own because otherwise moc won’t even look at it in the first place.
The Q_DECLARE_PRIVATE(Class) macro is just the counterpart to the Q_DECLARE_PUBLIC(Class) macro except it creates a method named d_func() to access the private class. Likewise, there is a Q_D macro for use inside method implementations so you can use d as a pointer to the private class.
Private Slots Qt 5 8The Q_PRIVATE_SLOT macro takes a pointer to your private class, and we will take advantage of the Private Slots Qt 5 0d_func() method to get it. Next is the signature of the private class’ method to call. One interesting thing about this macro is that it directly expands to no actual code - it simply a trigger for moc.
We are also creating a QTimer to call this slot repetadly for the example.
Now our implementation:
file: slotTest.cpp

The most important thing to notice here is that we have listed the moc-generated moc_slotTest.cpp file as an include, doing this ensures that slotTestPrivate has been defined before the moc file is processed. If you were to not include of this file here, you may get an error like:

Here, we just set up our private class, a timer, and connect the timer to call our slot. Notice that even though the method we are calling is inside our private class, we still use this in the connection because the slot is technically a slot of our public class, even though the implementation is in the private class.
Now lets make it runnable:
filename: main.cpp

And one final very important thing, the project file...

Private Slots Qt 5 12It’s very important that slotTest.h is listed before slotTest_p.h otherwise you will end up with the following error:
Private Slots Qt 500
Once you have it built, you should see ’bob!’ printed to the screen every second from the bob() method in the private class.
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