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1. Signal And Slot In Qt
2. Boost Signals And Slots
3. Qt Signal Slot With 2 Arguments
4. Qt Connect Signal Slot
5. Qt Vs Boost

Qt's meta-object system provides the signals and slots mechanism for inter-object communication, run-time type information, and the dynamic property system. The meta-object system is based on three things: The QObject class provides a base class for objects that.

12 Sep 2004

The article describes an efficient way to implement delegates in C++ using Signal and Slot pattern.

Introduction

In my previous article “Generic Observer Pattern and Events in C++”, we discussed classical “Observer Pattern” and its implementation in C++ using templates. An event class CppEvent introduced in that article allowed us to bind together loosely coupled classes, by connecting one class containing CppEvent to other class member functions. Member functions could be of any number and types of arguments, and no relations or special inheritance for model and view classes are required.

The only problem here is that we can’t delete our view without detaching it from the model first without risk of crashing our application on the next call of the event notification. It’s becoming annoying when we have many models created and deleted dynamically. To resolve this problem, we need a delegate which removes itself from the model when the view is deleted. To accomplish this task, we use Signal and Slot concept. This concept had been introduced in Trolltech Qt library and Boost C++ library.

Using Signal and Slots

To demonstrate how signals and slots work, we create a model class containing CppSignal member and a view class containing CppSlot.

Note that we use CppSignal1 to specify signal with one parameter; for two and three parameters there are CppSignal2 and CppSignal3. We could avoid this name multiplicity for standard C++ compiler, it’s done for portability with VC++ 6, which is still in use.

Note that slot member is initialized in the constructor of the view class. It’s initialized with pointer to the view class and member function. Now, to connect model to the view, we need to connect its signals with slots.

Now we can create another view and connect it to the same model.

In order to track return values from view's callback functions, we can use collector function object. For example, an object counting return values will be:

And when it is passed as a second parameter of emit_sig method, it will return sum of view responses.

Now we can delete one of the views and the sum of responses will change accordingly.

Signals and Slots implementation

Implementation of CppSignal and CppSlot is very similar to CppEvent in “Generic Observer Pattern and Events in C++”.

Signal And Slot In Qt

Signals and slots is a language construct introduced in Qt for communication between objects^[1] which makes it easy to implement the observer pattern while avoiding boilerplate code. The concept is that GUI widgets can send signals containing event information which can be received by other widgets / controls using special functions known as slots. This is similar to C/C++ function pointers, but signal/slot system ensures the type-correctness of callback arguments.^{[citation needed]}

The signal/slot system fits well with the way graphical user interfaces are designed. Similarly, the signal/slot system can be used for other non-GUI usages, for example asynchronous I/O (including sockets, pipes, serial devices, etc.) event notification or to associate timeout events with appropriate object instances and methods or functions. It is easy to use and no registration/deregistration/invocation code need to be written, because Qt's metaobject compiler (MOC) automatically generates the needed infrastructure.

A commonly used metaphor is a spreadsheet. A spreadsheet has cells that observe the source cell(s). When the source cell is changed, the dependent cells are updated from the event.

Boost Signals And Slots

Alternative implementations[edit]

There are some implementations of signal/slot systems based on C++ templates, which don't require the extra metaobject compiler, as used by Qt, such as libsigc++, sigslot, vdk-signals, nano-signal-slot, neosigslot, Signals, boost.signals2, Synapse, Cpp::Events, Platinum and JBroadcaster. Common Language Infrastructure (CLI) languages such as C# also supports a similar construct although with a different terminology and syntax: events play the role of signals, and delegates are the slots. Another implementation of signals exists for ActionScript 3.0, inspired by C# events and signals/slots in Qt. Additionally, a delegate can be a local variable, much like a function pointer, while a slot in Qt must be a class member declared as such. The C based GObject system also provides similar functionality via GSignal.In D it is implemented by std.signals.

See also[edit]

Qt Signal Slot With 2 Arguments

Libraries[edit]

Java: sig4j - multi-threaded, type-safe, based on the FunctionalInterface annotation introduced in Java 8.

C++: vdk-signals - thread-safe, type-safe, written in C++11 with atomic variables.

References[edit]

Qt Connect Signal Slot

1. ^'Signals & Slots - QtCore 5.1'. Qt Project. 2013-07-04. Retrieved 2013-07-04.

Qt Vs Boost