Qt: How to connect static signal from different class to slot?

Question

So let's suppose I've got a regular Qt class MyQtClass and a class MyClass with ONLY static members. If I want to access the ui of MyQtClass I have to use Signals and Slots. So I create a static Signal (static so I can just invoke it like MyClass::mySignal();) and a slot in the Qt class. How can I connect the static signal from MyClass with the slot from the Qt class, without having an object of MyClass, since it has only got static members? I know that classes with only static members isn't considered as a good design in c++ but I'm too far into the project by now and I want to know if there's a way to do it with only static members. Thanks in Advance! Code: MyQtClass.h:

#include "ui_MyQtClass.h"

class MyQtClass : public QMainWindow
{
    Q_OBJECT

public:
    MyQtClass(QWidget *parent = Q_NULLPTR);

    Q_SLOT void mySlot();

private:
    Ui::MyQtClassClass ui;
};

MyClass.h:

#pragma once
#include <QtWidgets/QMainWindow>

class MyClass : public QObject
{
public:
    static void myFunction1();
    static void myFunction2();
    /*--- More Stuff ---*/


    Q_SIGNAL static void mySignal();
};

score 2 · Accepted Answer · answered Jan 11 '19 at 22:37

2

As indicated in this thread it is not possible to emit static signals since it is always associated with a QObject, as an alternative they create a singleton that would be equivalent to what you want.

#include <QtCore>
class Receiver: public QObject
{
    Q_OBJECT
public:
    using QObject::QObject;
    Q_SLOT void mySlot(){
        qDebug()<< __PRETTY_FUNCTION__;
        QCoreApplication::quit();
    }
};
class Sender: public QObject
{
    Q_OBJECT
    using QObject::QObject;
public:
    static Sender& instance(){
        static Sender m_instance;
        return m_instance;
    }
    static void myFunction1(){
        emit instance().mySignal();
    }
    Q_SIGNAL void mySignal();
};
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    Receiver r;
    QObject::connect(&Sender::instance(), &Sender::mySignal, &r, &Receiver::mySlot);
    QTimer::singleShot(1000, &Sender::myFunction1);
    return a.exec();
}
#include "main.moc"

answered Jan 11 '19 at 22:37

eyllanesc

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I tried using your method, and everything works, except the slot. My code is basically the same as yours, but my main is different: ```#include "MyQtClass.h" #include "Sender.h" #include #include int main(int argc, char *argv[]) { QApplication a(argc, argv); MyQtClass w; w.show(); const bool connected = QObject::connect(&Sender::instance(), &Sender::mySignal, &w, &MyQtClass::mySlot); qDebug() << "Connection established?" << connected; Sender::myFunction1(); return a.exec(); } ``` via qDebug I found out, that the slot is never invoked. – Jan 12 '19 at 09:53
@itssimsch First of all, have you tested my code without modifications? On the other hand I think that you have made the mistake in another part so I need to review your code so it would be great to share it through github – eyllanesc Jan 12 '19 at 12:32
Well, I don't use Github, and you'd also have to install the visual Studio Qt-Addon. Here's the whole code: https://pastebin.com/JFCay8cH Thanks for your help – Jan 12 '19 at 14:48
@itssimsch As I see your implementation of my solution has had inconveniences such as implement a signal (that must be done by the MOC), and others. https://stackoverflow.com/questions/54173098/qt5-though-connected-signal-emmited-but-slot-not-being-called So I think my solution is valid, can you mark it as correct? – eyllanesc Jan 15 '19 at 19:46
Just did that, your solution worked out perfectly fine, the mistake was that I mustn't define a function body for the signal. Thank you very much for your time! – Jan 15 '19 at 20:00
@itssimsch I recommend for future occasions first test my solution without modifications, I strive to provide a minimum code that implements the logic. As you see I do not implement the signals, only the functions and slots. :-) – eyllanesc Jan 15 '19 at 20:02

Qt: How to connect static signal from different class to slot?

1 Answers1