How Qt Signals and Slots Work - Part 2 - Qt5 New Syntax. In the function FunctionPointer::call, the args0 is meant to receive the return value of the slot. If the signal returns a value, it is a pointer to an object of the return type of the signal, else, it is 0. If the slot returns a value. Signals and Slots. Signals and slots are used for communication between objects. The signal/slot mechanism is a central feature of Qt and probably the part that differs most from other toolkits. In GUI programming we often want a change in one widget to be notified to another widget.

Is there a way trigger a signal from another signal in Qt? Ask Question Asked 4 years, 2 months ago. Qt signals and slots, threads, app.exec, and related queries. Why is wired Ethernet losing its speed advantage over wireless?
This way the reference remains valid through all slot execution. I'm mentioning it only because it's possible, but I don't recommend using non-const references in signals/slots. There's no way to guarantee how a user connects them and in case of Qt::QueuedConnection everything @Paul-Colby and @kshegunov said apply.
Understanding Signals and Slot in Qt is not very difficult. Signals and slots are the basic foundation of Qt C GUI Application. In this QT tutorial we will learn signal and slots tutorial.

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.

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 Signals And Slots Tutorial

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

Retrieved from 'https://en.wikipedia.org/w/index.php?title=Signals_and_slots&oldid=839724350'

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Signals and Slots in Depth

Qt Signal Slot Example

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This chapter is from the book

Qt Signal Slot With 2 Arguments

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C++ GUI Programming with Qt4, 2nd Edition

This chapter is from the book

This chapter is from the book 

Signals and Slots in Depth

The signals and slots mechanism is fundamental to Qt programming. It enables the application programmer to bind objects together without the objects knowing anything about each other. We have already connected some signals and slots together, declared our own signals and slots, implemented our own slots, and emitted our own signals. Let's take a moment to look at the mechanism more closely.

Slots are almost identical to ordinary C++ member functions. They can be virtual; they can be overloaded; they can be public, protected, or private; they can be directly invoked like any other C++ member functions; and their parameters can be of any types. The difference is that a slot can also be connected to a signal, in which case it is automatically called each time the signal is emitted.

The connect() statement looks like this:

where sender and receiver are pointers to QObjects and where signal and slot are function signatures without parameter names. The SIGNAL() and SLOT() macros essentially convert their argument to a string.

In the examples we have seen so far, we have always connected different signals to different slots. There are other possibilities to consider.

One signal can be connected to many slots:
When the signal is emitted, the slots are called one after the other, in an unspecified order.
Many signals can be connected to the same slot:
When either signal is emitted, the slot is called.
A signal can be connected to another signal:
When the first signal is emitted, the second signal is emitted as well. Apart from that, signal–signal connections are indistinguishable from signal–slot connections.
Connections can be removed:
This is rarely needed, because Qt automatically removes all connections involving an object when that object is deleted.

To successfully connect a signal to a slot (or to another signal), they must have the same parameter types in the same order:

Exceptionally, if a signal has more parameters than the slot it is connected to, the additional parameters are simply ignored:

If the parameter types are incompatible, or if the signal or the slot doesn't exist, Qt will issue a warning at run-time if the application is built in debug mode. Similarly, Qt will give a warning if parameter names are included in the signal or slot signatures.

So far, we have only used signals and slots with widgets. But the mechanism itself is implemented in QObject and isn't limited to GUI programming. The mechanism can be used by any QObject subclass:

Notice how the setSalary() slot is implemented. We emit the salaryChanged() signal only if newSalary != mySalary. This ensures that cyclic connections don't lead to infinite loops.

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Alternative implementations[edit]

See also[edit]

Libraries[edit]

References[edit]

Qt Signals And Slots Tutorial

Qt Signal Slot Example

Qt Signal Slot With 2 Arguments

This chapter is from the book

This chapter is from the book 

Signals and Slots in Depth

Related Resources

Qt Signals And Slots Speed