What is the difference between specifying a Widget parent with the Class and with the constructor?

[DevinQT]

Hey,

You have two ways you can specify a parent. After public in the Class declaration:
class Widget : public QWidget

And through the constructor:
RockWidget::RockWidget(QWidget* parent) : QWidget(parent)

What is the difference?
(p.s. I am new to this..)

[dheerendra]

No two ways specifying the parent. Both are two different things.

1. In case of declaration it is not parent. QWidget is base class of RockWidget. It is class relationship.
2. In the case of constructor, QWidget type object is passed as parent to object which is constructed from RockWidget. Here it is object->object relationship

[DevinQT]

Oh that's nice.

Does an object's parent always have to be another object of the same class as the child's object class has as a class-parent?

And: When the constructor says QWidget(parent) in the initializer list (and basically says QWidget = parent, right? (Parent being input from us)), what does that statement make a parent or how does that statement make or create a parent?
I am trying to find out if this is some Qt specific behavior, a Qt construct, or that this is just C++. Is there something in the keyword parent? Or is that just a name and is just saying QWidget = something, enough to make a parent? (If the constructor argument was also 'something'.

[jsulm]

@DevinQT said in What is the difference between specifying a Widget parent with the Class and with the constructor?:

Does an object's parent always have to be another object of the same class as the child's object class has as a class-parent?

No, it must be the same type as specified for the parent (or a derived class):

RockWidget::RockWidget(QWidget* parent) : QWidget(parent)

In this case parent must be a QWidget or a class derived from QWidget.

[dheerendra]

In addition to what @jsulm said please note. What ever you do in qt is all c++. Parent is just a variable name. It can be 'something' as well. It is just a object relationship ship.

[DevinQT]

So "RandomClassName* Name = RandomObjectOfThatClass" in the constructor of an object makes that object the child of the object specified in the constructor in every C++ program?

[dheerendra]

C++ does not have any class like that. If you write class like the way specified it will work.

[jsulm]

@DevinQT No. Parent - child relationship is a Qt thing, it is not C++ specific.
Also

RandomClassName* Name = RandomObjectOfThatClass

is not really valid C++ code, I'm not sure what you want to ask here. And if RandomObjectOfThatClass is a pointer to a RandomClassName instance you would simply declare a local pointer to that instance with this line of code (though ; is missing).
This is how a parent is specified:

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

So, you pass a pointer to the parent as parameter to the child. The pointer can be nullptr, in this case there is no parent - child relationship.
You should read http://doc.qt.io/qt-5/objecttrees.html

[DevinQT]

Sorry with 'RandomObjectOfThatClass' i meant that that is what's basically happening in the constructor when it's called... So it is the argument of the new object.

RockWidget* NewRW = new RockWidget(RandomObjectOfThatClass)

And the constructor would be the same as you point out. So i supplanted 'parent' with Name, and QWidget with RandomClassName.

RockWidget::RockWidget(RandomClassName* Name) : RandomClassName(Name)

So that was how i meant my previous comment.

My question is; I think it's just not clear to me why setting an object in the constructor to be equal to another existing object makes a parent.. The same way that setting an (w) integer in the the constructor to set the existing width variable makes width not a parent. Is it just with objects that this is happening? (I know integers couldn't be something like a parent, but i hope you get my point) Or is there is some magic happening here? How does Qt know that with 'QWidget = parent' i mean to create a parent? I should note that i haven't seen this concept object-to-object relationships in normal C++ tutorials yet.

_

@jsulm said in What is the difference between specifying a Widget parent with the Class and with the constructor?:

No, it must be the same type as specified for the parent (or a derived class):

With specified you mean specified in the constructor? or in the class? (The "class RockWidget : public QWidget"?) Are they related? If i try to make an 'OwnClass' a parent i get an error:

[jsulm]

@DevinQT said in What is the difference between specifying a Widget parent with the Class and with the constructor?:

With specified you mean specified in the constructor?
RockWidget is derived from QWidget, right?
QWidget is derived from QObject and hence implements parent/child relationship.
Now, if you go to http://doc.qt.io/qt-5/qwidget.html#QWidget you will see that parent must be a QWidget:

QWidget::QWidget(QWidget *parent = nullptr, Qt::WindowFlags f = ...)

"If i try to make an 'OwnClass' a parent i get an error" - yes, because RockWidget is not derived from OwnClass but from QWidget..