signals-slots

Can someone explain to me the basic idea of Qt signals&slots mechanism IMPLEMENTATION? I want to know what all those Q_OBJECT macros do "in plain C++". This question is NOT about signals&slots usage. added: I know that Qt uses moc compiler to transform Qt-C++ in plain C++. But what does moc do? I tried to read "moc_filename.cpp" files but I have no idea what can something like this mean void *Widget::qt_metacast(const char *_clname) { if (!_clname) return 0; if (!strcmp(_clname, qt_meta_stringdata_Widget)) return static_cast<void*>(const_cast< Widget*>(this)); return QDialog::qt_metacast(

Can someone explain in simple terms the "signals and slots" pattern? Signals and slots are a way of decoupling a sender (the signal) and zero or more receivers (the slots). Let's say you a system which has events that you want to make available to any other part of the system interested in those events. Rather than hard-wiring the code that generates event to the code that wants to know about those events, you would use a signals and slots pattern. When the sender signals an event (usually by calling the function associated with that event/signal) all the receivers for that event are

What happens if the same signal and slot is connected twice? How is the mechanism handled? San Jacinto A few weeks ago, we had an intern accidentally connect a signal to a slot more than once. The idea was that under one condition, you'd have the slot connected to the signal, and under another condition you'd disconnect it. When you changed modes, you'd do the appropriate work. Well, he forgot to to disconnect when appropriate. So every time you changed modes, you had a new connection to the slot. The end result? 1 connection == 1 call to slot. 2 connections == 2 calls to slot. 3 connections =

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 inside code, and this causes the unwanted signal. Is there any way to prevent firing signal under some conditions? Job You can use the clicked signal because it is only emitted when the user actually clicked the check box, not when you manually check it using setChecked . If you just don't want the signal to be emitted at one specific time, you can

I am making an abstract-base-class and was thinking I might want a pure virtual signal. But when I compiled I get a warning for the pure virtual signals I have defined: ../FILE1.h:27: Warning: Signals cannot be declared virtual ../FILE1.h:28: Warning: Signals cannot be declared virtual Is it valid to define a pure virtual signal in C++/Qt? Is it valid to define a virtual signal? Qt's signal and slot documentation page says you can define virtual slots but doesn't talk about signals. I can't seem to find good information on pure virtual signals. Signals don't ever have an implementation[1] (i.e

There are discrepancies between respected answers here on SO and the actual Qt docs. I've read this question and I want some further clarification. Can anyone confirm: A signal is always protected , therefore it can be emitted only by the class or any of its subclasses. I'm not sure this is true; the question above shows answers supporting this statement. But the Qt docs say: Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses. So which is it? Slots are just functions, and thus may be

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 PyQt5.QtWidgets import (QWidget, QLCDNumber, QSlider, QVBoxLayout, QApplication) class Example(QWidget): def __init__(self): super().__init__() self.initUI() def printLabel(self, str): print(str) def logLabel(self, str): '''log to a file''' pass def initUI(self): lcd = QLCDNumber(self) sld = QSlider(Qt.Horizontal, self) vbox = QVBoxLayout() vbox.addWidget(lcd)