As per the main.cpp which you have sent you have made a class Test and and in the constructor you have gone for making a process with the connection of signal and slots.When an Object t of class test is made the process is initiated and the signal is eminated to be taken in by slot read().To remind you your code is down below:-
Qt Code: 
  1. #include <QApplication>
  2. #include <QProcess>
  3. #include <QStringList>
  4. #include <QMessageBox>
  5. #include <QtDebug>
  6. class Test : public QObject
  7. {
  8.    Q_OBJECT
  9.    public:
  10.       Test()
  11.       {
  12. 	 _proc = new QProcess( this );
  13.          _proc->setReadChannelMode( QProcess::MergedChannels );
  14. 	 connect( _proc, SIGNAL( readyReadStandardOutput() ), this, SLOT( read() ) );
  15. 	 connect( _proc, SIGNAL( finished(int, QProcess::ExitStatus) ), QCoreApplication::instance(), SLOT( quit() ) );
  16. 	_proc->start( "cjpeg", QStringList() << "aaa" << "bbb" );
  17.       }
  18.  
  19.    private slots:
  20.       void read()
  21.       {
  22.            QMessageBox::information( 0, "test", _proc->readAllStandardOutput() );
  23.       }
  24.    private:
  25.       QProcess *_proc;
  26. };
  27. int main( int argc, char **argv )
  28. {
  29.    QApplication app( argc, argv );
  30.    Test t;
  31.    return app.exec();
  32. }
  33. #include "main.moc"
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In my existing code I tried the similar thing.First of all I have class steg declared in steg.h but it is public QDialog whereas you have gone for public QObject.Do I have to stick to Public Qobject only.

Qt Code: 
  1. #ifndef STEG_H
  2. #define STEG_H
  3. #include "ui_stegform1.h"
  4. #include <QProcess>
  5. #include <QMessageBox>
  6. class steg  : public QDialog 
  7.  {
  8.      Q_OBJECT
  9.  public:
  10.      steg(QWidget *parent = 0); 
  11.      QProcess *_proc;   
  12.      steg(QStringList s2)
  13.       {
  14. 	   _proc = new QProcess( this );
  15.        _proc->setReadChannelMode( QProcess::MergedChannels );
  16.  
  17. 	 connect( _proc, SIGNAL( readyReadStandardOutput() ), this, SLOT( read()));
  18. 	// connect( _proc, SIGNAL( finished(int, QProcess::ExitStatus) ), QCoreApplication::instance(), SLOT( quit() ) );
  19. 	 _proc->start(("cjpeg"),s2);
  20.       }
  21. private slots:
  22.         void stego();
  23.         void read()
  24.       	 {
  25.               QMessageBox::information( 0, "stego", _proc->readAllStandardOutput());
  26.      		 }
  27. private:
  28.      Ui::steg ui;
  29.  
  30.  };
  31. #endif
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Thereafter I overloaded the constructor by passing the Qstringlist s2 ( basically to pass arguments to cjpeg). Thereafter in the function stego() written in steg.cpp I
initiated the object steg t(s2)and called for the function t.read().

Qt Code: 
  1. void steg::stego()  /*** THIS MODULE  HIDES  CODED.TXT ***/
  2. {
  3. 	 if(QFile::exists("coded.txt")) 
  4. 			{  
  6. 			s2 << "-steg" << "coded.txt" << "-q" << ui.compressionspinBox->text() << "cleanimage.ppm" << ui.jpeglineEdit ->text() ;
  7. 		 	steg t (s2);
  8. 	}
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I do get the Qmessage box but no message is there
It is blank .You also notice that I pass the signal form "stego" to slot read. Is it OK.

Where is the problem.