Confusion between the usage of 'this' and 'parent'

Swati777999

What do you mean by top-level widget? Is it the one declared in main.cpp ; i.e quit button?

JKSH

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

@JKSH

What do you mean by top-level widget? Is it the one declared in main.cpp ; i.e quit button?

A top-level widget is a widget that has its own window.

In your code, button1 and button2 are not top-level widgets because they don't have their own windows. Instead, they are inside w's window.

Swati777999

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

@Christian-Ehrlicher
There's only one example in the documentation.

The following code is something I tried on my own.

layoutexample.h

#ifndef LAYOUTEXAMPLE_H
#define LAYOUTEXAMPLE_H

#include <QMainWindow>
#include <QWidget>
#include <QTableWidget>
#include <QGridLayout>
#include <QPushButton>
#include <QMenuBar>

class LayoutExample : public QMainWindow
{
    Q_OBJECT
    QWidget *box;
    QGridLayout *gridLay;
    QPushButton *button1;
    QPushButton *button2;

public:
    LayoutExample(QWidget *parent=nullptr);
    ~LayoutExample();
};
#endif // LAYOUTEXAMPLE_H

layoutexample.cpp

#include "layoutexample.h"
#include <QGridLayout>
#include <QWidget>
#include <QTableWidget>
#include <QPushButton>
#include <QMenuBar>

LayoutExample::LayoutExample(QWidget *parent)
    : QMainWindow(parent)
{
    box = new QWidget();
    button1=new QPushButton("Click Here");
    button2 = new QPushButton("Sign up");
    gridLay = new QGridLayout();
    box->setFixedSize(100,100);

    gridLay->setRowMinimumHeight(5,4);
    gridLay->setHorizontalSpacing(20);
    gridLay->addWidget(box);
    gridLay->addWidget(button1);
    gridLay->addWidget(button2);

    this->setLayout(gridLay);
    //this->parentWidget()->setLayout(gridLay);
}

LayoutExample::~LayoutExample()
{
}

In the above code, in place of 'this' when I write parent , my program crashes. This code does not add widgets to the main window. Only the effects of codes of main.cpp are reflected in the main window.

How can I use w parent Qobject from main.cpp in layout.cpp? [I am so confused about the structure of writing codes in these 3 files and parent-child implementation]

Does parent in layout.cpp refer to w ?
main.cpp

#include "layoutexample.h"

#include <QApplication>
#include <QWidget>
#include <QTableWidget>
#include <QMenuBar>


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

    LayoutExample w;  // w: parent QObject
    QPushButton quit("Quit",&w);      // quit: child QObject
    quit.move(100,100); // x and y coordinates of widgets wrt to the mainwindow
    w.setWindowTitle("Layout Example");
    quit.setBaseSize(100,100);
    quit.setAutoFillBackground("yes");

    QGridLayout grid(&w);    
   //w.setLayout(grid);

    w.show();
    return a.exec();
}

}

Edit: I've changed the name of the project from layout to layoutExample to avoid confusion.

@JKSH
Another observation here,
In layoutExample.cpp , [ in main.cpp , w.show() is present as it is ]
CASE-1
In the presence of

this->show();

following result is obtained ;

CASE-2
When I comment

//this->show();

I get the following result

My doubt here is ; in CASE-2, why should these buttons; Sign up and Click Here appear as this->show() is commented?

In CASE-1 , why does quit button from main.cpp appear in the same window?

Please explain the reason.

Thanks!

Pl45m4

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

In CASE-1 , why does quit button from main.cpp appear in the same window?

Because you've set w as parent of your quit button. So it appears in parent coodinates. If you remove the &w from your quit button's c'tor and still move it to, say (100, 100) and show it afterwards, it should appear as standalone widget at 100,100 from TOP-LEFT corner of your SCREEN.

Btw: You should really use your layouts, not create them and then just throw your widgets (buttons) on your parent (LayoutExample window/widget) without actually putting them in a layout.

CASE 2

where is your this->show() located?! Cant tell right now...
The other 2 buttons are there because they are children of LayoutExample widget.... and you still show it with w.show() in your main()

Swati777999

@Pl45m4 said in Confusion between the usage of 'this' and 'parent':

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

CASE 2

where is your this->show() located?! Cant tell right now...

As mentioned in my previous reply, this->show() is located in layoutExample.cpp file.

Pl45m4

@Swati777999

Shouldn't make any difference since this in your LayoutExample class and w in main are pretty much the same objects/instances. It's just the point when you show it. Using this->show at the end of c'tor shows the widget right after it is constructed. Then you do some stuff in main() and then w.show() would display it anyway.

Swati777999

@Pl45m4

My doubt is , even if I comment this->show() in layoutExample.cpp , the widgets in layoutExample.cpp still appear on the screen. How is it possible?

Swati777999

@Pl45m4 said in Confusion between the usage of 'this' and 'parent':

@Swati777999 said in Confusion between the usage of 'this' and 'parent':
Btw: You should really use your layouts, not create them and then just throw your widgets (buttons) on your parent (LayoutExample window/widget) without actually putting them in a layout.

I've used gridlayout for the widgets in layoutExample.cpp , you can check it below(as a part of layoutExample.cpp) but it seems that the layout has not been applied successfully.

