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Questions tagged [signals-slots]

Signals and slots is a mechanism for implementing the observer pattern.

Signals and slots is a mechanism for implementing the observer pattern. It has many implementations in various languages:

See also:

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Does Qt support virtual pure slots?

My GUI project in Qt has a lot of "configuration pages" classes which all inherit directly from QWidget. Recently, I realized that all these classes share 2 commons slots (loadSettings() and saveSettings()). Regarding this, I have two…
ereOn
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Qt "private slots:" what is this?

I understand how to use it, but the syntax of it bothers me. What is "private slots:" doing? I have never seen something between the private keyword and the : in a class definition before. Is there some fancy C++ magic going on here? And example…
Justin
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C++ signal to QML slot in Qt

I want to send a Signal from C++ to a Slot in my QML File. I already got it working without and primitive type parameters, although if I want to send a QString to my QML Slot I get an error whilst connecting. I connect in main.cpp QObject…
alex
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8 answers

Prevent Firing Signals in Qt

We have a QCheckBox object, when user checks it or removes check we want to call a function so we connect our function to stateChanged ( int state ) signal. On the other hand, according to some condition we also change the state of QCheckBox object…
metdos
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How to emit cross-thread signal in Qt?

Qt documentation states that signals and slots can be direct, queued and auto. It also stated that if object that owns slot 'lives' in a thread different from object that owns signal, emitting such signal will be like posting message - signal emit…
grigoryvp
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Can Qt signals return a value?

Boost.Signals allows various strategies of using the return values of slots to form the return value of the signal. E.g. adding them, forming a vector out of them, or returning the last one. The common wisdom (expressed in the Qt documentation) is…
Marc Mutz - mmutz
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Qt Signals and Slot connected twice... what happens?

What happens if the same signal and slot is connected twice? How is the mechanism handled?
itapadar
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QT : Templated Q_OBJECT class

Is it possible to have a template class, which inherit from QObject (and has Q_OBJECT macro in it's declaration)? I would like to create something like adapter for slots, which would do something, but the slot can take arbitrary number of arguments…
BЈовић
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Is the PySide Slot Decorator Necessary?

I've seen some example code for PySide slots that uses the @QtCore.Slot decorator, and some that does not. Testing it myself, it doesn't seem to make a difference. Is there a reason I should or should not use it? For example, in the following…
JasonFruit
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PyQt proper use of emit() and pyqtSignal()

I am reading through some documentation on PyQt5 to come up with a simple signal-slot mechanism. I have come to a halt due to a design consideration. Consider the following code: import sys from PyQt5.QtCore import (Qt, pyqtSignal) from…
Max
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Is it possible to emit a Qt signal from a const method?

In particular, I am implementing a QWizardPage ("MyWizardPage") for a QWizard, and I want to emit a signal ("sigLog") from my override of the QWizardPage::nextId virtual method. Like so: class MyWizardPage : public QWizardPage { …
DataGraham
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Which C++ signals/slots library should I choose?

I want to use a signals/slots library in a project that doesn't use QT. I have pretty basic requirements: Connect two functions with any number of parameters. Signals can be connected to multiple slots. Manual disconnection of signal/slot…
kshahar
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How can I emit a signal from another class?

I have a problem with my Qt application. I'm trying to emit a signal from within another class (it is a nested class of the one in which the signal is placed). I already connected the signal with a slot, which should be fine. But when I try to emit…
lagoru
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Does large use of signals and slots affect application performance?

The question is just done for educational purpose: Does the use of 30-50 or more pairs of signals and slots between two object (for example two threads) affect the application performance, runtime or response times?
lucaboni
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My signal / slot connection does not work

I repeatedly see people having problems with slots not being called. I would like to collect some of the most common reasons. So maybe I can help people and avoid a lot of redundant questions. What are reasons for signal / slot connections not…
Silicomancer
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