confusion using 'this' pointer

viniltc

Hi all,

I'm opening a new window upon clicking a push button from a QmainWindow as follows:

void stageProgram::on_pushButton_programKeyGrip_clicked()
{
   this->hide();
   keygrip = new ProgramKeyGrip(); //---> here 'this' pointer not passed 
   keygrip -> show();
}

If I dont pass this pointer as above, I can see the Paintevent (drawing two lines in the window) in the ProgramKeyGrip (given below):

#include "programkeygrip.h"
#include "ui_programkeygrip.h"

ProgramKeyGrip::ProgramKeyGrip(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::ProgramKeyGrip)
{
    ui->setupUi(this);
}
ProgramKeyGrip::~ProgramKeyGrip()
{
    delete ui;
}

void ProgramKeyGrip::paintEvent(QPaintEvent *e)
{
    QPainter painter1(this); 

    QPen linepen1(Qt::darkRed);
    linepen1.setWidth(2);

    painter1.setPen(linepen1);
    painter1.drawLine(p11,p12);
    painter1.drawLine(p12,p13);
}

Where as If I do as follows , PaintEvent is not visible in the new window:

void stageProgram::on_pushButton_programKeyGrip_clicked()
{
   this->close(); // or hide()
   keygrip = new ProgramKeyGrip(this); //---> here 'this' pointer passed 
   keygrip -> show();
}

Any idea, what's happening here?

Thanks in advance

SGaist

Hi,

In your second case, keygrip becomes a child of the stageProgram and will thus appear in its hierarchy. Since you are closing or hiding its parent, theres no reason for keygrip to be chown.

viniltc

@SGaist Thanks for your response.
My doubt is still- how this affects the paintEvent in the keygrip?

keygrip = new ProgramKeyGrip(); --->this case paintEvent is visible
keygrip = new ProgramKeyGrip(this); ----> here paintEvent not visible (all other widgets are visbile)

SGaist

@viniltc said in confusion using 'this' pointer:

keygrip = new ProgramKeyGrip(); --->this case paintEvent is visible

-> not parented so is shown because it ignores the widget were it was created in because it doesn't even know it exists.

@viniltc said in confusion using 'this' pointer:

keygrip = new ProgramKeyGrip(this); ----> here paintEvent not visible (all other widgets are visbile)

Parented to a widget that is hidden so not shown since all the children of said widget will also be hidden.

JKSH

@viniltc said in confusion using 'this' pointer:

keygrip = new ProgramKeyGrip(this);

To add to @SGaist, the code above is equivalent to

keygrip = new ProgramKeyGrip();
keygrip->setParent(this);

viniltc

Thank you @SGaist , @JKSH :)