Unable to connect signal to a function inside main()

Question

I am aware that to use the signals and slots mechanism of Qt inside a class, the class must include the Q_OBJECT macro, but I am attempting to use signals and slots in main(), without using any class.

Here is my code so far:

#include <QApplication>
#include <QWidget>
#include <QTextEdit>
#include <QtGui>

void saveText();

int main(int argv, char **args)
 {
    QApplication app(argv, args);
    QTextEdit textEdit;
    QPushButton saveButton("Save!");
    QPushButton exitButton("Exit!");
    QObject::connect(&exitButton,SIGNAL(clicked()),qApp,SLOT(quit()));
    QObject::connect(&saveButton,SIGNAL(clicked()),qApp,SLOT(saveText()));

    QVBoxLayout vlyt;
    vlyt.addWidget(&textEdit);
    vlyt.addWidget(&exitButton);
    vlyt.addWidget(&saveButton);

    QWidget mainWindow;
    mainWindow.setLayout(&vlyt);
    mainWindow.show();

    return app.exec();
}

void saveText()
{
    exit(0);
}

Here is the GUI window generated:

From the above code, the exit button is connected to quit(), which is a Qt function, when clicked it works. The save button assigned to the function saveText(), is configured to exit, but does not do so.

Please tell me where I have gone wrong in understanding signals and slots in Qt.

Marketa

connect(&saveButton, &QPushButton::clicked, [](){saveText();}); // qt5.9.6 – Hirai

Answer 1

Qt4...

All classes that inherit from QObject or one of its subclasses (e.g., QWidget) can contain signals and slots.¹

So, you can not use slots where placed outside of QObject children.

You can connect signals to the slots which are in classes where derived from QObject. Put your slot in a class which is in a separated .h/.cpp file:

class MyClass : public QObject
{
   Q_OBJECT

   ...

   public slots:
     void saveText();

};

According to Qt5: New Signal Slot Syntax in Qt 5. You can connect to those type of global functions. (Thanks to @thuga's comments)

Duotone

So why is exit button working ? Is the quit() function inside of a class inheriting from QObject ? – Marketa

quit() is a slot inside qApp and qApp is an object which derived from QObject. – Duotone

You can do something like this in Qt5 though – Cele

@MM. Here it says The new syntax can even connect to functions, not just QObjects. You can also see this question. – Cele

@Cele , thanks a lot man !! Wish you had given an answer , by editing my code to demonstrate it , but thanks anyways , the links have been of great help – Marketa

Answer 2

I'll just put example here.

main.cpp:

#include <QCoreApplication>
#include <iostream>
#include <QObject>
#include "siggen.h"

void handler(int val){
  std::cout << "got signal: " << val << std::endl;
}

int main(int argc, char *argv[])
{
  SigGen siggen;
  QObject::connect(&siggen, &SigGen::sgAction, handler);
  siggen.action();

  QCoreApplication a(argc, argv);
  std::cout << "main prog start" << std::endl;

  return a.exec();
}

siggen.h:

#ifndef SIGGEN_H
#define SIGGEN_H

#include <QObject>

class SigGen : public QObject
{
  Q_OBJECT

public:
  explicit SigGen(QObject *parent = 0);
  void action(void);

signals:
  void sgAction(int value);
};

#endif // SIGGEN_H

siggen.cpp:

#include "siggen.h"

SigGen::SigGen(QObject *parent) : QObject(parent)
{}

void SigGen::action()
{
  emit sgAction(42);
}

Respect

Answer 3

QObject::connect(&saveButton, &QPushButton::clicked, [](){saveText();});  // qt5.9.6

or as mentioned in the masoud's answer

QObject::connect(&saveButton, &QPushButton::clicked, saveText);  // qt5.9.6

Hirai

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review – Successful

"This does not provide an answer to the question." - what question ? – Hirai

your answer is short it should be a comment, not an answer. please refer the guideline of StackOverflow. – Successful

@AashishJ "your answer is short it should be a comment" - Just because an answer is short doesn't mean it should be a comment, this is clearly an attempt to answer the question, whether or not it is correct is not the job of the LQP review queue, that's what up/downvoting is for. – Stunner

Answer 4

It is possible to connect a signal to function inside main function. This has been tested in Qt5.15. Here is the simple example where the QPushButton 'Clicked' signal is used to trigger a function (here I used lamda's, but regular functions can also be used).

int main(int argc, char *argv[])
{
    // Created QApplication
    QApplication a(argc, argv);
    
    // Created the splashscreen(which is QObject)
    QPixmap pixmap(":/images/Sample.png");
    QSplashScreen splash(pixmap);

    // Created the pushbutton and added to splashscreen
    QPushButton b(&splash);
    b.setGeometry(50,50, 100, 50);
    b.setText("FPS");
    
    // variable to be modified inside a lamda function
    int i = 0;
    
    // Connection for button clicked signal executes lamda function
    QObject::connect(&b, &QPushButton::clicked, 
    [i = static_cast<const int&>(i), &splash = static_cast<QSplashScreen&>(splash)]()mutable 
    {i = i+1; splash.showMessage("clicked: "+ QString::number(i));});
    
    // Adding properties and displaying the splashscreen
    splash.setGeometry(0,0, 1920, 1080);
    splash.setEnabled(true);
    splash.show();
    
    a.processEvents();  
    return a.exec();
}

Elrod