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!