The example program on this page may be used, distributed and modified without limitation.

A graphical hello world

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.
//
// Qt Sample Application: connect.cpp
//
// Draws lines that connect points.
//

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

const int MAXPOINTS = 2000;                     // maximum number of points
const int MAXCOLORS = 400;

//
// 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
};

//
// Initialize ConnectWidget.
//

ConnectWidget::ConnectWidget( QWidget *parent, const char *name )
        : QWidget( parent, name )
{
    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;
}

//
// Paint/refresh widget
//

void ConnectWidget::paintEvent( QPaintEvent * )
{
    QPainter paint;
    paint.begin( this );                        // start painting
    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
        }
    }
    paint.end();                                // painting done
}

//
// mousePressEvent receives all mouse presses
//

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

//
// mouseReleaseEvent receives all mouse releases
//

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

//
// mouseMoveEvent receives all mouse move/drag events
//

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

//
// Create and display a ConnectWidget
//

int main( int argc, char **argv )
{
    QApplication a( argc, argv );
    ConnectWidget connect;
    connect.show();
    return a.exec( &connect );
}

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