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

jsulm

@Swati777999 I upvoted your last post. Now you should be able to answer faster :-)

Swati777999

@jsulm
So kind of you! :)

JKSH

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

is it not the standard structure of any Qt project?

A widget can have 0 or 1 parent, and it can have many children. This is what https://doc.qt.io/qt-5/objecttrees.html says.

In your code, w is the parent of quit. In other words, quit is a child of w.

Tell me: Who is the parent of w?

JKSH

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

*parent points to the null pointer [ done in its declaration] intentionally

This is wrong. The declaration contains the default value. Please see https://www.programiz.com/cpp-programming/default-argument

Compare the following constructor declarations:

// layout.h
layout(QWidget *parent = nullptr)

// qpushbutton.h
QPushButton(const QString &text, QWidget *parent = nullptr)

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

Why are you trying to access the parent in the child at all?

That's not the intention, I believe. OP just has a big misunderstanding that "w is the parent object in this code, therefore the pointer parent should point to w.

@Swati777999, note that in your code parent->setLayout(gridLay); is the same as this->parentWidget()->setLayout(gridLay);

Swati777999

Tell me: Who is the parent of w?

I think w is the primary parent of this parent-child-ladder. It's from where this hierarchy of parent-child originates, I think.

Swati777999

@JKSH

On running the above code! Layout_Example.PNG, I get the following result.

This is the result that I'm getting because of the code that I wrote in main.cpp .

The codes are written in layoutExample.cpp , is not making any change in the main window, even if I write like below..

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

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

All I want to know, is how can I add widgets in this mainWindow where you can see a pushbutton?

JKSH

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

Tell me: Who is the parent of w?

I think w is the primary parent of this parent-child-ladder. It's from where this hierarchy of parent-child originates, I think.

Yes, that's correct: w is at the top of this parent-child-ladder.

So the next question is: Which part of your code in main.cpp set w as quit's parent? Or, asked in a different way: How does main.cpp know that quit is not the the "primary parent"?

All I want to know, is how can I add widgets in this mainWindow where you can see a pushbutton?

To answer this question, there are actually 2 different things you need to know:

How to set widget parents
How to use layouts

These are different different topics.

#1 is handled by my questions above. Try to answer those questions.
#2 can be studied from the Basic Layouts Example: https://doc.qt.io/qt-5/qtwidgets-layouts-basiclayouts-example.html Run the example and see.

Swati777999

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

So the next question is: Which part of your code in main.cpp set w as quit's parent? Or, asked in a different way: How does main.cpp know that quit is not the the "primary parent"?

Doesn't LayoutExample w in main.cpp declare w as the parent object?

QPushButton quit("Quit",&w);

This sentence sets the parent of quit to be w.

JKSH

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

QPushButton quit("Quit",&w); 
This sentence sets the parent of quit to be w.

Correct.

This line constructs a QPushButton called quit, and sets w as the parent of the QPushButton object. This happens because the address of w is passed into the QPushButton constructor.

Doesn't LayoutExample w in main.cpp declare w as the parent object?

Sort of... I understand what you mean and you are correct, but please be careful with how you use the word "parent".

This line constructs a LayoutExample object called w, and sets nothing as the parent of the LayoutExample object. This happens because nullptr is passed into the LayoutExample constructor.

button1=new QPushButton("Click Here",parent);

Please fill in the blanks (see above for examples):

This line constructs a QPushButton called button1, and sets ________ as the parent of the QPushButton object. This happens because ________ is passed into the QPushButton constructor.

Swati777999

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

@Swati777999 said in Confusion between the usage of 'this' and 'parent':
QPushButton quit("Quit",&w); 
This line constructs a LayoutExample object called w, and sets nothing as the parent of the LayoutExample object. This happens because nullptr is passed into the LayoutExample constructor.
button1=new QPushButton("Click Here",parent);
Please fill in the blanks (see above for examples):

This line constructs a QPushButton called button1, and sets ________ as the parent of the QPushButton object. This happens because ________ is passed into the QPushButton constructor.

