How to draw a horizontal line??

[NIXIN]

I want to draw a horizontal line through code and I tried something like below

drawline.h
#ifndef DRAWLINE_H
#define DRAWLINE_H

#include <QWidget>
#include <QLineF>
#include <QPainter>
#include <QDebug>
#include <QGridLayout>
class DrawLine : public QWidget
{
public:
DrawLine(QWidget *parent = 0);
~DrawLine();

void lineEvent();
void paintEvent(QPaintEvent *);

private:
QPainter paint;
QLineF line;
QGridLayout *gLayout;
};

#endif // DRAWLINE_H

drawline.cpp

#include "drawline.h"

DrawLine::DrawLine(QWidget *parent):QWidget(parent)
, paint(this)
{
gLayout = new QGridLayout;
this->setLayout(gLayout);
qDebug() << "Inside Constructor";
}
DrawLine::~DrawLine()
{
qDebug() << "Inside Destructor";
}

void DrawLine::paintEvent(QPaintEvent *)
{
qDebug() << "Inside paintEvent()";

paint.setPen(QPen(Qt::black, 10));

line.setP1(QPointF(80,80));
line.setAngle(45);
line.setLength(50);

paint.drawLine(line);

}

main.cpp

#include "drawline.h"
#include <QApplication>

int main(int argc, char **argv)
{
QApplication a(argc, argv);

DrawLine *dLine = new DrawLine;
dLine->show();


return a.exec();

}

This code is building well but when I run it, following errors, occurs:
QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1
Inside Constructor
setGeometry: Unable to set geometry 22x22+363+124 on QWidgetWindow/'QWidgetClassWindow'. Resulting geometry: 116x22+363+124 (frame: 8, 30, 8, 8, custom margin: 0, 0, 0, 0, minimum size: 22x22, maximum size: 16777215x16777215).
Inside paintEvent()
QPainter::setPen: Painter not active
Inside paintEvent()
QPainter::setPen: Painter not active

Can somebody help me fix it??

[Chris Kawa]

There are a couple of errors here and some weird unused code. Lets go over it:

• setGeometry: Unable to set geometry (...) - this is just a warning. A widget has a 0 minimum size and minimum size hint by default. You used a layout on it so it has some margins (11 px in this case) Because of the frame of the window a window manager can't create a window that small so it resizes it to a smallest possible size and lets you know with a warning. To fix it either use setMinimumSize() or override minimumSizeHintfor your widget.

• QPainter related messages - you're creating a painter as a class member variable. Don't do that. Create the painter locally in the paint event with a proper surface to use it on:

void DrawLine::paintEvent(QPaintEvent *) {
    QPainter p(this);
    //paint using p
    ...

• There's no such thing as a lineEvent() - what is that?

• What's the layout for? Are you gonna put something in it? If yes then remember that it will obscure whatever you paint in your paint event.

• You're not deleting the dLine variable in main(). It's leaking. Either create it on the stack (which is the easiest and recommended), set a Qt::WA_DeleteOnClose attribute on it or delete it manually after the a.exec() call returns (pointless manual work).

• just a hint - when overriding virtual methods use override specifier. It will save you a lot of typo related bugs:

void paintEvent(QPaintEvent *);          //compiles fine and overrides the base implementation
void pAintEvent(QPaintEvent *);          //compiles fine and never gets called because of a typo
void pAintEvent(QPaintEvent *) override; //doesn't compile and lets you know. yay!