1. Qt Signal Slot Template Classes
2. Qt Signal Slot Template Classification
3. Qt Signal Slot Template Classic
4. Qt Signal Slot Template Classifieds

• Old syntax Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget) connect (sender, SIGNAL (valueChanged (QString, QString)), receiver, SLOT (updateValue (QString))); New: connecting to QObject member.
• Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type.

I’ve been asked multiple times how I would implement a signal / slot mechanism in modern C++. Here is the answer!

What’s the Signal / Slot Pattern?

The receivers of signals are called Slots in Qt terminology. A number of standard slots are provided on Qt classes to allow you to wire together different parts of your application. However, you can also use any Python function as a slot, and therefore receive the message yourself. A QObject only holds data and logic you can use in QML as properties, signals and slots. When registering a QObject class as a type for QML, keep this restriction in mind. To create a QML Item with C which should support a visual representation with all default properties, derive from QQuickItem instead.

So basically it allows for event based inter-object communication. In my opinion it’s intuitive to use and produces easily readable code when used in a moderate amount. And the big plus: It can be added to your program with one simple template class!

There are many libraries around (refer to the linked Wikipedia article) implementing this pattern, but it’s so easy to implement on you own that I would recommend to do this without an additional dependency. All you need is the header I posted below. And it’s a good exercise.

The signal template class

Below you can find the entire class. Because this class is using variadic templates you can define signals which pass any kind of data to their slots. Basically you can create signals which allow for arbitrary slot signatures. The emit method will accept the same argument types you declared as template parameters for the Signal class. The class is documented with comments and should be quite understandable. Further below you will find two usage examples.

Simple usage

The example below creates a simple signal. To this signal functions may be connected which accept a string and an integer. A lambda is connected and gets called when the emit method of the signal is called.

When you saved the Signal class as Signal.hpp and the above example as main.cpp you can compile the example with:

And if you execute the resulting application you will get the following output:

Advanced usage

This example shows the usage with classes. A message gets displayed when the button is clicked. Note that neither the button knows anything of a message nor does the message know anything about a button. That’s awesome! You can compile this example in the same way as the first.

You may also connect member functions which take arguments (Thank you FlashingChris for pointing out how to do this without std::placeholders!). In the following example Alice and Bob may say something and the other will hear it:

Issues & next steps

There are two drawbacks in this simple implementation: It’s not threadsafe and you cannot disconnect a slot from a signal from within the slot callback. Both problems are easy to solve but would make this example more complex.

Using this Signal class other patterns can be implemented easily. In a follow-up post I’ll present another simple class: the Property. This will allow for a clean implementation of the observer pattern.

Have some fun coding events in C++!

Edit on 2020-10-19:

• Add move-copy-constructor and move-assignment-operator
• Add emit_for_all_but_one and emit_for methods.
• Remove previously added std::forward.

Manage connections between VTK events and Qt slots. More...

#include <vtkEventQtSlotConnect.h>

Detailed Description

Manage connections between VTK events and Qt slots.

vtkEventQtSlotConnect provides a way to manage connections between VTK events and Qt slots. Qt slots to connect with must have one of the following signatures:

• MySlot()
• MySlot(vtkObject* caller)
• MySlot(vtkObject* caller, unsigned long vtk_event)
• MySlot(vtkObject* caller, unsigned long vtk_event, void* client_data)
• MySlot(vtkObject* caller, unsigned long vtk_event, void* client_data, void* call_data)
• MySlot(vtkObject* caller, unsigned long vtk_event, void* client_data, void* call_data, vtkCommand*)

Definition at line 57 of file vtkEventQtSlotConnect.h.

Member Typedef Documentation

◆ Superclass

Definition at line 61 of file vtkEventQtSlotConnect.h.

Constructor & Destructor Documentation

◆ vtkEventQtSlotConnect()

◆ ~vtkEventQtSlotConnect()

Member Function Documentation

◆ New()

◆ IsTypeOf()

◆ IsA()

Return 1 if this class is the same type of (or a subclass of) the named class.

Returns 0 otherwise. This method works in combination with vtkTypeMacro found in vtkSetGet.h.

Reimplemented from vtkObjectBase.

◆ SafeDownCast()

◆ NewInstanceInternal()

◆ NewInstance()

◆ PrintSelf()

Print the current connections between VTK and Qt.

Reimplemented from vtkObject.

◆ Connect()

Connect a vtk object's event with a Qt object's slot.

Qt Signal Slot Template Classes

Multiple connections which are identical are treated as separate connections.

◆ Disconnect()

Disconnect a vtk object from a qt object.

Passing no arguments will disconnect all slots maintained by this object. Passing in only a vtk object will disconnect all slots from it. Passing only a vtk object and event, will disconnect all slots matching the vtk object and event. Passing all information in will match all information.

◆ GetNumberOfConnections()

Allow to query vtkEventQtSlotConnect to know if some Connect() have been setup and how many.

◆ RemoveConnection()

Friends And Related Function Documentation

◆ vtkQtConnection

Definition at line 95 of file vtkEventQtSlotConnect.h.

Member Data Documentation

◆ Connections

Qt Signal Slot Template Classification

Definition at line 94 of file vtkEventQtSlotConnect.h.

Qt Signal Slot Template Classic

Qt Signal Slot Template Classifieds

• GUISupport/Qt/vtkEventQtSlotConnect.h