Connect the Points

This example shows very simple mouse-based user interaction and painting without any world transform matrix or other advanced features. Run the program, click the button, move the mouse, release the button, and watch the lines get drawn.
Implementation:
/****************************************************************************
** $Id: qt/connect.cpp   3.3.8   edited Jan 11 14:37 $
**
** Copyright (C) 1992-2007 Trolltech ASA.  All rights reserved.
**
** This file is part of an example program for Qt.  This example
** program may be used, distributed and modified without limitation.
**
*****************************************************************************/

#include <qwidget.h>
#include <qpainter.h>
#include <qapplication.h>
#include <stdlib.h>


const int MAXPOINTS = 2000;                     // maximum number of points
const int MAXCOLORS = 40;


//
// ConnectWidget - draws connected lines
//

class ConnectWidget : public QWidget
{
public:
    ConnectWidget( QWidget *parent=0, const char *name=0 );
   ~ConnectWidget();
protected:
    void        paintEvent( QPaintEvent * );
    void        mousePressEvent( QMouseEvent *);
    void        mouseReleaseEvent( QMouseEvent *);
    void        mouseMoveEvent( QMouseEvent *);
private:
    QPoint     *points;                         // point array
    QColor     *colors;                         // color array
    int         count;                          // count = number of points
    bool        down;                           // TRUE if mouse down
};


//
// Constructs a ConnectWidget.
//

ConnectWidget::ConnectWidget( QWidget *parent, const char *name )
    : QWidget( parent, name, WStaticContents )
{
    setBackgroundColor( white );                // white background
    count = 0;
    down = FALSE;
    points = new QPoint[MAXPOINTS];
    colors = new QColor[MAXCOLORS];
    for ( int i=0; i<MAXCOLORS; i++ )           // init color array
        colors[i] = QColor( rand()&255, rand()&255, rand()&255 );
}

ConnectWidget::~ConnectWidget()
{
    delete[] points;                            // cleanup
    delete[] colors;
}


//
// Handles paint events for the connect widget.
//

void ConnectWidget::paintEvent( QPaintEvent * )
{
    QPainter paint( this );
    for ( int i=0; i<count-1; i++ ) {           // connect all points
        for ( int j=i+1; j<count; j++ ) {
            paint.setPen( colors[rand()%MAXCOLORS] ); // set random pen color
            paint.drawLine( points[i], points[j] ); // draw line
        }
    }
}


//
// Handles mouse press events for the connect widget.
//

void ConnectWidget::mousePressEvent( QMouseEvent * )
{
    down = TRUE;
    count = 0;                                  // start recording points
    erase();                                    // erase widget contents
}


//
// Handles mouse release events for the connect widget.
//

void ConnectWidget::mouseReleaseEvent( QMouseEvent * )
{
    down = FALSE;                               // done recording points
    update();                                   // draw the lines
}


//
// Handles mouse move events for the connect widget.
//

void ConnectWidget::mouseMoveEvent( QMouseEvent *e )
{
    if ( down && count < MAXPOINTS ) {
        QPainter paint( this );
        points[count++] = e->pos();             // add point
        paint.drawPoint( e->pos() );            // plot point
    }
}


//
// Create and display a ConnectWidget.
//

int main( int argc, char **argv )
{
    QApplication a( argc, argv );
    ConnectWidget connect;
#ifndef QT_NO_WIDGET_TOPEXTRA   // for Qt/Embedded minimal build
    connect.setCaption( "Qt Example - Draw lines");
#endif
    a.setMainWidget( &connect );
    connect.show();
    return a.exec();
}