LayoutExample::LayoutExample(QWidget *parent)
    : QMainWindow(parent)
{
    box = new QWidget(this);
    button1=new QPushButton("Click Here",this);
    button2 = new QPushButton("Sign up",this);
    button1->move(200,200);
    button2->move(100,100);
    gridLay = new QGridLayout(this);
    box->setFixedSize(50,50);

    gridLay->addWidget(box);
    gridLay->addWidget(button1);
    gridLay->addWidget(button2);

    this->setLayout(gridLay);

    this->show();

    //this->parentWidget()->setLayout(gridLay);
}

JKSH

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

My doubt here is ; in CASE-2, why should these buttons; Sign up and Click Here appear as this->show() is commented?

In CASE-1 , why does quit button from main.cpp appear in the same window?

Please explain the reason.

I think you've got CASE-1 and CASE-2 mixed up. quit will show if this->show() is commented.

The reason is this: When you call show() on a widget, it also automatically calls show() on all of the widget's children.

When you show w before setting it as the parent of quit, then quit won't be shown. You must call quit.show() manually.
When you show w after setting it as the parent of quit, then quit will be automatically shown.

But more importantly: You don't need to spend time worrying about things like this. If you just put ALL of your child widgets inside layouts, then you won't encounter these strange behaviours!

I've used gridlayout for the widgets... but it seems that the layout has not been applied successfully.

Look, both @SimonSchroeder and I have told you already: You must specify the row and column when you call QGridLayout::addWidget(): https://doc.qt.io/qt-5/qgridlayout.html#addWidget-1

It irritates us when you ignore our advice and then repeat the same issue.

Swati777999

@JKSH

Yes, I remember your advice of giving a dimension to my layout to make it appear on the screen, I was mistakenly putting row,column parameters while creating gridLay as a result of which I was getting errors repeatedly but now , I understood the concept.

gridLay->addWidget(button1,100,120,Qt::Alignment=0x0020);

The third argument gives me an error. How should the value of alignment be used?
Qt::Alignment=0x0020

J.Hilk

@Swati777999

gridLay->addWidget(button1,100,120,Qt::Alignment=0x0020);

100,120 does not mean the pixels on your screen, but row/column 100/120

try placing your first widget on column/row 0 and use the actual enum not the integer value as 4th paramter

gridLay->addWidget(button1,0,0,Qt::AlignTop);

Swati777999

@J-Hilk

After modifying the existing code, gridLay->addWidget(button1,0,0,Qt::AlignTop);
I can't find any change in my display. :(

JKSH

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

My doubt is , even if I comment this->show() in layoutExample.cpp , the widgets in layoutExample.cpp still appear on the screen. How is it possible?

Because you called w.show().

After modifying the existing code, gridLay->addWidget(button1,0,0,Qt::AlignTop);
I can't find any change in my display. :(

First: Instead of QMainWindow, make your widget inherit QWidget. This makes things simpler.

Second: You only put one button in the layout. You should put ALL buttons in the layout. Remember to use different rows/columns for each button.

Swati777999

@JKSH

This is the LayoutExample.cpp

LayoutExample::LayoutExample(QWidget *parent)
    : QMainWindow(parent)
{
............
}

In place of QMainWindow, if I put QWidget, it gives an error. For example:

LayoutExample::LayoutExample(QWidget *parent)
    : QWidget(parent)
{
............
}

Swati777999

@JKSH said in Confusion between the usage of 'this' and 'parent':

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

My doubt is , even if I comment this->show() in layoutExample.cpp , the widgets in layoutExample.cpp still appear on the screen. How is it possible?

Because you called w.show().

After modifying the existing code, gridLay->addWidget(button1,0,0,Qt::AlignTop);
I can't find any change in my display. :(

First: Instead of QMainWindow, make your widget inherit QWidget. This makes things simpler.

Second: You only put one button in the layout. You should put ALL buttons in the layout. Remember to use different rows/columns for each button.
This is the complete code, please refer to it.

LayoutExample.cpp

LayoutExample::LayoutExample(QWidget *parent)
    : QMainWindow(parent)

{   gridLay = new QGridLayout(this);
    box = new QWidget(this);
    button1=new QPushButton("Click Here",this);
    button2 = new QPushButton("Sign up",this);
    button1->move(200,200);
    button2->move(100,100);

    gridLay->addWidget(box,60,60);
    gridLay->addWidget(button1,0,0,Qt::AlignTop);
    gridLay->addWidget(button2,50,100,Qt::AlignBottom);

    this->setLayout(gridLay);
    this->show();


}

LayoutExample::~LayoutExample()
{
}

JKSH

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

In place of QMainWindow, if I put QWidget, it gives an error.

You must update your .h file too.

gridLay->addWidget(box,60,60);
gridLay->addWidget(button1,0,0,Qt::AlignTop);
gridLay->addWidget(button2,50,100,Qt::AlignBottom);

You want to specify row/column number. Not pixel number.

I suggest you use a QVBoxLayout for now. It's simpler than QGridLayout.

Swati777999

@JKSH said in Confusion between the usage of 'this' and 'parent':

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

You must update your .h file too.
Done! It works now!

gridLay->addWidget(box,60,60);
gridLay->addWidget(button1,0,0,Qt::AlignTop);
gridLay->addWidget(button2,50,100,Qt::AlignBottom);

You want to specify row/column number. Not pixel number.
Noted!

I suggest you use a QVBoxLayout for now. It's simpler than QGridLayout.
Okay

Pl45m4

@Swati777999

As I've said, because you still show your LayoutExample in your main with w.show().... and this will show the MainWindow widget and ALL its children including your buttons.

Edit:
@JKSH sry :) Missed the sentence in your reply where you also said that

JKSH

@Pl45m4 said in Confusion between the usage of 'this' and 'parent':

@JKSH sry :) Missed the sentence in your reply where you also said that

No prob! It's good to have multiple confirm the same thing