Unable to connect signal to a function inside main()

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);
}

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)