Answer to above fill in the blanks: parent, parent (which is set to null pointer)

Swati777999

@JKSH

In layoutExample.cpp , with the following modification:
setting parent of box, buttons widgets as this and using this->show(), I am able to get the result as below.
However, the layout part seems to have not worked for these widgets.

I observed that if I comment line this->show() , quit pushbutton from main.cpp seems to be overlapping signup pushbutton as they're in the same location.

From above observations, I can conclude that whatever design made in mainwindow.cpp overshadows the Main Window of the project. Correct me, if I'm wrong.

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':

Answer to above fill in the blanks: parent, parent (which is set to null pointer)

OK, and what happens when you set nullptr as a widget's parent? (Hint: Your LayoutExample object also has nullptr as its parent)

button1=new QPushButton("Click Here",this);

Good, now you have set your LayoutExample object as the QPushButton's parent. Previously, it had no parent.

However, the layout part seems to have not worked for these widgets.

When you use QGridLayout, you should also specify the row and column: https://doc.qt.io/qt-5/qgridlayout.html#addWidget-1

I observed that if I comment line this->show() , quit pushbutton from main.cpp seems to be overlapping signup pushbutton as they're in the same location.

That is because the QPushButton from main.cpp was not put in a layout. It is good practice to put all child widgets in a layout.

Swati777999

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

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

Answer to above fill in the blanks: parent, parent (which is set to null pointer)

OK, and what happens when you set nullptr as a widget's parent? (Hint: Your LayoutExample object also has nullptr as its parent)

When the widget's parent is set to null, that means it has no parent, maybe it starts acting as a parent.

JKSH

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

OK, and what happens when you set nullptr as a widget's parent? (Hint: Your LayoutExample object also has nullptr as its parent)

When the widget's parent is set to null, that means it has no parent,

Correct.

maybe it starts acting as a parent.

That's not quite right. When a widget has no parent, it can act as a top-level window (say "top-level window", not "parent"). That means whenever you call show() on a widget that has no parent, then that widget appears in a new window.

// button1 and button2 have a parent, and will appear inside your widget
button1 = new QPushButton("Click Here", this);
button2 = new QPushButton("Sign up",this);

// button3 has no parent, and will open in a new window
button3 = new QPushButton("Hello");
button3->show(); // If you don't call show(), button3 will be invisible

Swati777999

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

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

button1 and button2 appear on the window because I have used this->show() at the end.

// button1 and button2 have a parent, and will appear inside your widget
button1 = new QPushButton("Click Here", this);
button2 = new QPushButton("Sign up",this);

// button3 has no parent, and will open in a new window
button3 = new QPushButton("Hello");
button3->show(); // If you don't call show(), button3 will be invisible

I am not worried about button3, all I am concerned about is quit button of main.cpp file.

I made following change in main.cpp

quit.move(80,80)

and w.show() is already there in main.cpp

Then, why isn't quit button showing up in my window whereas other two buttons signup and 'Click Here' appear?

SimonSchroeder

You shouldn't move and resize widgets (buttons are also widgets) when you want to use layouts. Layouts will position and resize all their widgets themselves. Most likely layouts will override everything else in the end.

When you are using a QGridLayout you should always specify the row and column. I am not sure what would happen otherwise. Only with the box layouts it makes sense to not specify a position in the layout as they will then only be appended (to the right in horizontal layouts or to the bottom in vertical layouts).

About the parents in Qt: Unfortunately, it is a little bit more confusing when you are using layouts than what you have discussed so far. Whenever you add a widget to a layout it will be reparented to the layout's parent. The reason behind this is that for widgets the parent-child relationship also applies to the visual representation. When you show a parent all its children will be shown as well. I personally don't provide a parent when creating child objects (like in new QPushButton("Button")) but rely on the parent being set when I add widgets to a layout (and I exclusively use layouts instead of fixed positioning). Though this is just my personal preference.

JKSH

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

Then, why isn't quit button showing up in my window whereas other two buttons signup and 'Click Here' appear?

I agree with @SimonSchroeder: Since your top-level widget has a layout, do not add a child widget that is not part of that layout. It is not designed to work that way.

Swati777999

@JKSH

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!