Why Layout is reparenting to the top most widget through addChildLayout?

leonardoMB

Hi I have the following:

QWidget *widget_1 = new QWidget(this);
QGridLayout *layout_1 = new QGridLayout;
widget_1->setLayout(layout_1);
QGridLayout *layout_2 = new QGridLayout;
layout_2->addWidget(_qButton, 0, 0, 1, 2, Qt::AlignHCenter);
layout_1->addLayout(layout_2,0,1);

So I would expect that layout_1 and _qButton are child of widget_1, meanwhile layout_2 should be child of layout_1.
Here is why: addLayout call adoptLayout that calls
addChildLayout(layout_2) and here we have a l->setParent(this); So layout_2 should be child of layout_1, Since layout_1 has a parent, it calls

if (QWidget *mw = parentWidget()) {
        l->d_func()->reparentChildWidgets(mw);
    }

that will make _qButton child of widget_1. Where did I go wrong? printing it I have that all are child of widget_1

J.Hilk

@leonardoMB Layouts are not qwidgets ~~nor qobjects in general~~ therefore they can not be the parent of anything -> the parent responsibility is passed on

Correction: they are indeed QObjects !

leonardoMB

@J-Hilk but what is with this function so?

void QLayout::addChildLayout(QLayout *l)
{
    if (Q_UNLIKELY(l->parent())) {
        qWarning("QLayout::addChildLayout: layout \"%ls\" already has a parent",
                 qUtf16Printable(l->objectName()));
        return;
    }
    l->setParent(this);
    if (QWidget *mw = parentWidget()) {
        l->d_func()->reparentChildWidgets(mw);
    }
}

This is setting the parent of a lyout to a layout

J.Hilk

@leonardoMB
thank you for looking up and correcting me :D

I was wrong, QLayout has QObject as a base class
https://doc.qt.io/qt-6/qlayout.html and does take ownership and manages lifetime etc

mmh,

can you post/explain a bit more of your example case, I don't think I understood it correctly!

leonardoMB

@J-Hilk No problem, thanks too for helping me.
For sure, I noticed that I forgot one line of code in the question. I edited, sorry:

widget_1->setLayout(layout_1); //This will call the following function:
void QWidget::setLayout(QLayout *l) //This function will set layout_1 as a child of widget_1

The next step is:

layout_2->addWidget(_qButton, 0, 0, 1, 2, Qt::AlignHCenter); //this one will call QLayout::addWidget

void QLayout::addWidget(QWidget *w)
{
    addChildWidget(w); //will call addChildWidget but since layout_2 hasn't parent nothing will change
    addItem(QLayoutPrivate::createWidgetItem(this, w));
}

In the last line we have:

layout_1->addLayout(layout_2); 
//that calls addLayout(), that calls QWidget::adoptLayout(QLayout *layout) that calls QWidget::addChildLayout(QLayout * layout)

So, for me this last function QWidget::addChildLayout(QLayout * layout) will reparent the items of layout_2 to widget_1 and will set the parent of layout_2 to layout_1. But it's not what happens, everything becomes child of widget_1 in the end

J.Hilk

@leonardoMB
this

    if (QWidget *mw = parentWidget()) {
        l->d_func()->reparentChildWidgets(mw);
    }

~~is clearly a wrong if condition, I think it evaluates to always true, but that may be compiler dependent.~~

~~I would blame all strange behaviour on that :D~~

~~First I thought its a copy and past error on your side, but it seems to be in the latest dev branch, If I checked that correctly.~~

long day.

JonB

@J-Hilk said in Why Layout is reparenting to the top most widget through addChildLayout?:

is clearly a wrong if condition, I think it evaluates to always true, but that may be compiler dependent.

Sorry, why?
It's just an assign-and-test, so it sets mw to parentWidget(). IF THAT IS != nullptr it goes into if body, if it's nullptr it does not execute the body. Therefore it will never reparentChildWidgets(nullptr). Is that what you were talking about?
The only compiler-dependency is whether it chooses to issue a warning (not error) message for naked-assignment-within-condition.

leonardoMB

@JonB So if it's not compile dependent I really didn't understand why I didn't get what I expected

JoeCFD

@leonardoMB said in Why Layout is reparenting to the top most widget through addChildLayout?:

What will make _qButton child of widget_1. Where did I go wrong? printing it I have that all are child of widget_1

What will make _qButton child of widget_1?
_qButton = new QPushButton( widget_1 ) or _qButton->setParent( widget_1 ); will make _qButton the child of widget_1.
The layouts of widget_1 can have children of other widgets.