connect(sender, SIGNAL(signal), receiver, SLOT(slot));

1-  #ifndef CONVENTOR_H
2-  #define CONVENTOR_H
3- 
4-  #include <QWidget>
5-
6-  class QLineEdit;
7-
8-  class Conventor:public QWidget
9-  {
10-	Q_OBJECT
11-	
12-	public:
13-		Conventor(QWidget* parent=0);
14-			
15-	public slots:
16-		void setUpper(const QString&);
17-		void setLower(const QString&);
18-		
19-	private:
20-		QLineEdit* upperCaseLineEdit;
21-		QLineEdit* lowerCaseLineEdit;	
22- };
23-
24- #endif // CONVENTOR_H 

#include <QLineEdit>
#include <QLabel>
#include <QGridLayout>
 
#include "conventor.h"
 
Conventor::Conventor(QWidget* parent):
QWidget(parent)
{
	// Create Widgets
	upperCaseLineEdit=new QLineEdit;
	lowerCaseLineEdit=new QLineEdit;
	QLabel* upperCaseLabel=new QLabel("Upper Case:");
	QLabel* lowerCaseLabel=new QLabel("Lower Case:");
 
	// Establish Connection
	connect(upperCaseLineEdit,SIGNAL(textEdited(const QString&)),this,SLOT(setLower(const QString&)));
	connect(lowerCaseLineEdit,SIGNAL(textEdited(const QString&)),this,SLOT(setUpper(const QString&)));
 
	// Create Layouts
	QGridLayout* layout=new QGridLayout;
	layout->addWidget(upperCaseLabel,0,0);
	layout->addWidget(upperCaseLineEdit,0,1);
 
	layout->addWidget(lowerCaseLabel,1,0);
	layout->addWidget(lowerCaseLineEdit,1,1);
 
	setLayout(layout);
	setWindowTitle("Upper/Lower case Conventor v1.0");
}
 
void Conventor::setUpper(const QString& str)
{
	QString tmp=str.toUpper();
 
	if( upperCaseLineEdit->text() == tmp )
		return;
 
	upperCaseLineEdit->setText(tmp);
}
 
void Conventor::setLower(const QString& str)
{
	QString tmp=str.toLower();
 
	if( lowerCaseLineEdit->text() == tmp )
		return;
 
	lowerCaseLineEdit->setText(tmp);
}

connect(upperCaseLineEdit,SIGNAL(textEdited(const QString&)),this,SLOT(setLower(const Qstring&)));
 
connect(lowerCaseLineEdit,SIGNAL(textEdited(const QString&)),this,SLOT(setUpper(const Qstring&)));

#include <QApplication>
#include "conventor.h"
 
int main(int argc,char* argv[])
{
	QApplication app(argc,argv);
 
	Conventor c;
	c.show();
 
	return app.exec();
}