Qt5 Tutorial QPen Cap Style - 2020

In this tutorial, we will learn QPen's Cap style.

Here is an example of Cap stylefrom Qt Doc QPen.

OK. Let's make own own example.

Qt->Qt Gui Application:

We will override QWidget::paintEvent(). That's because whenever there is a paint event, we can draw whatever we want. In other words, we let Qt know what to draw on our dialog box at the paint event.

Here is the modified dialog.h:

// dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QPainter>

namespace Ui {
class Dialog;
}

class Dialog : public QDialog
{
    Q_OBJECT
    
public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();
    
private:
    Ui::Dialog *ui;

protected:
    void paintEvent(QPaintEvent *e);
};

#endif // DIALOG_H

Here, we're going to reimplement paintEvent().

Here is the implementation file, dialog.cpp:

#include "dialog.h"
#include "ui_dialog.h"

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
}

Dialog::~Dialog()
{
    delete ui;
}

void Dialog:: paintEvent(QPaintEvent *e)
{
    QPainter painter(this);
    QPen pen(Qt::black);
    pen.setWidth(20);

    QPoint st(50,50);
    QPoint end(350,50);
    pen.setCapStyle(Qt::FlatCap);
    painter.setPen(pen);
    painter.drawLine(st, end);

    st.setY(100);
    end.setY(100);
    pen.setCapStyle(Qt::SquareCap);
    painter.setPen(pen);
    painter.drawLine(st, end);

    st.setY(150);
    end.setY(150);
    pen.setCapStyle(Qt::RoundCap);
    painter.setPen(pen);
    painter.drawLine(st, end);
}

In the paintEvent(), we created QPolygon object, fill the points, and draw it using drawPolygon().

If we run the code, we get: