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

Swati777999

@Christian-Ehrlicher

I named mainwindow as the name of the project;layout while creating it. Later, I found it to be confusing.

Then how can I add widgets to the mainwindow; the object of layout which is w that I've created?

jsulm

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

Then how can I add widgets to the mainwindow

You do it in mainwindow, not in its children.
In mainwindow you create its children and add them to the layout.

JKSH

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

In the above code, in place of 'this' when I write parent , my program crashes.

Your code crashes because parent was nullptr. You must not operate on null pointers.

layout w;

The line of code above is the same as layout w(nullptr); Can you tell me why?

Swati777999

@JKSH

ohh , I see, you're correct , in the constructor the parent has been set to null pointer but is it not the standard structure of any Qt project?

jsulm

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

but is it not the standard structure of any Qt project?

parent can be nullptr if the object does not have a parent.
So, you should always check the parent pointer before using it!

if (parent) {
    parent->...
}

JonB

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

I am more concerned about accessing the parent object in child class via parent

Then how can I add widgets to the mainwindow

You are asking/worrying about the wrong thing! Please re-read the replies from @jsulm above and think about them

But why would you want to do so?

Child should not change its parent, else it is bad design.

And again: you should NOT manipulate parent in child! It is really bad design.

Why are you trying to access the parent in the child at all? 99% of the time children should never need to look at/access parent, and you will be in that 99%! The correct place to manipulate things inside the parent/main window is inside that parent/main window, and not in any child!

Swati777999

@JKSH

Now, I am somehow able to connect the dots and understand the connectivity well.

In the constructor implementation [layout.cpp] , *parent points to the null pointer [ done in its declaration] intentionally so that we can independently create objects of parent and child in layout.cpp for simplicity.

jsulm

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

intentionally so that we can independently create objects of parent and child in layout.cpp for simplicity.

No.
parent pointer can be nullptr simply because not every object has a parent.
I really hope you will spend some minutes to read the documentation I gave you...

Swati777999

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

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

accessing the parent object in child class via parent or this keyword

Via "this" you access your object, not parent.
And again: you should NOT manipulate parent in child! It is really bad design.
If you want to know what parent is used for please read https://doc.qt.io/qt-5/objecttrees.html

Yes, I read this document but when it comes to implementing concepts that I read, I make a lot of blunders(please bear with me ) and red underlines are enough to scare off a beginner like me.

The sequence of my non-instantaneous reply in this forum is adding woes to the confusion. I can't post a reply within 10 minutes of my last reply and by the time, I am eligible to post my next reply, I had already received responses by other members. It's lil frustrating for not being able to reply immediately.

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);