Unable to connect signal to a function inside main() - c++

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.

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)

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);
}

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

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();
}