Qt/C - Lesson 024. Signals and Slot in Qt5. Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided.

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.

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]

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]

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

Hi People,

Signals And Slots Qt

I have a problem with Qt meta-type system and the signal and slot connections.
I try to connect a signal and slot with each other. The signal looks like this:

@
signals: void sigSaveFileName(QString&);
@

and the slot:
@
private slots: void slotPutSaveFileName(QString& name);
@

Before I connect those, I would like register the QString with qRegisterMetaType() method then call connect method:

@
qRegisterMetaType<QString>('QString');
connect(&_worker, &Worker::sigOpenFileName, this, &MainWindow::slotPutOpenFileName);
@

If I run my application, I watch the application output, and I get the following warning:
QObject::connect: Cannot queue arguments of type 'QString&'
(Make sure 'QString&' is registered using qRegisterMetaType().)
QObject::connect: Cannot queue arguments of type 'QString&'
(Make sure 'QString&' is registered using qRegisterMetaType().)

Signals And Slots Qt Tutorial

and doesn't happen anything :(

Can tell me somebody what I make wrong?

Qt Signals And Slots Example

Regards,
Norbert