C++ Qt 3 - Intro to GUI programming
Messaging and Signaling in C++ published at 20.
This time it is about how to notify one part of our application that something has happened somewhere else.
I will start with Qt, as it brings with signals and boost slots and signals a mechanism to do exactly that.
But, as I have the goal not to use Qt mainly in the UI Layer, I will also look on how to notify other parts of the application, when things are changing.
The last episode was about.
The video for this episode: Signals and Events in Qt But lets start with Qt.
Usually you have to overwrite a method to receive such events, or use an event filter, like I showed in my last post for QFocusEvents.
Some classes translate QEvents to signals, such as the TreeView, which has a signal for displaying context menus.
But as this blog post is more on signaling then system events.
Qt has had its own signaling mechanism for a long time now, so when you use Qt, you also will use QSignals.
Qt also uses its own keywords for this: signals, slots and emit.
This allows to use 3rd party libraries which use these terms, e.
Maybe I will touch this in a later post.
For now, lets see the basics of using signals and slots check this out Qt.
There is a fifth defaultet parameter: the.
The last line contains the new, lambda based connection option, where you again have the sender and its slot, this time as a method-pointer, boost slots and signals then followed by a lambda acting as the receiving slot.
This boost slots and signals can lead to a rare error, when ever a signal is overloaded, like QComboBox::currentIndexChanged, which is available with an int or QString parameter.
} ; In this case I didn't even needed the https://games-deposit-bonus.website/and-slots/qt-connecting-signals-and-slots.html from the slot.
It is fairly easy to use your own signals and slots, all you need is a QObject derived class, which is processed by the moc.
Slots are often used to react to certain events in the UI, like the currentIndexChanged signal in this case.
In the widget editor of QtCreator you get an overview of boost slots and signals signals when right clicking and selecting "go to slot.
There is also the option to map certain widgets to certain values when a signal fires, this is done via QSignalMapper.
The QSignalMapper is a member variable, and each QCheckBox connects its clicked signal to the map slot of QSignalMapper.
With setMapping the connection between the sender and the value is set up.
QVariant or a generic interface is not provided by Qt.
In the clicked slot I simply toggle the bit for the corresponding flag.
Deriving your classes from templates CRTP e.
While Qt is fairly well prepared to manage its own messaging needs, what alternatives exist, that could be used in the non Qt related code?
The C++ standard offers currently only std::function, which can be used to implement a callback mechanism.
But this has its limitations, of a 1:1 or 1:many connection this is a viable option.
I use it to notify my MainWindow class that a node in the tree has changed its name.
Also its useful to implement classes which execute a callback in a certain context, like EventFilter in the last blog post in this series.
But std::function is not an implementation of the observer pattern, and implementing your own with it would be reinventing the wheel.
Boost has had for a long time a signal library, which now is available as version 2: boost::signals2.
Using boost::signals2 Honestly, if I could avoid using signals2, I would, as it has one certain disadvantage: build times increase.
So far my project is kind of small, has only a few classes, which most of are less then 100 loc.
Adding to a class makes it hard to build a project quickly for debugging or just seeing if the work of https://games-deposit-bonus.website/and-slots/poker-and-slots.html past hour still compiles.
There is one LayoutPanel to edit, create and delete layouts in LayoutItem, and each PagePanel has a QComboBox, so that the user can select the layout for the page.
But then, this important business logic of removing a layout will only work through the UI.
But once it is in the vector, it will stay there.
But boost::signals2 can do far more.
I have no use for code that depends on the order of slots called, but you can specify this with signal::contect int group, slot : boost::signals2::signal sig; sig.
This combiner then also overwrites the return value of the signal, which is now std::vector instead of float.
Alternatives to boost::signals2 If you know very well what you are doing, you could use boost::signal instead of its new version, signals2.
This might improve your compile times, but boost::signals is not any more maintained.
Also, while signals2 is header-only, signals is not.
The thread safety is a key feature of signals2, which at some time sooner or later will come into play in your code base.
But seems to do the job.
Virtual function calls are the base of this library it seems, at least for method slots, which the call has to be derived from sigc::trackable.
The only disadvantage of boost::signal2 is really its impact on compile and link time, which can be reduced through pimple and other isolation techniques, so that a recompilation is only triggered when really needed.
I'm not sure how this would work out, but boost::signals2 seems to be pretty well build to do this, a lot of template parameters have default values which then configure the library, and are hidden from the day to day usage.
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Qt signal/slot + Boost.Coroutine. GitHub Gist: instantly share code, notes, and snippets.
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